materials Article
A Study of Polycrystalline Silicon Damage Features Based on Nanosecond Pulse Laser Irradiation with Different Wavelength Effects Jiangmin Xu 1, *, Chao Chen 1 , Tengfei Zhang 1 and Zhenchun Han 2 1 2
*
School of Mechanical Engineering, Jiangsu University of Science and Technology, Zhenjiang 212013, Jiangsu, China;
[email protected] (C.C.);
[email protected] (T.Z.) School of Mechanical Engineering, Jiangsu University, Zhenjiang 212013, Jiangsu, China;
[email protected] Correspondence:
[email protected]
Academic Editor: Martin M. Frank Received: 11 February 2017; Accepted: 28 February 2017; Published: 3 March 2017
Abstract: Based on PVDF (piezoelectric sensing techniques), this paper attempts to study the propagation law of shock waves in brittle materials during the process of three-wavelength laser irradiation of polysilicon, and discusses the formation mechanism of thermal shock failure. The experimental results show that the vapor pressure effect and the plasma pressure effect in the process of pulsed laser irradiation lead to the splashing of high temperature and high density melt. With the decrease of the laser wavelength, the laser breakdown threshold decreases and the shock wave is weakened. Because of the pressure effect of the laser shock, the brittle fracture zone is at the edge of the irradiated area. The surface tension gradient and surface shear wave caused by the surface wave are the result of coherent coupling between optical and thermodynamics. The average propagation velocity of laser shock wave in polysilicon is 8.47 × 103 m/s, and the experiment has reached the conclusion that the laser shock wave pressure peak exponentially distributes attenuation in the polysilicon. Keywords: laser wavelength; polysilicon; laser damage; thermal shock
1. Introduction A PVDF (piezoelectric sensing techniques) piezoelectric sensor, which has been developed in recent years with a high response frequency (nano_second level) and above a 20 GPa pressure measurement range, is an ideal test component of laser shock waves [1]. Researchers have successively measured the attenuation law of laser shock waves in the solid target, shock wave velocity and stress wave waveform [2]. Silicon materials are excellent optical materials, usually used in the filter and substrate materials of optical systems, and they are widely applied in the microelectronics industry, optoelectronic industry and other fields. Because the band width of silicon materials is narrower than other materials (1.12 eV at 300 K), they have a large intrinsic absorption of the infrared band laser. Meanwhile, they are brittle materials with a narrow plastic region, so they are prone to be damaged in the infrared wavelength of strong light irradiation [3]. Chen et al. [4] used a laser to etch microchannels on the surface of polysilicon to increase photoelectric conversion efficiency of polysilicon solar cells. The results show that the efficiency of polysilicon solar cells and microchannels is increased by 0.23%–1.50%. The fill factor of microchannel scanning also improves polycrystalline silicon solar cells. Karnakis et al. [5] used nanosecond Nd:YAG (Neodymium-doped Yttrium Aluminium Garnet) at 355 nm high-intensity laser etching of monocrystalline silicon. The results show that an abnormally high etching depth is observed on the silicon surface when the intensity of the incident laser exceeds a
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certain threshold. Mainly due to a high-intensity laser explosive boiling mechanism with the secondary heating of the plasma, the laser energy changes and comes up with two different erosion mechanisms. Dobrzanski et al. [6] used the wavelength of a 1064 nm laser to treat the micro-texture of the solar polysilicon surface, which improved the solar cell trapping, effectively reducing the reflectivity and improving the efficiency of solar cells. Kumar et al. [7] studied the microstructure evolution of a silicon surface by laser etching. Therefore, it is of great practical significance to study the damage mechanism of silicon with different wavelengths. 2. Theoretical Analysis of Thermodynamic Effects of Polycrystalline Silicon Irradiated by Single Pulse Laser 2.1. Analysis of Vapor Pressure Effect When the laser is irradiated to the surface of polysilicon, the laser is absorbed strongly. When the time waveform of output laser is Gaussian, the change of temperature rise with pulse time can be described as follows: 2AI0 √ x T ( x, t) = DIer f c √ , t < tp. (1) k 2 Dt When the pulse is completed, the temperature rise equation is: q √ 2AI0 x x , t ≥ tp. T ( x, t) = DIer f c √ − D (t − t p ) Ier f c q k 2 Dt 2 D (t − t p )
(2)
When the laser is continuously irradiated to polycrystalline silicon, the surface temperature rises to the evaporation temperature of the target, which will build up pressure on the target as the target material evaporates. Assuming that the pυ ( Ts ) temperature is the equilibrium vapor pressure, the expression is expressed as [8,9]:
Lυ pυ ( Ts ) = P∞ exp k B Ts
Ts −1 Tb
,
(3)
where Ts —target surface temperature, Tb —boiling point temperature, p∞ —equilibrium vapor pressure at Tb —temperature, Lυ —latent heat of vaporization, and k B —Boltzmann constant. It is obvious that target surface vapor pressure is related to temperature, and it is a function of temperature. 2.2. Plasma Shock Wave Pressure Effect Plasma generation occurs when a pulsed laser is irradiated on a target, which includes a plasma flash and a plasma blast process to produce a shock wave [10]. A plasma shock wave propagates against the incident direction in the form of ultrasound, which will have a certain pressure effect on an irradiated area of the target. According to Phipps’s pressure load analysis formula, we can get the pressure formula of the plasma shock wave on the surface of laser target area [11]:
√ n P = bI ( Iλ τ) .
