Processing Technology of Embedded Thin-Film Resistor ... - IEEE Xplore

Processing Technology of Embedded Thin-Film Resistor Materials Lifei Laia,b,c, Rong Sun a,*,Tao Zhao a, XiaoLiang Zeng a,ShuHui YU a Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055 b Graduate University of Chinese Academy of Sciences, Beijing 100049,China c NingBo University of Technology,NingBo315016,China a

Abstract It is relatively rare for us to research Embedded Thin-Film Resistor (ETFR) Materials in domestic. This paper describes the development history and present situation of the ETFR, the whole process technology is told,the urgency of domestic products is revealed, the thermal stability and microscopic characteristics of the Ni-Cr (80/20wt.%) ETFR material that we have researched are introduced, the possible development and application is also tentatively discussed. 1.Introduction Embedded Thin-Film Resistor (ETFR) was put forward a resistance pattern in 90 time, which was thin-film resistor embedded printed circuit board ( PCB ) internal. Such as Fig.1 shows that the thin film resistor is sandwiched between the copper foil and dielectric, then pressed into PCB layer upon layer after etched. The ETFR can shorten the transmission distance, reduce electromagnetic interference, decrease the installation cost, save the space of the circuit board. The primark consulting company of America had a statistic in 2000 and pointed out that the chip resistors had exceeded 10000 in PCB average per second in the world…… Resistance and other passive devices occupy about 40% of the PCB space [1-2], so it is necessary to develop the ETFR. This paper focuses on the processing technology of ETFR material, and introduces the thermal stability, characterization and influence factors of the Ni-Cr ETFR material which we have studied, then the next step research directions are discussed tentatively.

Fig.1 Schematic diagram of Embedded Thin-Film Resistor (a) Embedded Thin-Film Resistor before etched(b) Multi-layer Embedded Thin-Film Resistor after etched.

2.The ETFR material 's research overview

Studying on ETFR material starts earlier in aboard, which can be traced back to the 90's, Dupont company began research the embedded passive component materials and packaging technology in 1995 [1]. In addition to, the CTS,3M, OakMitsui, Sanminal and other companies have also begun to develop embedded passive components and materials in the late 90's [2]. So far, the ETFR and ETFR materials in USA has developed maturely, the representative company include DuPont Electronic Technologies[3], Ohmega ,Ticer[4-5], Sheldahl, W.L.GORE & ASSOCIAT and Georgia lnst.of Tech. [1]. In Asia, the ETFR and ETFR materials have also been researched in this century, such as South Korea's Suok-Min Na et al. studying Ta- N and Ta /Ta-N ETFR materials [6], North Korean's S.M.Kang et al. researching Ta-N thin film resistor material [7], Taiwan's C.K.Chung et al. studying Ta-Al thin films resistance material [8], Hongkong's H.F.Lee et al. exploring the processing technology of ETFR [9]. At home, ETFR materials made by screen printing technology is relatively mature, The homemade conductive polymer paste of thin-film resistor has been listed in 2002, and the research on the metal thin-film resistance has made some progress, such as the NiCr thin-film resistance was studied by Shenyang University of Technology, Central South University, Xiamen University, Eastchina Research Institute of Photo-Electronic [10-13],the CrSi thin-film resistance was studied by CNGC Institute No.214 of China Arms Industry Group Comporation, Institute NO.771 of China Aerospace Times Electronics Co.,Ltd, Chongqing University of Posts and Telecommunications[1416], but the resistance film are generally deposited on microcrystalline glass, Si or other insulator substrate, its purpose is not to do ETFR, and mainly used for general integrated circuit or as pressure sensor. In fact, at present, studying ETFR is really begun in Domestic. some scientific research units and enterprises have done a lot of work, and made

