Excimer Ultraviolet Sources for Thin Film Deposition

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narrow-band emission at various wavelengths(λ) between 108 - 351 nm. The use of dielectric-barrier ... Keywords: dielectric-barrier discharge, ultraviolet radiation, excimer, thin films. 1. INTRODUCTION ..... 633–638, 1996. 2. U. Kogelschatz ...

Invited Paper

Excimer Ultraviolet Sources for Thin Film Deposition: A 15 Year Perspective Ian W. Boyd a

a

and Irving I. Liaw

b

Melbourne Centre for Nanofabrication, 151 Wellington Road, Clayton, VIC 3168, Australia b School of Chemistry, University of Melbourne, VIC 3010, Australia ABSTRACT

High intensity intensity ultraviolet (UV) and vacuum ultraviolet (VUV) radiation provide a singular dominant narrow-band emission at various wavelengths(λ) between 108 - 351 nm. The use of dielectric-barrier discharges in its embodiment of an excimer lamp as a photon-source provides a novel method to induce surface modification. From its in relatively humble beginnings in ozone generation, the excimer lamp has found new applications in the field of low-temperature processing of surfaces. Herein, a 15 year perspective of work done at the Materials & Devices Group at University College London between 1992 and 2007 is presented. The excimer lamps’ application to the modification of surfaces for materials processing include: photo-induced formation of high-κ dielectric thin films and more recently the UV-induced photo-doping of silicon substrates, amongst others. With its robust yet inexpensive setup and flexibility of geometric configurations, they are easily coupled in parallel resulting in the provision of high photon fluxes over large areas. These sources also have an incoherent and almost monochromatic selectivity for application to process chemical pathway specific tasks by simple variation of the discharge gas mixture. These sources are an interesting addition to and an alternative to lasers for scalable industrial applications and have potential for a myriad of applications across different fields. Keywords: dielectric-barrier discharge, ultraviolet radiation, excimer, thin films

1. INTRODUCTION The concept of a dielectric barrier discharge (DBD) was first demonstrated by Werner Von Siemens in 1857 to generate ozone. Ozone (O3 ) is a well understood oxidant capable of oxidizing many organic and inorganic compounds. Major industrial deployment of DBDs may be found in ozone generating facilities mainly used for the treatment of water. High density rare gases at about atmospheric pressure possess an ability to efficiently convert electron kinetic energy initially stored in excited atomic and ionic states through rapid funneling to a few low lying atomic and excimer energy states which react to form excimer molecules via a three-body reaction. Inert noble gases such as xenon and krypton are the usual dimers used anddo not usually form chemical compounds. When excited by an electrical discharge or high-energy electron beams producing pulses of intense energy, they can form temporarily-bound molecules with themselves (dimers). These inert gases may also be coupled with halides (complexes) such as fluorine and chlorine. The excited compound then releases its excess energy by undergoing spontaneous or stimulated emission which UV radiation is emitted in a very tight, quasi monochromatic spectral range. Depending on the choice of gas, different narrow band UV spectra are produced, predominantly in a single spectral line. Simple and efficient, excimer lamps are capable of producing high power, high efficiency and narrow band radiation from the near UV (λ = 351 nm) to the deep UV (λ = 108nm). With typical halfwidths of 10-15 nm using rare gas, halogens and gas halides through the formation of heteronuclear diatomic molecules1 based on the concept of the excited dimer (excimer), tailored emission peaks may be generated in a number of ways. These lamps may then be easily integrated or scaled for current processing methodologies. This paper concentrates on the use of such lamps to the field of electronic materials processing and its integration into vacuum processing chambers they are usually deployed in. Further author information: (Send correspondence to I.W.B.) I.W.B.: E-mail: [email protected], Telephone: +61 (03) 9902 0493 I.I.L.: E-mail: [email protected], Telephone: +61 (03) 8344 6485 Laser Applications in Microelectronic and Optoelectronic Manufacturing XV, edited by Hiroyuki Niino, Michel Meunier, Bo Gu, Guido Hennig, Jan J. Dubowski, Proc. of SPIE Vol. 7584, 75840C · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.847091 Proc. of SPIE Vol. 7584 75840C-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 03/26/2014 Terms of Use: http://spiedl.org/terms