(4)
In the formula, λ (nm) is the laser wavelength, Ia (GW/cm 2 is the laser power density, τ (ns) is the laser pulse width, and b is the parameter determined for the material n = −0.3, b = 5 [12]. 3. Experimental Equipment and Methods The experiment uses the polysilicon sample of 30 mm × 30 mm × 0.25 mm and removes oil on the sample surface with acetone, cleans it with ethanol, and dries it with cold air. Taking the effect of laser thermal effects into account, according to the literature [13] on temperature and location relationship of laser ablation silicon material, when the temperature at a depth of 1.0 × 10−6 m is 300 K, its thermal
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300 K, its thermal impact on PVDF can be ignored. The target surface has no absorbing and impact on PVDF can but be ignored. The target surface has are no absorbing anditconstrained layers, buttoPVDF constrained layers, PVDF piezoelectric sensors attached to and perpendicular each piezoelectric sensors are attached to it and perpendicular to each other, and the back surface of thea other, and the back surface of the target is closely fitted with PVDF through the clip. PVDF has target is closely withabsorb PVDF the through clip. through PVDF has a veryThe thick base that can absorb the very thick base fitted that can shockthe wave PVDF. Nd:YAG solid-state lasers shock wave through PVDF. The Nd:YAG solid-state lasers (1064 nm, 532 nm, 355 nm), pulse width (1064 nm, 532 nm, 355 nm), pulse width of 10 ns, laser spot diameter of 2 mm, and pulse laser energy of 10 ns, laser spot of the 2 mm, and pulse energydevice rangeof ofshock 100–200 mJpressure are used in the range of 100–200 mJdiameter are used in experiment. Thelaser acquisition wave signal is experiment. The1.acquisition device of shock wave pressure signal is shown in Figure 1. shown in Figure
Figure 1. Shock experiment device device schematic. schematic. Figure 1. Shock wave wave measurement measurement experiment
4. Experimental 4. Experimental Results Results and and Analysis Analysis 4.1. Vapor Pressure Effect on the Target Target Surface Surface When using laser to irradiate the target, the energy absorbing particles collide with each other and the the temperature temperature of the the material material surface will be significantly increased after to transfer energy, and being heated by laser energy [14]. As shown in Figure 2, sputtering and wave-like ripples appear outside the boundaries of the three-wavelength laser irradiated regions due to the high temperature of the material in the irradiated region and the propagation of waves to the surroundings. When the polysilicon, a laser breakdown breakdown energy density of the laser exceeds the breakdown threshold of the polysilicon, occurs. The vapor formed on the surface of the material forms a substance vapor with the outward splashing substance and continues continues to to absorb absorb the the energy, thus it is partially ionized to form a high temperature and high pressure plasma. In the process pulse, the splashing phenomenon of and high pressure plasma. In the processofofa alaser laser pulse, the splashing phenomenon high temperature and high of high temperature and highdensity densitymolten moltenmaterial materialshows showsthat thatthe the melted, melted, gasified gasified and ionized is discharged dischargedacutely. acutely.Due Duetotothe thefurther further enhancement laser intensity, surface of material is enhancement of of thethe laser intensity, the the surface of the the irradiated polysilicon vaporizing, the ionized breakdown of the ambient gas, or the gasified irradiated polysilicon vaporizing, the ionized breakdown of the ambient gas, or the gasified substance substanceaproduces a high-speed laser detonation wave (LSD), at which time thegas expansion and produces high-speed laser detonation wave (LSD), at which time the expansion and thegas surface the surface layer interact. First, air high-speed air stamps molten liquid and then couples to the melted layermelted interact. First, high-speed stamps molten liquid and then couples to the solid part to solid part to form the pressure, causing liquid–solid interface deformation. At this point, the strain of form the pressure, causing liquid–solid interface deformation. At this point, the strain of the media thethe media result mechanical of thermal mechanical coupling. The wave laser shock causes the pressure is resultisofthe thermal coupling. The laser shock causeswave the pressure wave in the wave in the melt layer to emit back when it encounters the solid–liquid interface. As the laser energy melt layer to emit back when it encounters the solid–liquid interface. As the laser energy is large, the is large, layer the solution will peelsurface off theand solid surface splashinout, as shown in Figure 3. solution will peellayer off the solid splash out,and as shown Figure 3. The melting and The meltingof and in theregion laser irradiation region provides conditions gasification thegasification polysilicon of in the the polysilicon laser irradiation provides the conditions for the removal of for material, the removal of the material, the vaporgreatly pressure contributes to the migration of the but the vapor pressurebut contributes to the migration greatly of the material. − 1 the material. At 300 K, the light penetration depth coefficients (α ) of polysilicon materials with different At 300 K,(1064 the light penetration coefficients ( α 1 ) respectively of polysilicon[15]. materials with different wavelengths nm, 532 nm, 355 depth nm) were 0.01, 1, 1000, The bandgap width − 1 wavelengths (1064 nm, nm)onwere 1, 1000, width Eg Eg of the polysilicon is 532 1.12nm, eV. 355 Based the 0.01, Equation: E(respectively eV ) = hν = [15]. 1.24 The · λ bandgap , the laser photon , the3.49 laser energies of the polysilicon is 1.12 eV. Based on the E (nm) eV ) are h1.165, 1.242.33 λ 1and energies of the three wavelengths (1064 nm,Equation: 532 nm, 355 eV.photon The absorption of the three wavelengths (1064 nm, 532 nm, 355 nm) are 1.165, 2.33 and 3.49 eV. The absorption
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coefficients of of the the polycrystalline polycrystalline silicon silicon material material at at 1064, 1064, 532 532 and and 355 355 nm nm were were 10 1033,, 6.68 6.68 ×× 10 1033 and and coefficients 6 cm−1 −1 [16]. Under this energy, the wavelength of 1064 nm is mainly due to hot melting 3 3 6 10 damage in coefficients the polycrystalline silicon materialofat1064 1064, 532 355due nm to were , 6.68 damage × 10 and 10 cm [16].ofUnder this energy, the wavelength nm is and mainly hot 10 melting in 6 − 1 thecm photothermal mode. Duringthe thewavelength irradiationof process, the moltendue material greater, and the the 10 [16]. Under this energy, 1064 nm is mainly to hotisis melting damage in the photothermal mode. During the irradiation process, the molten material greater, and spray phenomenon occurs under the action of the vapor pressure and more obvious ripples are the photothermal mode. During thethe irradiation the pressure molten material is greater, and the spray spray phenomenon occurs under action ofprocess, the vapor and more obvious ripples are formed. The Thewavelength wavelength of 532 532nm nmisismainly mainly dueto topressure hotmelting melting damage andphotothermal photothermal damage phenomenon occurs under the action of the vapor anddamage more obvious ripples are damage formed. formed. of due hot and (thewavelength high photon photon energy opens the the to chemical bond of the the material directly and causes causes The of 532 nm is opens mainly due hot melting damage and photothermal damage (the high (the high energy chemical bond of material directly and aa photochemical reaction to cause the debris of the material to be ejected in a small or gaseous photon energy opens the to chemical bonddebris of theof material directlytoand reaction photochemical reaction cause the the material becauses ejecteda photochemical in a small or gaseous manner [17]). Themain main form ofdamage damage hotmelt melt damage andthe themanner meltisisrelatively relatively less than 1064 to cause [17]). the debris of theform material to be ejected in a small or gaseous [17]). Theless main form of manner The of isishot damage and melt than 1064 nm, but also forms a clear ripple. The wavelength of 532 nm is mainly due to hot melting damage and damage is hotforms melt adamage and the is relatively lessnm than 1064 nm, forms adamage clear ripple. nm, but also clear ripple. Themelt wavelength of 532 is mainly duebut to also hot melting and photothermal damage, butthe themainly maindamage damage photothermal damage. Thedegree degreeof ofdamage, rippleformation formation The wavelength of 532 but nm is due to hot melting damage and photothermal but the photothermal damage, main isisphotothermal damage. The ripple isrelatively relatively small. is small. main damage is photothermal damage. The degree of ripple formation is relatively small.