60 2011 International Symposium on Advanced Packaging Materials (APM 2011) 978-1-4673-0149-7/11/$26.00 ©2011 IEEE

certain achievements, such as Jiangnan Electronic Institute, Wus printed circuit Co.,LTD (Kunshan), GuangzhouViasystems, Shengyi technology Co., LTD, as well as Shennan Circuit Co.,LTD, but the products are not mature enough, ETFR material is mainly rely on imports, which greatly increase the cost, so it is very important to develop the ETFR material of domestic independent production. 3. The ETFR material 's preparation 3.1 Preparation method The ETFR material usually has the following four ways: (1) conductive polymer coating method Graphite and carbon black are used as conductive phase of resistance paste. Phenolic Resin, Benzoguanamine Resin, Epoxy Resin et al. are elected as the binder. Pulvistalci, Carbon dioxide, titanium dioxide, glass powder used as filler, ethylacetate, methanol and ethanol et al. used as Solvent. But the Resistor paste has a higher error and poor environment coefficient, and Sheet Resistance is too single. For example, the Simov resistance paste produced by Siemens company of German, carbon is used as the conductive phase, sheet resistance is in the range of 20~150ohm/sq, when sheet resistance in 0~100ohm/sq, error is ±25%, and more than 100ohm /sq, error is ±40% [17-18]. (2)The chemical plating method In the electrolytic process, the metal or alloy will is deposited on the substrate to form a thinfilm, its thickness is from several micrometers to tens of micrometers. For example, Ohmega-Ply ® produced from the Ohmega company, Ni-P is used as plating material, copper foil is used as substrate, the thickness of the thin-film in 0.1 ~ 0.4 micron [19-20]. The chemical plating method usually has certain limitations:the thickness of film could not be too thin, efficiency is not high, and a large number of industrial wastewater will be produced to pollute the environment. (3)The chemical vapor deposition (CVD) method CVD method is let a reaction agent of gases or liquid which contain the thin film element and other gases required into a reaction chamber, and the film is formed on the substrate surface by

chemical reaction. It has some characteristics, such as deposition temperature is low, component is easy to control, film thickness is proportional to time, good uniformity and repeatability. Limitations: 1) To ensure that the reactants have sufficient vapor pressure, and introduce into the reaction chamber at a appropriate rate; 2) Reaction product not only can form solid material, but also be volatile; 3) Thin films deposited and the substance material must have sufficiently low vapor pressure. For example: Shipley's CVD production of InsiteTM is formed on the surface of copper foil, Lead as a main component of film, it is reported the sheet resistance can reach 1000ohm/Sq[21]. (4) Physical vapor deposition ( PVD ) method PVD method may be divided into vacuum evaporation and sputtering method. 1) Vacuum evaporation is that container containing the thin film materials is heated in vacuum evaporation, so that the gasification of atoms or molecules overflow from surface forming a vapor stream, and shoot to a surface of the substrate, at last form a thin solid film by condensation. Features: equipment simple, easy to operate, high purity and good quality, thickness controlled accurately, forming film rapidly, efficiency highly, and growth mechanism simply. The main disadvantages: the adhesion is poor between film and substrate, composition and microstructure is difficult to be controled, repeatability isn't good enough. 2) Sputtering coating is that rarefied gas in abnormal glow discharge produce plasma to bombard the cathode surface of target in electric field, so that the molecular, atomic, ionic and electronic on surface with a certain kinetic energy is sputtered toward the surface of the substrate along certain directions, then the resistance film is formed on the substrate. At present , sputtering method is often used including magnetron sputtering and ion beam assisted deposition. Sputtering coating technology compared with other technologies has the following characteristics: target material is wide, almost all metals, alloys and ceramic materials can be made into target; and it


can realize multiple target to sputter, which is suitable for industrial mass production. Sputtering technology has the advantages of the following: ķ Sputtering quickly; ĸ The substrate temperature is low; Ĺ Crystallization compact, film closely and adhesion strong; ĺ Film uniformity; Ļ The components is easy control; ļ Target is no phase change, compounds and compositions are not easy to change, purity and quality are high. ĽGood repeatability, low cost [22]. According to the properties of metals, the resistance of the metal material is more suitable for using the method of PVD, therefore the ETFR material is mainly adopted the sputtering method. 3.2The ETFR material 's selections The metal alloys are usually used as the resistive layer, such as the Ohmega using Ni-P alloy, sheet resistance is in the range of 25~500ohm/sq [19], Ticer using Ni-Cr alloy, NiCr-Al-Si alloy, and Si-Cr-O, sheet resistance is in10~1000ohm/sq [23], Sheldahl developing CrSi material, the sheet resistance is 40ohm/sq, W.L.GORE & ASSOCIAT developing TiW material, the sheet resistance is in the range of 2.4~3.2 ohms / sq, Georgia lnst. of Tech developing Ni-P, Ni-W-P, the sheet resistance is in the range of 10~50ohm/sq [1]. The thin-film resistor material can also use the Ta-N, Ta-Al, Cr-Si, Ti-Si, Ti-W, Ta-N etc.