2. THE EXCIMER SYSTEM In the past three decades, several excellent reviews on the development and applications of excimer lamps have been published. Of which, papers by Kogelschatz2–5 (The von Engel Prize6 awardee in 2001 for his contributions to the principles and application of DBDs) and Oppenlander7 are amongst the most prominent. The following sections cover the basic fundamentals of the discharge process. Originally, DBDs had planar configurations consisting of a pair of parallel separated electrodes, with at least one covered by a dielectric. This served as an insulator thus not passing a dc current and requiring ac voltages for operation. The dielectric constant, thickness and frequency of the applied voltage determined the the amount of displacement current that could be passed through the dielectric(s). With a high enough electric field, a critical stage is reached where the local “eigenfield” induces breakdown and exceeds the corresponding reduced Paschen field resulting in a multitude of filaments (microdischarges). Each channel corresponds to a single transit breakdown or streamer with diameters of about 100 µm and a lifetime of under 100 ns. The electron density in a microdischarge is typically of the order of 1014 cm−3 and mean energies of the electrons are a few electron volts.8 The discharge conditions can be influenced by gap size, gas pressure, gas mixture, and characteristics of the dielectric materials and can be optimized for the formation of the desired excimer species.1, 9 Preferred materials for the dielectric barrier are glass with suprasil quartz being by far the most popular. In some special cases: ceramics, enamel or polymer layers are also feasible.

Figure 1. Cylindrical Excimer UV Source. Insets : Micro-discharges in a discharge tube with external 2 mm x 2mm mesh electrode.

The fill gas determines the UV emissions of the DBD system. The most extensively used excimer lamps are those based on rare gas dimers and rare gas halides known from their prior use in excimer lasers. The measured energy conversion efficiencies (UV output / electrical input) for these lamps are reported to be as high as 15%. Open or windowless excimer systems have been studied10, 11 to overcome the attenuated transmission of the suprasil envelopes. In these set-ups, the samples are placed within the reactive gas and subjected to VUV radiation without an isolating window.

2.1. Excimer Excitation: Rare Gas (Rg ) Emissions generated at wavelengths: 126, 146 and 172 nm are from stimulation of rare gas dimers : argon, krypton and xenon respectively. The bandwidths of these emissions are typically around 10 - 20 nm. The emission process is illustrated in the figure??. Microdischarges possess energetic electrons present which excite and ionise the Rg atoms (Eqs. (1) and (2)). Above 50 mbar, the rapid formation of the molecular ions

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Figure 2. Emission energy process for Xe∗2 .

leads to the formation of excited neutrals (Eqs. (3) and (4)). A three body reaction of an electronically excited rare gas atom Rg∗ with two others in a ground state (Eq. (5)) results in the creation of the rare gas dimer, while the Rg∗2 excimer consequently dissociates into Rg atoms and in turn emits a 172 nm photon (Eq. (6)). The absence of any bound ground state for the Rg2 dimer, ensures that no self-adsorption of the radiation by the gas phase occurs in this reaction.

e− + Rg → Rg ∗ + 2e−

(1)

+



e + Rg → Rg + e

(2)

Rg2+ ∗∗

+ Rg

(3)

+ e → Rg

+ Rg

(4)



Rg + 2Rg → +

Rg2+ ∗



Rg + Rg + Rg → Rg2∗

Rg2∗

+ Rg

→ 2Rg + hν(U V )

(5) (6)