Figure 2.2. The ripples of three wavelengths after laser irradiation. Figure2. Theripples ripplesof ofthree threewavelengths wavelengthsafter afterlaser laserirradiation. irradiation. Figure The
Figure3.3.Interactions Interactionsof ofthe thesilicon siliconmelting meltinglayer layerand andsolids solidsby byLSD LSDwave. wave. Figure
When the laser laser irradiates irradiates the the surface of of the target, target, the laser laser will melt melt the surface surface layer of of the the When When the the laser irradiates the surface surface of the the target, the the laser will will melt the the surface layer layer of the target, and the pulse laser will generate the heat wave again, which causes the periodic change of the target, heat wave again, which causes thethe periodic change of the target, and and the thepulse pulselaser laserwill willgenerate generatethe the heat wave again, which causes periodic change of targetsurface surface[18]. [18].Under Underthe theaction actionof ofdetonation detonation wave waveimpulse, impulse,aalarge largeradial radialvapor vaporpressure pressurewill will target the target surface [18]. Under the action of detonation wave impulse, a large radial vapor pressure be generated, generated, and and the the liquid liquid will will be be pushed pushed to to the the edge edge of of the the irradiated area area to form form a wave-like be will be generated, and the liquid will be pushed to the edge of theirradiated irradiated areato to form aa wave-like wave-like phenomenon. The high-pressure and high-speed expansion airflow stamps molten liquid and and phenomenon. The Thehigh-pressure high-pressure and high-speed expansion airflow stamps liquid phenomenon. and high-speed expansion airflow stamps moltenmolten liquid and couples couplesto tothe thesolid solidtarget targetpart, part,producing producingthe theback backpressure, pressure,which whichisisperpendicular perpendicularto tothe thesurface, surface, couples to the solid target part, producing the back pressure, which is perpendicular to the surface, to form to form the elastic wave source and produce the surface shear wave. Then, the propagation of the the to thewave elastic waveand source and the produce the surface shear wave. Then, the propagation of theform elastic source produce surface shear wave. Then, the propagation of the transverse transverse wave causes the liquid–solid interface deformation. At the same time, the temperature transverse causes the liquid–solid interface deformation. At the time, the temperature wave causeswave the liquid–solid interface deformation. At the same time, thesame temperature gradient of the gradient of of the the molten molten material material forms forms aa surface surface tension tension gradient, gradient, which which aggravates aggravates the the ripple ripple gradient phenomenon. In the modulated laser irradiation, the high temperature region of the liquid on the phenomenon. In the modulated laser irradiation, the high temperature region of the liquid on the
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molten liquid surface is pulled to the low temperature region; it is the result of optical and molten material forms a surfaceThe tension gradient, which theory aggravates theused ripple In the mechanical coherent coupling. thermal wave model can be to phenomenon. analyze it, aiming at modulated laser irradiation, the high temperature region of thethe liquid on the molten liquid surface is the phenomenon of corrugation on the silicon surface. Under experimental conditions, the laser pulledwidth to theof low temperature1064, region; is the andthe mechanical coherent coupling. pulse wavelengths 532itand 355result nm isof10optical ns; thus, thermal wave frequency canThe be thermal wave can. Assuming be used to analyze aiming at thedphenomenon of corrugation on the f 1model / 2τ theory 50 MHz that the it, stripe spacing is wavelength λ of the thermal obtained: silicon the surface. thecan experimental conditions, the laserEquation pulse width of wavelengths 1064, 532 and wave, waveUnder velocity be obtained by the following (5) [18]: 355 nm is 10 ns; thus, the thermal wave frequency can be obtained: f ≈ 1/2τ = 50 MHz. Assuming Tm g 2 that the stripe spacing d is wavelength λvof thermal the wave velocity can be obtained(5) by the tanh mh wave, m ρ the following Equation (5) [18]: , g Tm 2 v = gtanhmh + , (5) where in m 2π / d , T is the surface tension, acceleration, and ρ is density, m is the gravitational ρ 3. Since h d / 2 , d is small, the above Equation (6) and theindensity of the material is 2.3 g/cm where m = 2π/d, T silicon is the surface tension, g is the gravitational acceleration, and ρ is density, and is as:the silicon material is 2.3 g/cm3 . Since h ≥ d/2, d is small, the above Equation (6) is thesimplified density of simplified as:
T v 2 2π T 2 v = 2πρd .. ρd
(6) (6)
Putting , ν = into Equation (5),(5),thethesurface 500500 m/sm/s Putting dd =2 2μm µm, into Equation surfacetension tensioncan canthen thenbe be obtained obtained as: as: 183 T= 183N/m N/m. . According to wewe summarize thethe interaction process between a high According to the theexperiment experimentphenomenon, phenomenon, summarize interaction process between a energy pulse laser and polysilicon material, as shown in Figure 4. When distributed laser of Gaussian high energy pulse laser and polysilicon material, as shown in Figure 4. When distributed laser of irradiates irradiates polysilicon, the energythe of the lightofspot is the largest, so the center irradiation Gaussian polysilicon, energy the center light spot center is the largest, so of thethe center of the area is the area of peak parameter. As shown in the Figure 4 laser irradiation area, due to the effect irradiation area is the area of peak parameter. As shown in the Figure 4 laser irradiation area, dueoftoa higheffect energy silicon material changes fromchanges solid tofrom liquid on to theliquid surface target, the of alaser, high the energy laser, the silicon material solid on of thethe surface ofand the moves toward the light spot the edgelight under theedge actionunder of plasma pressure steampressure pressure.and Then, the target, and moves toward spot the action of and plasma steam melt recrystallizes again and becomes solid on the edge of the light spot. On top of the liquid state pressure. Then, the melt recrystallizes again and becomes solid on the edge of the light spot. On top material is thestate steam, not fully ionized. of the steamOn is the layer. Because the area is a of the liquid material is the steam,On nottop fully ionized. topplasma of the steam is the plasma layer. completely the plasma layer will produce high pressure compress the pressure steam layer, Because theionized area is material, a completely ionized material, the plasma layer willtoproduce high to and accelerate the ionization of the molecular state material. Under the liquid state is the heat affected compress the steam layer, and accelerate the ionization of the molecular state material. Under the zone and under is the solid area that is under not affected bysolid the laser. liquid state is theitheat affected zone and it is the area that is not affected by the laser.
Figure Figure 4. 4. Schematic Schematic diagram diagram of of irradiation irradiation upon upon the the polysilicon polysilicon of of the the pulse pulse laser. laser.
4.2. Effect of Plasma Shock Wave Pressure upon the Surface of the Target 4.2. Effect of Plasma Shock Wave Pressure upon the Surface of the Target 4.2.1. Material Surface Surface under under the the Laser Laser Thermal Thermal 4.2.1. Shock Shock Damage Damage Location Location of of the the Target Target Material As the above above picture, picture, the irradiation damage As in in the the irradiation damage phenomenon phenomenon under under the the power power density density 2 of 2 6.3 GW/cm three wavelength lasers are shown. Because of the polysilicon materials on theon(111) 6.3 GW/cm of three wavelength lasers are shown. Because of the polysilicon materials the surface, bonding strength is the lowest between atomic bonds, and a cross type of crack damage area
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(111) surface, bonding strength is the lowest between atomic bonds, and a cross type of crack damage can be seen fromfrom Figure 5. Under the effect of theoflaser shockshock wave,wave, the priority, and destroyed front area can be seen Figure 5. Under the effect the laser the priority, and destroyed are mutually vertical. The angle is 900, and this is due to (111) obeying the C2 symmetry, which is front are mutually vertical. The angle is 900, and this is due to (111) obeying the C2 symmetry, which called brittle intergranular fracture. As shown in Figure 5, however, the cleavage damaged area does is called brittle intergranular fracture. As shown in Figure 5, however, the cleavage damaged area not give priority to light spotspot center, andand thethe maximum degree the does not give priority to light center, maximum degreeposition positionisison onthe the verge verge of of the irradiated area. The main reason is the effect of laser shock wave pressure, and we will analyze the irradiated area. The main reason is the effect of laser shock wave pressure, and we will analyze the longitudinal stress stress signals signals during during the the process process of of shock shock waves waves and and lateral lateral strain strain in in the the following following part. part. longitudinal
Figure Figure 5. 5. Cleavage Cleavage failure failure under under laser laser shock shock waves. waves.