Table 1 summarises the different resonance lines and continua of Ar, Kr and Xe rare gases, created by discharge excitation. The resonance lines (I/II) arise from the transitions of the 3 P1 → 1 S0 and the 3 P2 →1 S0 states. Two “first continua” emitted at the shorter wavelengths result from the transition of the two lowest excited dimer states (1 σu+ ,3 σu+ ) of the neutral molecule Rg∗2 to the repulsive ground state. The third continua corresponds to transitions between the Rg2+ ↔ Rg states and this can usually only be initiated by Rg excitation with protons, particles and by electron beam. Short pulse excitations produce the best excitation. Liu and Neiger12 have reported much higher efficiencies using unipolar pulse excitations. Shorter wavelengths emissions of Ar∗2 and Kr∗2 lamps (< 160 nm) requiring special transmitting window materials such as (MgF2 , LiF or CaF2 ) to prevent attenuation has seen the Xe∗2 variety of rare gas lamp become the most important over the last decade. The VUV emission is generated via 1 + the radiative decay of four excited xenon states: Xe∗ 6s[3/2]1 (1s4 ) resonance level (147 nm), the Xe∗2 O+ u ( Σu )(v) ∗ + 1 + ∗ + 3 + vibrationally excited level (150 nm) and the Xe2 Ou ( Σu ) & Xe2 Ou ( Σu ) excimers both at 172 nm. The resonance level contributes weakly as its radiative lifetime is lengthened by radiation trapping well beyond the typical conversion rate to excimers. Further, the high purity silica (suprasil) which forms the lamp envelope transfers its radiation (at 172 nm) efficiently and attenuates both the lower emissions. The highest VUV efficiency of commercial xenon excimer lamps is reported to reach close to 40%. An excellent simulation of pulsed discharges in Xe∗2 has been done by Carman et al.13 where efficiencies of up to 71% has been reported.

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Table 1. Resonance lines and continua of rare-gas (Rg) dimers at high pressures

Rg Ar

I/II

I

II

III

Resonance line (nm)

Continuum (nm)

Continuum (nm)

Continuum (nm)

106.4/104.8

110

126

188

Kr

123.6/116.5

125

146

220...270

Xe

146.96/129.56

150

172

300

2.2. Excimer Excitation: Rare Gas Halide (Rg X∗ ) The reaction mechanisms governing the formation of rare gas halides are more complex. This involves several ground state atomic and molecular species, several ionic species and large numbers of excited atomic and molecular species. With the additional reactant X representing the halogen (e.g. F, Cl, Br, I), the high energy electrons

Figure 3. Emission process for rare gas halides.

ionize and excite the rare-gas and other halogen species. e− + Rg → Rg ∗ + e−

(7)

e− + Rg → Rg + + 2e−

(8)

e− + X2 → X + X −

(9)

RgX∗ exciplexes (excited complexes) can be created by a three-body recombination of the positive rare-gas ions and the negative halogen ions (Eq. (10)) or the harpooning reaction (Eq.(11)) in which the excited rare gas species transfers its loosely bound electron to the halogen molecule or halogen-containing compound to form an electronically excited RgX∗14 whereby a collisional third partner (termed M) which in many cases can be an atom or molecule of the active species or even of the buffer gas. Rg + + X − + M → RgX ∗ + M

(10)

Rg ∗ + X2 → RgX ∗ + X

(11)

The excimer or exciplex molecules now are not very stable and rapidly decompose (within nanoseconds), giving up their excitation energy in the form of a UV photon (Eq. (12)). This radiation process competes with

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several quenching processes with halogen-beating species (Eq. (13)) being the dominant quenching mechanism at low pressures. RgX ∗ → Rg + X + hν(U V )

(12)

RgX + X2 → Rg + 3X

(13)



However, at high pressures three-body reactions involving the rare-gas atoms quench the excited rare-gas halides forming tri-atomic species (Eq. (14)). RgX ∗ + 2Rg → Rg2 X ∗ + RG

(14)

The X1/2 ground state arises from the ground state 1 S of the rare-gas and the 2 P level (s = 1/2, l = 1, m1 = 0) of the halogen atom. The A2 P1/2,3/2 state comes from 1 S rare-gas and 2 P (s= 1/2, l = 1, m1 = ±1) halogen atom. The three close lying excited states (B1/2 , C3/2 and D1/2 ) are generated by a positive rare gas ion 2 P and a negative halogen ion 1 SX− (X = F, Cl, Br, I). The transitions from B1/2 , C3/2 , and D1/2 to A1/2,3/2 and X1/2 and their associated wavelengths are shown in Table 2. Table 2. Main peak wavelengths (nm) of rare-gas (Rg) halide excimers at high pressures14, 15

Rg

X

RgX∗

RgX∗

RgX∗

RgX∗

D1/2 → X1/2

B1/2 → X1/2 108

C3/2 → A3/2

110

B1/2 → A1/2

193

203

204

Ne

F

106

Ar

F

185

Ar

Cl

175

Ar

Br

165

111

195 172

RgX∗ ∼ 145

290±25 245±15

183

Kr

F

220

248

275

272

400±35

Kr

Cl

200

222

240

235

325±15

Kr

Br

207

222

228

Kr

I

190

195

225

∼ 318

Xe

F

264

351

460

410

610±65

Xe

Cl

236

308

345

340

450±40

Xe

Br

221

282

300

325

440±30

Xe

I

203

253

265

320

∼375

Different intensity and spectral half-width values of the radiation generated from various transitions. The D1/2 → X1/2 transition generates the shortest wavelength whilst its intensity is much less than that of the B1/2 → X1/2 transition (which is the strongest due to the initial and final pσ orbitals in which the electron occupies have the largest overlap of any of the valence orbitals14 ), indicating that the upper level is quenched by collisions to lower states in the ionic manifold. The B1/2 → X1/2 transition, indicates that the upper level is quenched by collisions to a lower state. Radiations may also be generated from the formation of Rg2 X∗ species, such as triatomic Kr2 Cl∗ . Table 2 further shows emissions are shown to be much broader.

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3. UV PROCESSING SETUP The principle of dielectric barrier discharge makes it possible to construct UV sources of various geometries. Early designs were flat and planar and over the years the influence of the application determined the geometrical shape. Cylindrical implementations using a special grade of fused silica doped with fluorine commercially marketed as suprasil has been extensively employed in photolytic enhanced processes. With reported transmission at 90%, these UV transparent tubings also possess high dielectric strengths even at high temperatures act as the dielectric barrier. Sources operating in the VUV (< 160 nm), necessitate a need for the removal of the quartz envelope to overcome the attenuation of the glass. This has given rise to the implementation of windowless designs. Straight annular cylindrical tubings as illustrated in Fig.1 are the most common configuration today with effective lengths up to 2 m in length available. The barrier discharges are operated over a large pressure and frequency, with filling pressures typically between 0.1 - 1 bar (required for the three body reaction), sinusoidal feeding voltages between 200 to 500 kHz are used with higher power lamps requiring higher supply frequencies. Discharge gap widths of a few mm are used in most excimer lamps though special applications which require less or more intense VUV irradiation have been operated with extremely narrow (0.1 mm) or extremely wide (5 cm) gaps respectively. For high power applications, it is necessary to cool the inner electrode. With the material of the inner electrode reaching temperatures of up to 3300◦ C, this electrode which is usually made of molybdenum or copper needs to be cooled in order to prevent burn out of these fragile electrodes. This is usually achieved through a recirculated flow of de-ionised water.

3.1. UV Lamp Performance Long lifetimes of up to at least 4000 h in the case of 222 and 308 nm lamps with low fluctuations (less than 0.2%) and immediate UV output after the ignition can be achieved in excimer VUV sources. We studied the stability and lifetime of these sources and found there was no variation in the intensity emitted from the 222, 308 and 172nm variations of the lamps run at an electrical power of 40 W and temperature of 25◦ C as shown in the following figure:

Figure 4. Stability of UV emission over operation time

It was noted however, that within the first 60 h of operation for 172 nm excimer lamps, color centres formed resulting in a marked reduction of their output intensity. These color centres are a result of irradiation induced changes in the molecular structure of the glass matrix and are caused by a two-photon process of 7.2 eV Xe photons surmounting the band gap generating an exciton capable of creating a color centre. The results show that excimer lamps possess far greater stability and extended lamp life when compared with conventional lamps. UV power densities (measured using chemical actinometry at high electrical power) up to 250 mW/cm2 were obtained showing such UV sources as a long life, cost-effective and high efficiency source of UV photons and worthy of consideration as an integrated process methodology.