4.2.2. 4.2.2. The The Dynamic Dynamic Curve Curve of of Laser Laser Shock Shock Wave Wave toward towardthe theTarget Target PVDF range from from 0 PVDF piezoelectric piezoelectric film, film, with with measuring measuring range 0 to to 20 20 Gpa, Gpa, with with aa nanosecond nanosecond as as its its frequency response, and dynamic calibration being simple and fast, is an ideal sensor of super-high frequency response, and dynamic calibration being simple and fast, is an ideal sensor of super-high pressure pressure measurement measurement [19]. [19]. At At time time t,t, PVDF PVDF measured measured voltage voltage signal signal V(t) V(t) and and the the shock shock pressure pressure on on 8 8 Pa] will meet the relation [20]: the surface of PVDF thin film P(t) within the scope of P ∈ [ 0, 3 × 10 the surface of PVDF thin film P(t) within the scope of P [0,3 10 Pa] will meet the relation [20]: P(Pt)(t = )
KK tt VV((t )t) dtdt. AA 00 RR Z
(7) (7)
.
8
2
2/μ·ccm For dynamic calibration coefficient, its value 10 Pa /µthe is the effective of c , Aeffective Pa·cm , A is area of area PVDF, For dynamic calibration coefficient, K, itsK, value is 6.6is× 6.6 108× PVDF, R is parallel resistance with PVDF, and the resistance is 50. By Equation (7), a voltage signal R is parallel resistance with PVDF, and the resistance is 50. By Equation (7), a voltage signal detected detected by an oscilloscope can be transformed to the laser-induced shock pressure wave pressure which, by an oscilloscope can be transformed to the laser-induced shock wave signal,signal, which, after after transformation, beactual the actual relative pressure values. transformation, turnsturns out toout beto the shockshock wavewave relative pressure values. When When the the pulse pulse laser laser irradiates irradiates target target material material and and produces produces shock shock waves waves on on the the workplace workplace surface surface and and produces produces reflection reflection and and transmission, transmission, the the transmission transmission wave wave becomes becomes the the form form of of stress stress wave in the target material. The stress wave transmits to the interior of the target material and bounces wave in the target material. The stress wave transmits to the interior of the target material and back andback forth.and When laser shock waves transmit the back theback target will produce a bounces forth. When laser shock wavesto transmit toofthe of material, the targetitmaterial, it will voltage on PVDF piezoelectric film, and thefilm, oscilloscope record the voltage signal. When producepulse a voltage pulse on PVDF piezoelectric and the will oscilloscope will record the voltage 2 , the first waveform 2 laser power density is 6.3 GW/cm cycle of the three wavelength laser shock is signal. When laser power density is 6.3 GW/cm , the first waveform cycle of the three wavelength shown in Figure 6. With the decrease of wavelength, oscilloscope detected shock wave piezoelectric laser shock is shown in Figure 6. With the decrease of wavelength, oscilloscope detected shock wave signal amplitude decreases. piezoelectric signal amplitude decreases.
The curve in Figure 6 and time integrates and becomes relative pressure curve Figure 7. From Figure 7 under the same power density, relative pressure of the wavelength of 1064 nm is the
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Materials 2017, 10, 260 Materials 10,in 260Figure The 2017, curve
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7 of 15 6 and time integrates and becomes relative pressure curve Figure 7. From Figure 7 under the same power density, relative pressure of the wavelength of 1064 nm is the greatest. greatest. greatest. With With the the decrease decrease of of the the laser laser wavelength, wavelength, shock shock wave wave pressure pressure of of laser laser ablation ablation becomes becomes With the decrease ofwavelength the laser wavelength, shock wave pressure of laser ablation becomes smaller. The smaller. The short laser has the following characteristics: the greater photon energy, smaller. The short wavelength laser has the following characteristics: the greater photon energy, the the short wavelength laser has the following characteristics: the greater photon energy, the greater the greater greater the the absorption absorption coefficient, coefficient, the the shallower shallower the the penetration penetration depth, depth, the the stronger stronger the the interaction interaction absorption coefficient, the shallower the penetration depth, the wavelength stronger the interaction mechanism mechanism mechanism of of laser laser photons-silicon photons-silicon becomes, becomes, making making aa short short wavelength laser laser and and polycrystalline polycrystalline ofsilicon laser photons-silicon becomes,wavelength making a short1064 wavelength laser and polycrystalline silicon couple silicon couple couple more more fully. fully. The The wavelength of of 1064 nm nm has has aa high high breakdown breakdown threshold. threshold. When When the the more fully. The wavelength of 1064 nm has a high breakdown threshold. When the laser energy laser laser energy energy density density is is aa constant constant value, value, as as the the wavelength wavelength from from 1064 1064 nm nm decreases decreases to to 355 355 nm, nm, the the density is a constant value, as degree the wavelength from 1064 nm decreases to 355shock nm, the photons and photons photons and and silicon silicon coupling coupling degree increases increases to to strengthen strengthen the the formation formation of of shock waves. waves. silicon coupling degree increases to strengthen the formation of shock waves. 3
3 1.5x10 1.5x10
1064nm 1064nm
3
3 1.0x10 1.0x10
532nm 532nm
2
Voltage/mV Voltage/mV
2 5.0x10 5.0x10
355nm 355nm
0.0 0.0 2
2 -5.0x10 -5.0x10
3
3 -1.0x10 -1.0x10
00
10 10
20 20
30 30
Time/(ns) Time/(ns)
40 40
50 50
60 60
Figure The laser shock wave piezoelectric signal three different wavelengths. Figure6.6. 6.The Thelaser lasershock shockwave wavepiezoelectric piezoelectricsignal signalofof ofthree threedifferent differentwavelengths. wavelengths. Figure 4
4 1.2x10 1.2x10 4
4 1.0x10 1.0x10 3
RelativePressure/(Pa) Pressure/(Pa) Relative
3 8.0x10 8.0x10 3
3 6.0x10 6.0x10
1064nm 1064nm 532nm 532nm 355nm 355nm
3 3
4.0x10 4.0x10
3
3 2.0x10 2.0x10
0.0 0.0 00
10 10
20 20
30 30
Time/(ns) Time/(ns)
40 40
50 50
60 60
Figure Three wavelengths relative pressure curves. Figure Figure7.7. 7.Three Threewavelengths wavelengthsofof ofrelative relativepressure pressurecurves. curves.