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Table 3. Lifetime of different industrial lamps (data sources: Hamamatsu & Optical Radiation Corporation)

Lamp

Wavelength, λ (nm)

Guaranteed life (h)∗

Low-pressure mercury lamp

185 and 254

2000

Medium-pressure mercury lamp

200-600

800-1000

High-pressure mercury lamp

300-600

600

Xenon lamps

UV to IR continuous spectrum 185-2000

600-1000

Mercury-xenon (HgXe) lamps

UV to IR continuous spectrum 185-2000

400-1000

Deuterium lamps

UV continuous spectrum 160-400

1000

Directly-heated deuterium lamps

UV continuous spectrum 115-400

≈500

3.2. Vacuum System Integration These excimer sources are easily integrated into a process system. By placing the lamps within a ultra-high vacuum system in a top down arrangement on top of a process chamber, our group integrated these photon sources for the purpose of processing surfaces. Two different systems were utilised. The first was based on a system developed initially by the JIPELEC group (now also known as Qualiflow)in France. A liquid injection system coupled with several lamps provided a basis for UV-enhanced CVD. A further variation of this system without the liquid injection source formed the basis for a UV annealing system. Pictures of the lamp and process chambers are shown:

Figure 5. Lamp and Process Chambers

A vacuum UV processing system designed for experimental work usually consists of an oil and particulate free vacuum valving, heating and sample manipulation such as Z-motion and rotation. Such systems must be free of vibration and an ultra high purity gas line which allows the controlled entry of process gases usually handled by mass flow controllers. Comprising of a lamp chamber and a process chamber under it, these may be separated by a quartz window in some cases in order to reduce physical contamination onto the lamps. Substrates could be heated on a z-motion stand which could be made to rotate to make for a more uniform exposure. An liquid injection may be incorporated in photo-assisted deposition setups driven by an array of flow controllers connected

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to a manifold. The lamp chamber is usually evacuated to a partial vacuum to prevent absorption and permits the lamps to emit fully without adsorption by particles within the lamp section. The injection system capable of injecting a liquid precursor which vapourizes instantly within the processing chamber. This is achieved by a fast micro electro valve which may be set to vary both its opening time and frequency. The injected liquid is converted into a vapour by a progressive heating system flash evaporating the injection into a vapour which has its chemical bonds broken by the UV photons emitted from the lamp chamber. An example of such a system is shown:

Figure 6. Liquid Injection system

Figure 7. UV Intensity Profile of three lamp system

The injection system allows for the precise control of the mass of precursor/solvent introduced to the processing chamber with the flow rate fixed by the injection rate and the aperture size. The required film growth thickness is then determined by the number of pulsed injections. Exposure to intense UV-light was found to

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aid surface processing. This was utilised in UV annealing and doping procedures. Combining excimer lamps in parallel not only provided the UV light to be more intense but also allowed for larger area processing. This parallel arrangement also ensured a more even intensity profile across the profile of the surface.

4. NANO-ELECTRONIC APPLICATIONS OF UV PHOTO-PROCESSING Many materials currently used in electronic applications absorb radiation at wavelengths shorter than 250 nm, which make UV or deep UV sources highly desirable for stimulating chemical processes. The setups shown in the previous section have been applied to a variety of problems in electronic materials processing by our group. A subset of these are presented in the following sections.

4.1. Treatement of Polymer Surfaces The tunability of UV intensity allows for the processing of surfaces. Different intensities of UV light allow different process steps in the processing of polymer surfaces to be achieved. The schematic below shows the intensity effect of UV exposure on a polymer surface: At low intensities, a surface cleaning effect may be obtained, progressively, a chemical change in the material may occur (e.g. polymerisation and oxidation).

Figure 8. UV Intensity Profile of three lamp system

4.2. Rapid oxidation of Si using 126nm excimer radiation We have also demonstrated the rapid oxidation of silicon surfaces using intense UV 126nm Ar∗2 rare gas dimers irradiation. The atomic oxygen produced by photolysis of O2 molecules with different wavelengths are shown below: 3 3 O2 (3− g ) + hν → O( P ) + O( P )

∆H0 = 493.56kJ/mol

(15)

3 1 O2 (3− g ) + hν → O( P ) + O( D)

∆H0 = 683.38kJ/mol

(16)

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→ 110.6nm < λ < 134.2nm 3 3 O2 (3− g ) + hν → O( P ) + O( P )

∆H0 = 897.80kJ/mol

(17)

Successful room temperature oxidation of Si using 126nm radiation was demonstrated with significant oxidation rates, as high as 8 nm/min achieved as shown by XPS and FTIR confirming stoichiometric SiO2 formation. Rapid oxidation was shown to be related to creation of aggressive O1S oxidant.

Figure 9. XPS of SiO2 formation using 126nm excimer source

SiO2 layers of up to 24 nm are readily obtained with no signs of growth saturation.

4.3. UV Deposition and Annealing of High-κ Layers Photo-assisted processing has recieved much attention since it operates a low temperatures which minimizes problems such as atom diffusion, dopant redistribution, and defect generation caused at higher processing temperatures. Furthermore, the processed surface is not subjected to the damage caused through ionic bombardment which occur in plasma-assisted systems. Photo-CVD had been performed originally using high power and expensive laser which provided only small area beams or by the use of lower power mercury lamps. Excimer lamp based systems are applicable in this area and capable of producing high power radiation over large areas. There has been extensive studies to find a potential replacement candidate to replace SiO2 and amongst the many candidates, Ta2 O5 has received considerable attention due largely to it chemical and thermal stability. An excellent review of this material is available from Chaneliere et. al.16 Utilising a processing setup similar to that described in the previous section of this review, we were able to deposit Ta2 O5 layers of thicknesses ranging from 10 - 300 nm on Si substrates using two separate methods. The formed layers were also subjected to a UV annealing process which is also described in this section later. 4.3.1. Ta2 O5 formed by photo-assisted sol-gel processing The formation from a sol-gel solution involves two simultaneous chemical processes: hydrolysis and polymerization. The alkoxide hydrolysis and polymerisation reactions occurred over several hours, during which the colloidal particles links together with the condensing metal species resulting in a three dimensional network. The detailed alkoxide hydrolysis and polymerization reactions of sol-gel processing is described elsewhere.17 This solgel solution is then prepared by a spin-on process after which it is irradiated at different temperatures forming tantalum oxide. This particular photo-induced approach not only enables reduced temperatures and processing times but also provides good electrical properties in the resulting films without the need for high temperature.

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4.3.2. Ta2 O5 formed by photo-CVD In the photo-CVD of Ta2 O5 from tantalum metal-organic precursor and nitrous oxide, the primary photochemistry of the N2 O involves the following reaction: N2 O + hν(λ = 222nm) → O(1 D) + N2 (X 1 σg+ )

(18)

The active oxygen species O(1 D) subsequently reacts with the tantalum metal-organic precursor causing its dissociation through a series of reactions leading to Ta2 O5 deposition on the substrate surface. The growth rate depends on the substrate temperature and increases with temperature with growth rates below 400◦ C is particularly slow and essentially negligible. It is pertinent to note that there is a difference in the reaction mechanisms for pyrolytic and photolytic depositions being governed by different limiting processes. This reduced activation energy for the photo-induced process has shown clear evidence for the reduced importance of the substrate temperatures, allowing temperatures as low as 100◦ C to be used. 4.3.3. UV annealing induced reduction in the leakage current in Ta2 O5 films UV irradiation using a 172 nm excimer lamp was shown to improve the leakage current characteristics of these films after formation annealing was carried out. It is a well known concept that the UV activated oxygen species readily oxidize Si at low temperatures. These species could then readily react with the Si leading to the formation of SiO2 and/or the Ta or any suboxides resulting in Ta2 O5 thereby removing certain defects and oxygen vacancies present in the Ta2 O5 . This step can play an important role in the improvement of the formed layer’s properties. The possible mechanism for the UV annealing effect is attributed to the active oxygen species produced by the UV irradiation as follows: (19) O2 + hν(λ = 172nm) → O(3 p) + O(1 D) O2 + O(3 p) + M → O3 + M (where M is a third body)