However, the decrease of results in of critical power However, decrease of wavelength wavelength in the the decrease decrease critical breakdown breakdown power However, thethe decrease of wavelength resultsresults in the decrease of criticalof breakdown power threshold threshold and decreases laser-induced shock wave pressure, which limits the formation of threshold andlaser-induced decreases laser-induced wave pressure, limits the of shock shock and decreases shock wave shock pressure, which limitswhich the formation of formation shock waves [21]. √ n[11]: ( C b( Iλ τ) nn b = 5, waves [21]. According to the empirical formula presented by Phipps Cmm= 0.3), b( Iλtheτ)related b = 5, waves [21]. According the empirical presented According to the empiricalto formula presentedformula by Phipps [11]: Cm by = bPhipps ( Iλ τ) [11]: b = 5,( n nn == 0.3), the related parameters of silicon are put in and are shown in Figure 8. parameters of silicon are put in and are shown in Figure 8. 0.3), the related parameters of silicon are put in and are shown in Figure 8. As can be seen from Figure laser wavelength has great influence on the impulse coupling As Ascan canbe beseen seenfrom fromFigure Figure8,8, 8,laser laserwavelength wavelengthhas hasaaagreat greatinfluence influenceon onthe theimpulse impulsecoupling coupling coefficient. When the laser power density and laser pulse width constant, with the increase the coefficient. coefficient.When Whenthe thelaser laserpower powerdensity densityand andlaser laserpulse pulsewidth widthisis isconstant, constant,with withthe theincrease increaseofof ofthe the laser wavelength, the impulse coupling coefficient decreases. The wavelength of a 355 nm laser and laser wavelength, the impulse coupling coefficient decreases. The wavelength of a 355 nm laser and laser wavelength, the impulse coupling coefficient decreases. The wavelength of a 355 nm laser and polycrystalline polycrystalline silicon silicon coupling coupling degree degree is is the the largest largest and and has has highest highest efficiency. efficiency. However, However, with with the the increase increase of of laser laser power power density, density, the the impulse impulse coupling coupling coefficient coefficient is is declining declining to to an an equilibrium equilibrium state. state. The The main main causes causes of of this this phenomenon phenomenon are are due due to to an an excessive excessive steam steam layer layer and and plasma plasma spray spray produced by a high energy laser. Subsequently, the laser cannot penetrate and produces produced by a high energy laser. Subsequently, the laser cannot penetrate and produces plasma plasma
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polycrystalline silicon coupling degree is the largest and has highest efficiency. However, with the increase of laser power density, the impulse coupling coefficient is declining to an equilibrium state. The main causes of this phenomenon are due to an excessive steam layer and plasma spray produced Materials 2017, 10, 260 8 of 15 by a high energy laser. Subsequently, the laser cannot penetrate and produces plasma shielding. The explosion of steam and plasma produced by laser irradiation on polysilicon make the target material shielding. The explosion of steam and plasma produced by laser irradiation on polysilicon make the surfacetarget produce recoilsurface pressure and impulse. High energy nanosecond pulse laser works pulse on thelaser surface material produce recoil pressure and impulse. High energy nanosecond of the works target material and the pressure pulse duration is very short, and its mechanical effect on the its be Materials 2017,surface 10, 260 of the target material and the pressure pulse duration is very short, 8 of 15and can characterized with impulse, impulse is oneimpulse, of the important parameters ofimportant shock wave mechanics’ mechanical effect can beand characterized with and impulse is one of the parameters shielding. The explosion of steam and plasma produced by laser irradiation on polysilicon make the shock wave which can beZexpressed as [20]: effect, of which can be mechanics’ expressed effect, as [20]: target material surface produce recoil pressure and impulse. High energy nanosecond pulse laser F =and PP((tpressure works on the surface of the target material pulse duration is very short, and its F the t))dt. dt . mechanical effect can be characterized with impulse, and impulse is one of the important parameters of shock wave mechanics’ effect, which can be expressed as [20]:
-3
F P (t )dt
3.0x10
.
-3
355nm 532nm 1064nm
-3
2.0x10 -1
Cm/(NW ) -1
(8)
(8)
355nm 532nm 1064nm
3.0x10
(8)
-3
Cm/(NW )
2.0x10 -3
1.0x10
-3
1.0x10
0.0 0.0
9
9
9
3.0x10
9
6.0x10
3.0x10
0.0 0.0
9.0x10
2
I/(W/cm )
9
9
6.0x10
9.0x10
2
I/(W/cm )
The relationship between laser parametersand and coupling coupling coefficient the polysilicon. FigureFigure 8. The8.relationship between laser parameters coefficientofof the polysilicon. Figure 8. The relationship between laser parameters and coupling coefficient of the polysilicon.
Figure 9 shows the relations of ablation impulse of three wavelength single pulse lasers and Figure shows the relations ofisablation impulse of threewavelength wavelength single lasers and 2.of Figure shows relations of ablation single pulse lasers and times times 9 when the9the laser power density 6.3impulse GW/cm It three is through integration of pulse shock wave pressure 2 2 . It is times when the density laser power is 6.3 GW/cm isseen through integration shock wave pressure when of thedifferent laser power is density 6.3 GW/cm through integration of shock wave pressure wavelengths and time curves. As can. It be from Figure 9,ofwith the increase of pulse of of different wavelengths and time curves. As can be seen from Figure 9, with the increase of pulse time, the impulse and is to time linearly increase. theseen increase wavelength, impulse different wavelengths curves. As With can be fromofFigure 9, withablation the increase ofstrength pulse time, time, the impulse is to linearly increase. With the increase of wavelength, ablation impulse strength increases. Due to the longer wavelengths, ablation impulse is greater than the short wavelength, the impulse is to linearly increase. With the increase of wavelength, ablation impulse strength increases. Due to the longer wavelengths, ablation impulse is greater than the short wavelength, increases. to the most fracture degree of the thesurroundings surroundings of the 1064 nm wavelength towavelengths, the mostserious serious fractureimpulse degree of the 1064 nm wavelength Due toleading theleading longer ablation is greater than theofshort wavelength, leading to the irradiation area. irradiation area.
most serious fracture degree of the surroundings of the 1064 nm wavelength irradiation area.
5
2.0x10 2.0x10
Relative Force/(N)
Relative Force/(N)
5
355nm 355nm 532nm 532nm 1064nm 1064nm
5
1.5x10 5
1.5x10
5
1.0x10 5
1.0x10
4
5.0x10 4
5.0x10
0.0
0.0 20
30
20
30
40
Time/(ns) 40
50
60
50
60
The relationship betweenimpulse impulse and by by different wavelengths. Time/(ns) Figure 9.Figure The 9. relationship between andtime time different wavelengths.
Figure 9. The relationship between impulse and time by different wavelengths.
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Using PVDF piezoelectric film sensors to collect the dynamic wave signal of the laser irradiation 4.2.3. Dynamic Strain of Laser Irradiation Light Spot of Target Edges polysilicon plane, the strain response of PVDF piezoelectric film is transverse strain, the longitudinal Usingand PVDF piezoelectric film sensors strain’s effect algebra and Equation (9): to collect the dynamic wave signal of the laser irradiation polysilicon plane, the strain response of PVDF piezoelectric film is transverse strain, the longitudinal strain’s effect and algebra and Equation Q = ((9): d31 ε1 + d32 ε2 ) EPVDF S. Q (d 31ε1 d32 ε 2 ) EPVDF S
.
(9)
(9)
Among them, ε1 and ε2 are the two perpendicular directions, and strain can be calculated through Among them, piezoelectric ε1 and ε 2 arefilm the on twothe perpendicular directions, andtransfer strain can be calculated the detection of PVDF charge transfer. The time between charge and through the detection of PVDF piezoelectric film on the charge transfer. The time transfer between voltage signal V(t) on PVDF piezoelectric film satisfies: charge and voltage signal V(t) on PVDF piezoelectric film satisfies: Z t V (t) t V (t ) Q (Q t)(t= dt. ) 0 Rdt 0 R .
(10)
(10)
The output voltage signal V(t) theoscilloscope oscilloscope Equations (8) and (9), εthe− εt 1 − t The output voltage signal V(t)produced produced by by the in in Equations (8) and (9), the 1 curvecurve and the ε − t curve can be obtained. 2 ε − t curve can be obtained. and the 2 Figures 10 and 11, respectively, show the oscilloscope outputs’ corresponding surface target Figures 10 and 11, respectively, show the oscilloscope outputs’ corresponding surface target material piezoelectric signal curve andand strain curve when thethe wavelength is 1064 nm, and laser power material piezoelectric signal curve strain curve when wavelength is 1064 nm, and laser 2 . In Figure 2 density is 6.3 GW/cm 10, through the calculation by Equations (8) and (9) of the voltage power density is 6.3 GW/cm . In Figure 10, through the calculation by Equations (8) and (9) of the signal, the laser irradiation the polysilicon strain response curve can becurve produced. voltage signal, the laserofirradiation of thedynamic polysilicon dynamic strain response can beIt can produced. can be 10, found from the Figure 10, under actionlaser, of thethat pulse laser, that material be found fromItFigure under action of thethe pulse material along the along laser the speckle laser speckle phase radial strain (V1) and vertical direction strain (V2) change with the same trend. phase radial strain (V1) and vertical direction strain (V2) change with the same trend. Laser waveform Laser waveform is about ns,laser consistent with theInlaser pulse width. In action a singletime, pulsePVDF half-width is about 10half-width ns, consistent with10the pulse width. a single pulse laser laser action time, PVDF patch sensors detect firstly that surrounding material of the irradiated area patch sensors detect firstly that surrounding material of the irradiated area produces compressive produces compressive strain because of compression, and then compressive strain decreases in strain because of compression, and then compressive strain decreases in tensile strain. With the passage tensile strain. With the passage of time, the state of dynamic strain curve around the irradiation area of time, the state of dynamic strain curve around the irradiation area is leveled off. is leveled off.
Laser waveform
V1
V2
Figure 10. Piezoelectric signal curve when the wavelength is 1064 nm and laser power density is 6.3
Figure 10. Piezoelectric signal curve when the wavelength is 1064 nm and laser power density is GW/cm2 on the target surface. 6.3 GW/cm2 on the target surface.
In Figure 11, it takes the curve ε along the radial direction of laser speckle as an example to interpret phenomenon. irradiation in andirection unconstrained mode irradiates and to In Figurethe 11,above it takes the curveLaser ε along the radial of laser speckle as silicon an example produces shock wave pressure, and the material surface of the irradiation area produces thermal interpret the above phenomenon. Laser irradiation in an unconstrained mode irradiates silicon and expansion due to the strong absorption of laser energy, causing surface thermal stress waves.
produces shock wave pressure, and the material surface of the irradiation area produces thermal expansion due to the strong absorption of laser energy, causing surface thermal stress waves. Material
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particles in the irradiation area under the action of the plane wave expand, making the material in the irradiation area be in in the theirradiation compression detect that the compressive Material particles areastate. underTherefore, the actionPVDF of the sensors plane wave expand, making the strainmaterial and compressive strain area increase the increasestate. of laser shockPVDF wave sensors pressure. When in the irradiation be in with the compression Therefore, detect thatthe the laser shockcompressive wave is upstrain to theand peak pressure, strain compressive is corresponding a point the curve ε. compressive increase strain with the increase of laser to shock waveon pressure. thethe laser shock wave is up to compressive strain corresponding to adrops, point and Then,When within time pulse width, as the thepeak timepressure, continues to the end, the is laser temperature on the curve ε . Then, within the time pulse width, as the time continues to the end, the laser the laser shock wave pressure decreases, entering the stage of pressure unloading. Then, compressed temperature drops, and the laser shock wave pressure decreases, entering the stage of pressure material begins to rebound, leading to the compressive strain decrease. When the rarefaction wave unloading. Then, of compressed begins to rebound, leading totothe compressive strainwhen spreads to the center the impactmaterial zone, compressive strain decreases point B. However, decrease. When the rarefaction wave spreads to the center of the impact zone, compressive strain the longitudinal stress wave returns to the surface of the target from the back of the target material, decreases to point B. However, when the longitudinal stress wave returns to the surface of the target anti-media of high impedance (poly) spreads to anti-media of low impedance (air), which leads to the from the back of the target material, anti-media of high impedance (poly) spreads to anti-media of stresslow wave of the laser transforming an stress elasticwave rarefaction and makes the on the impedance (air), which leads into to the of the wave laser transforming intomaterial an elastic surface compress to point C under the action of tension. In addition, point D in the figure is the rarefaction wave and makes the material on the surface compress to point C under the action ofresult of impacts ofIn the following longitudinal wave. the attenuation of the shock wave, tension. addition, point D in the figure is theFinally, result ofwith impacts of the following longitudinal wave.strain withathe attenuation of the shock wave, strain tends to achieve a balance. tendsFinally, to achieve balance.
Laser waveform
D
ε
B C A
Figure 11. A strain curve of the target surface when the wavelength is 1064 nm and laser power
Figure 11. A strain curve of the target surface when the wavelength is 1064 nm and laser power density density is 6.3 GW/cm2. is 6.3 GW/cm2 .
It can be analyzed by combining with analysis of how PVDF detects the curve of stress waves thebe surface as follows: under the irradiation highPVDF energydetects laser, polysilicon induced to on Itoncan analyzed by combining with analysisofofahow the curve ofis stress waves produce a plasma shock wave by laser ablation, and the shock wave produces great pressure on the the surface as follows: under the irradiation of a high energy laser, polysilicon is induced to produce a target. As a result, the material at the edge of the radiating area transfers from compression to plasma shock wave by laser ablation, and the shock wave produces great pressure on the target. As a tension in a short time, in that it is firstly influenced by compressive stress of the thermal expansion result, the material at the edge of the radiating area transfers from compression to tension in a short wave. Furthermore, under the irradiation a Gaussian beam of light, the maximum tensile stress time, appears in that itatisthe firstly by compressive of the thermal wave. Furthermore, edge influenced of the area, forming the effectstress of steam pressure andexpansion that of plasma shock wave, underowing the irradiation a Gaussian beam of light, the maximum tensile stress appears at thelaser edge of to the ablation under the high energy laser. At the same time, the center of the the area, forming the effect of steam pressure and that of plasma shock wave, owing to the ablation irradiation area forms softening effects after receiving the high energy laser energy, and then it has underthe the high energy laser. At the same centerwaves, of themaking laser irradiation forms tendency to move forward under thetime, actionthe of shock the materialarea around thesoftening spot center its displacement. However, polysilicon belongs materials that have a narrow effects afterfollow receiving the high energy laser energy, and thentoitbrittle has the tendency to move forward plastic zone, and the transgranular cleavage is broken due to the sudden appearance of tensile stress. under the action of shock waves, making the material around the spot center follow its displacement. In addition, a softening effect to a certain extent of the material in the center of the laser However, polysilicon belongs to brittle materials that have a narrow plastic zone, and theirradiation transgranular area leads to a slight increase in the width of the plastic zone, and makes the material which is at the cleavage is broken due to the sudden appearance of tensile stress. In addition, a softening effect to edge of the area appear to have the narrowest plastic zone. Therefore, when the material that is at the a certain extent of the material in the center of the laser irradiation area leads to a slight increase center of the area has a tendency for displacement, the degree of damage is higher. In Figure 5a in the width of the plastic zone, and makes the material which is at the edge of the area appear to have the narrowest plastic zone. Therefore, when the material that is at the center of the area has a tendency for displacement, the degree of damage is higher. In Figure 5a (1064 nm), there appear a