(20)

The ozone is then decomposed by further absorption o VUV light at 172 nm, which produces excited state 1 D oxygen atoms, (21) O3 + hν(λ = 172nm) → O2 + O(1 D) The active oxygen species O(1 D) produced by the photolysis of ozone are absorbed on the Ta2 O5 surface and diffuse into the bulk and accept electrons finally occupying any vacancies as shown in the following figure. The decrease in the number of vacancies in the films leads to the reduction of leakage current density. On the contrary, the active oxygen species may react with the silicon either at the Ta2 O5 /Si interface or with any Si species which diffuse from the substrate to the Ta2 O5 surface leading to the formation of SiO2 at the interface and the surface of the Ta2 O5 films. Four of the most likely contributors for the reduction of leakage current of Ta2 O5 films are summarized as follows: 1. The active oxygen species assist in reducing or removing any suboxides present leading to an improved stoichometry. 2. The active oxygen species can decrease the density of defects and oxygen vacancies in the as-deposited films. 3. A thin SiO2 layer is formed at the Ta2 O5 /Si interface and on the surface of Ta2 O5 by the reaction between the active oxygen and Si during annealing, leading to improved interfacial quality. 4. Removal of any impurities present in the as-deposited films. All of these could result in an improved layer quality, especially for the reduction of leakage current density, although it is not clear at present which of these dominates.

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4.4. UV Doping A more recent application of the excimer lamp system was the use of UV annealing of low temperature deposited metallorganic layers of Phthalocyanines which form ordered films on most substrates. They remain intact and can include wide range of internal species. In our recent demonstration using MnPc,18 we demonstrated that these Mn atoms which form the central atom in the pthalocyanines may be implanted or doped into the silicon substrate matrix by utilising the UV lamps to remove organic ligands and providing it sufficient surface energy. This will have potential applications in spintronics. Common doping techniques such as ion implantation tend to result in high levels of surface damage and require high capital investments. More importantly though, existing methods provide little control on the lateral positioning of the dopants. A process flow of the steps taken is shown:

Figure 10. a. Structure of MnPc. b. Upright standing, c. Square planar and d. random MnPC arrays. Orgainic ligands are then removed by 172nm UV irradiation with metal species introduced into the Si forming e. linear, f. square or g. random patterns. This diagram ignores the effect of lateral dopant diffusion.

The presence of manganese was obtained using x-ray absorption near-edge structure (XANES) . The profile shown in fig11.a. contrasts UV-treated MnPc film following cleaning and is contrasted to a fresh MnPc film, foil and silicide. The comparisons give an approximate Mn concentration of 5X102 0 atoms/cm3 . This confirms that the UV treated sample is vastly different from the starting film with a notable absorption edge shift observed by 8.5eV proving that the metal is no longer bound to the organic ligand. In fig11.b., the EXAFS spectrum shows clearer that the Mn atoms are now surrounded by silicon, similar to the silicide reference, confirming its implantation.

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Figure 11. a. Normalised NEXAFS profile and b. EXAFS profile at the Mn K-edge. Spectra are offset for clarity. Notice that UV treated sample is similar to Mn Silicide, with subtle variations demonstrating the Mn introduction into the Si but does not form a full silicide.

5. OUTLOOK The simplicity of these incoherent excimer VUV/UV sources and their tuneable design make them an excellent processing pathway to many problems in interdisciplinary science and engineering. The excimer lamp has its origins in a technology for the industrial generation of ozone. Its merits and geometrical versatility saw it being applied to amongst others: the deposition of high-κ thin films and the surface modification of polymeric foils and more recently, its application in flat plasma display panels (PDPs).In particular - photochemistry and photochemical technology. With more temperature sensitive compound materials, downscaling of processes geometries and the upscaling of processes for efficiency and cost reduction, there is an increasing need to improve on the efficiency of these lamps and their manufacturing. This will open up new avenues in photo-processing.

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