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large number of micro cracks at the edge of the irradiation area, and the micro cracks in Figure 5c (1064have nm), there appear compared a large number micro cracks the edge of the area, (355 Materials nm) damage withofthe other two at wavelengths. In irradiation conclusion, the and reason 2017,less 10, 260 11 ofthe 15 why micro cracks in Figure 5c (355 nm) have less damage compared with the other two wavelengths. In transgranular cleavage is broken is that the shock wave effect, which is induced by the laser, pressure (1064 nm), there appear a large number of micro cracks at the edge of the irradiation area, and the conclusion, the reason why transgranular cleavage is broken is that the shock wave effect, which is effect and thermal coupling effect, act jointly. Figure 12 shows the surface topography of cleavage micro cracks in Figure 5c (355 nm) have less damage compared withact thejointly. other two wavelengths. In induced by the laser, pressure effect and thermal coupling effect, Figure 12 shows the damage fracture (111) and crystal structure. It can be seen from the diagram that the fracture, which conclusion, the reason transgranular cleavage is broken is thatstructure. the shockItwave effect, is surface topography of why cleavage damage fracture (111) and crystal can be seen which from the has the weakest binding force of atomic link in the silicon material, appears to be smooth and flat and induced pressure andweakest thermalbinding coupling effect, act jointly. 12 shows the diagramby thatthe thelaser, fracture, whicheffect has the force of atomic link inFigure the silicon material, will be damaged firstlyofunder thedamage action of the laser shock wave. Inaction the diagram, face ofthe [111] is surface cleavage fracture (111) and under crystal structure. It the can laser be the seen from appearstopography to be smooth and flat and will be damaged firstly the of shock wave. perpendicular to the thethe (111) diagram that fracture, has the weakest binding force of atomic link in the silicon material, In the diagram, faceplane. ofwhich [111] is perpendicular to the (111) plane. appears to be smooth and flat and will be damaged firstly under the action of the laser shock wave. In the diagram, the face of [111] is perpendicular to the (111) plane.
Figure 12. Transgranular cleavage failure section and structure.
Figure 12. Transgranular cleavage failure section and structure.
According to the analysis formula ofcleavage pressure overload put structure. forward by Phipps [11], the Figure 12. Transgranular failure section and
According to formula the analysis formula pressure which overload puttheforward Phippsby[11], semi-empirical of pressure can beofconcluded, is about pressureby produced the the According to the analysis formula of pressure overload put forward by Phipps [11], the laser plasma shock wave to the target surface in the laser irradiation area: semi-empirical formula of pressure can be concluded, which is about the pressure produced by semi-empirical formula of pressure can be concluded, which is about the pressure produced by the the laser plasma shock wave to the target surface P bI (in Iλ the τ) nlaser irradiation area: (11)
laser plasma shock wave to the target surface in the laser .irradiation area:
√
n
2 PP= bI is the (11) In Equation (11), λ (nm) is laser wavelength, bI ((IIIλ τ)τn ) . ) is the laser power density, τ (ns) (11) aλ (GW/cm . determined by the material—b = 5. laser pulse duration, and b are the parameters of the silicon material 2 2 In (11), λ13 (nm ) illustrate is is laser GW/cm is the laser power density, (ns ) is the λcan (nm) laserwavelength, wavelength, ) is the laser power density, τ (ns)τ is the In Equation (11), In Equation addition, Figure Equation (11).IIaa ((GW/cm
pulse duration, and b are theparameters parameters of of the the silicon determined by the = 5. = 5. laserlaser pulse duration, and b are the siliconmaterial material determined by material—b the material—b In addition, Figure 13 can illustrate Equation (11). 5 In addition, Figure 13 can illustrate Equation (11). 355nm 532nm 1064nm 355nm
5 4
532nm 1064nm
7 P/(10 Pa) P/(10 Pa)
4 3
7
3 2 2 1 1 0 0
0
1
2
3
4
5
2
6
7
8
9
10
6
7
8
9
10
Ia/(GW/cm ) 0
1
2
3
4
5
2 Figure 13. A diagram which shows the relationship of pressure laser wavelength and power density. I /(GW/cm ) a
Figure 13. A diagram which shows the relationship of pressure laser wavelength and power density. Figure 13. A diagram which shows the relationship of pressure laser wavelength and power density.
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The relationship between laser wavelength and pressure of target surface in the laser irradiation area can be fully expressed in Figure 13. In theory, with the increase of laser power density, the pressure is enhanced, and with the decrease of the wavelength, the pressure on the surface becomes greater. However, the energy of the laser photon that belongs to short wavelengths is great and a full consideration of the coupling between the short wavelength and material has been achieved. As a result, the breakdown threshold values of polysilicon are reduced, and when the laser energy is greater than the breakdown threshold, the excess energy will ionize the material in the irradiation area that is in the gaseous, atomic state quickly, so as to reduce the possibility of shock wave formation. Meanwhile, the experimental phenomena is consistent with the research results about regularities of distribution of silicon’s laser plasma pressure, which is carried out by Bao et al. [22]. The experimental phenomena shows that the spot center has the biggest pressure, and it will have a tendency of displacement due to the pressure, driving the material that is at the edge of the area. However, because of the brittle material’s compressive strength being greater than tensile ability, the material at the edge of area will crack due to the great tensile stress. 4.3. The Attenuation Rule of Laser Shock Wave in Polysilicon The piezoelectric waveform, whose laser power is 6.3 GW/cm2 and the wavelength is 1064 nm, has the biggest amplitude, and is similar to the other waveforms. Then, we take the 1064 nm wavelength as an example to analyze its periodicity. As shown in Figure 14, it can be found that the biggest peak is formed by a shock wave that is induced by a laser and the reflection of following wave. Meanwhile, the average speed of shock waves spreading in polysilicon can be calculated according to their periodicity. As shown in Figure 11, the duration between the two neighboring pulses of voltage waveform is t, which means the time interval between the two adjacent laser shock waves reaching the target material on the surface. It can be read from Figure 11 that tAB = 55 ns, tBC = 60 ns, and tCD = 61 ns. We can take the average: tave = 59 ns, and the thickness of the sample can be illustrated as follows: d = 0.25 mm. Therefore, according to the formula: V = 2d/t, the average speed of shock waves spreading in the sample of polysilicon can be concluded as v = 8.47 × 103 m/s. Because shock waves induced by laser transmit back and forth in the polycrystalline silicon sheet, the unconstrained and absorbed layer of a polysilicon target’s front surface is the free surface. However, the acoustic impedance of polysilicon is six orders of magnitude larger than the acoustic impedance of air. Thus, the free surface can be regarded as the free end of reflection, and the size of wave pressure, which is formed after reflection, remains the same. Then, as the area of a polysilicon sheet’s rear surface where the laser will function is pasted up with a PVDF membrane, part of the shock wave will reach the base after a transmission through PVD membrane. In addition, the base will absorb the shock wave that transmits through the PVDF. Compared with the thickness of polysilicon target material, the thickness of base can be ignored and the shock wave requires more time to reflect in the base than attenuate in the target material. Therefore, the transmission wave will not affect the accuracy of experimental results. However, the loss part of the shock wave that transmits into the PVDF piezoelectric membrane needs a certain amount of compensation while measuring shock wave pressure. Dividing the actual peak pressure values of shock waves, which are formed by reflecting on the polysilicon material surface many times, by the formula: F n , the peak voltage without transmission loss can be concluded [23]: Fn =
Zsi − ZPVDF Zsi + ZPVDF
n .
(12)
In Equation (12): Zsi = 1.239 × 106 g · cm−2 · s−1 , ZPVDF = 0.25 × 106 g · cm−2 · s−1 , F = 0.96.
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A A B B
C C
D D
Figure The piezoelectricwaveform waveform of of shock shock waves waves that are Figure 14.14. The piezoelectric are formed formed by bythe thelaser’s laser’s1064 1064nm nm Figure 14. The piezoelectric waveform of shock waves that are formed by the laser’s 1064 nm wavelength inducing silicon’starget targetsurface. surface. wavelength inducing silicon’s wavelength inducing silicon’s target surface.
mentioned above,the theshock shockwaves wavesinduced induced by by the the laser AsAs mentioned above, laser reflect reflectcontinuously continuouslyononthe theanterior anterior As mentioned above, the shock waves induced by the laser reflect continuously on anterior and posterior surface of the polycrystalline silicon sheet. Therefore, when the shock wave reflects onon and posterior surface of the polycrystalline silicon sheet. Therefore, when the shock wavethe reflects the anterior and posterior surface of polycrystalline silicon, the distance can be used to draw anan andanterior posteriorand surface of thesurface polycrystalline silicon sheet. Therefore, whencan thebe shock reflects the posterior of polycrystalline silicon, the distance usedwave to draw rule the peak pressure ofpolycrystalline shock waves. Insilicon, addition, the peak pressure wavesan on attenuation the anterior and posterior surface of of the distance can be of used to draw attenuation rule ofof the peak pressure shock waves. In addition, the peak pressure ofshock shock waves is illustrated inofTable 1. pressure of shock waves. In addition, the peak pressure of shock waves is attenuation rule the peak is illustrated in Table 1. illustrated in Table 1. Table 1. The peak voltage which is formed after laser shock waves’ reflection on 0.25 mm wafers. Table 1. The peak voltage which is formed after laser shock waves’ reflection on 0.25 mm wafers. Table 1. The peak voltage which is formed after laser shock waves’ reflection on 0.25 mm wafers.
Thickness/mm Thickness/mm Relative peak voltage/107 Pa Thickness/mm 0.25 Relative peak voltage/107 Pa Peak voltage withouttransmission loss/107 Pa
0.25 0.75 1.25 0.25 0.75 1.25 4.2 3.5 2.7 0.75 3.5 1.252.7 4.2 4.2 3.6 2.8 Peak voltage 4.2 4.2 3.5 3.6 2.72.8 Relativewithouttransmission peak voltage/107 Pa loss/107 Pa
1.75 2.25 2.75 1.75 2.25 2.75 2.2 1.7 1.4 1.75 2.25 2.75 2.2 1.8 1.7 1.4 2.3 1.5
2.3 1.8 1.5 2.2 1.7 1.4 7 Pa 4.2 3.6 2.8 2.3 1.8 1.5 Peak voltage withouttransmission loss/10 The attenuation rule of laser shock wave’s peak values in polysilicon is concluded by the The attenuation rule of laser wave’s method of exponential decay fittingshock (Figure 15): peak values in polysilicon is concluded by the method of exponential decay fitting (Figure 15): The attenuation rule of laser shock wave’s peak values in polysilicon is concluded by the method x P 5.37 exp 0.7 (13) max of exponential decay fitting (Figure 15): x3 Pmax 5.37 exp 0.7 . (13) 3x . In the formula, Pmax is the shockPwave’s pressure is the shock wave’s spreading distance.(13) 5.37 exp − and −x0.7 max = peak 3 In the formula, P is the shock wave’s peak pressure and wave’sisspreading distance. x is the shock According to Equation (12), we can conclude that the maximum peak value 4.67 × 107 Pa. max According to Equation (12), we can conclude that the maximum peak value is 4.67 × 107 Pa. 4.5
4.5 4.0 4.0 3.5
Pmax/(107Pa)
3.53.0
Pmax/(107Pa)
3.02.5 2.52.0 2.01.5 1.51.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
Distance/(mm)
1.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Figure15. 15.The Theattenuation attenuationrule rule of of shock shock wave Figure wave pressure pressurein inpolysilicon. polysilicon.
Distance/(mm)
Figure 15. The attenuation rule of shock wave pressure in polysilicon.
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In the formula,Pmax is the shock wave’s peak pressure and x is the shock wave’s spreading distance. According to Equation (12), we can conclude that the maximum peak value is 4.67 × 107 Pa. 5. Conclusions Based on the detection techniques of the PVDF piezoelectric membrane, this paper studies laser thermal shock’s devastating phenomenon in the process of laser irradiating polysilicon material and analyzes the function of wavelength effect in the process of shock waves, which is induced by a laser on the polysilicon material damage. Meanwhile, this paper also uses the test results of PVDF piezoelectric films to analyze the failure mechanism during the process of laser irradiating polysilicon and studies the propagation’s law of laser shock wave rules that is in the brittle materials. Thus, the main results are as follows: 1.
2.
3.
4.
In the process of pulsed laser action, the combined action of steam pressure and plasma contributes to splash phenomenon of melt, which has a high temperature. In addition, the melting and gasification of polysilicon in the area of laser irradiation provide conditions for the removal of material. Furthermore, the steam pressure and plasma pressure have made great contributions to the removal of material. In addition, the decrease of the laser wavelength enables stronger coupling between laser and target, reducing the breakdown threshold value and weakening laser shock wave strength. When the speedily expanding airflow of high pressure stamps melts and couples to the solid part of the target material, recoil pressure will be produced that is perpendicular to the surface, and elastic waves will be composed to form transverse waves on the surface. Furthermore, the spread of transverse waves will cause the deformation of the liquid–solid interface, which is similar to the formation of water waves. At the same time, the tension gradient that is on the surface of the target material causes the phenomenon of moiré, and the higher the temperature of the molten material is, the smaller the surface tension will be; in addition, the lower the temperature is, the greater the surface tension will be. Under the modulated laser irradiation, the liquid on the high temperature zone is pulled into the low temperature zone, which is a result of the coherence and coupling between optics and mechanics. Brittle material has a poorer ability to resist shear stress, so it is easy to find the phenomenon of cleavage destruction under the effect of laser shock waves, and the destroyed area is just the edge area of the laser irradiation. The average spreading speed of shock waves in the sample of polysilicon is v = 8.47 × 103 m/s, and the attenuation tendency of the laser shock wave’s pressure peak in polysilicon is in the form of exponential distribution.
Acknowledgments: This work was supported by a grant from the Important Research Programs of the Science and Technology Plan of Zhenjiang City (No. GY2016002). Author Contributions: J.X. and Z.H. conceived and designed the experiments; Z.H. performed the experiments; J.X. and Z.Y. analyzed the data; C.C. and T.Z. contributed analysis tools; J.X. and Z.H. wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
References 1. 2. 3.
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4. 5. 6. 7. 8. 9. 10.
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