Fig.2 Schematic diagram of continuous winding magnetron sputtering.

3.3The ETFR material substrate's selection In order to match the packaging technology, the ETFR material is generally deposited on Copper

foil by Magnetron Sputtering. Copper foil as substrate, not only can be used as multilayer core board, but also as electrode in packaging technology[20]. In addition to, the copper foil is flexible which can realize continuous winding coating(Figure2) and improve the efficiency. Thickness of copper foil is generally elected 0.5oz(17μm),1.0oz (34μm) and2.0oz (68μm). 4. Discussion on the Ni-Cr (80/20wt.%) ETFR material The NiCr (80wt/20wt.%) alloy is chosen the common materials of ETFR ,because of higher reliability, high electrical resistivity and small Temperature Coefficient of Resistance(TCR) [4] ,so it is widely used in high frequency hybrid integrated circuit and the precision of the circuit. It general is deposited on Copper foil by DC Magnetron Sputtering.

Fig.3(a). The temperature dependence of the Ni-Cr ETFR's Sheet Resistance(b) The temperature dependence of the NiCr ETFR's TCR.

4.1 Experiment The parameters of experiment was selected in sputtering state: the power of sputtering was 405W, the pressure of Argon was 0.85Pa,the temperature


of the substrate was 100ºC,the gas flow was 80sccm, the base pressure of chamber was 8.5×104 Pa,the distance between the target and the copper was 70mm,the speed of the sample holder was 20r/min, the sputtering time was 4min.The forming ETFR has been detected by the source meter (Keithley2410) matching the detection probing station(HFSE-PB4) in the temperature range from 60ºC to 250ºC.TCR of ETFR is defined as ( Rt Rt ) TCR (/ k ) Rt (t t 0 ) ,where t0 is the temperature of R the room, t0 is the sheet resistance in t0, Rt is the sheet resistance in t. 0

0

4.2 Thermal stability test of Ni-Cr ETFR Fig.3(a) shows the temperature dependence of the sheet resistance of ETFR, whether heating or cooling, the sheet resistance has minor change. Fig.3(b) shows the temperature dependence of the TCR, which has ups and downs in 0 ºC to 35ºC , in addition, it is relatively stable.

V

E f d0 d 2Q f d 0

(2)

where Lhkl is the grain size, K is the correction factor (0.89), O kD 1 is the X-ray wavelength(0.15406 nm), E 1 is the full-width at half-maximum (FWHM) values, and T is the Bragg diffraction angle, where d0,d are interplanar distance in no stress and stress , E f is Young's modulus and Q f is Poisson's ratio, Such as Ni ˖ E f

Qf

2.12 u 10 5 MPa ˈ

0.31 . The crystallite dimension of Ni(011) is

about 27.8nm , the FWHM values is about 0.305,and the stress is about 252.2 MPa. And the crystallite dimension of Cr(110) is about 24.9nm , the FWHM values is about 0.359, the stress is about 29631MPa. The stress may effect on TCR.

Fig.4. XRD pattern of the Ni-Cr ETFR.

4.3 Microscopic characteristics of Ni-Cr ETFR Fig. 4 shows the XRD pattern of Ni-Cr ETFR: there are Ni (011),Ni (103) of the hexagonal structure and Cr(200) of the Cubic structure, there hasn’t usually Ni(111) of the face-centered cubic structure in the XRD curve, which shows that the materials of substrate have influence to the crystal orientation. The diffraction peak is strong in the direction of Ni(011) and Cr(110).The crystallite dimension are estimated by the Debye–Scherrer formula (1), The stress Vertical film surface are calculated by formula (2) kO kD 1 (1) Lhkl E 1 cos T

Fig.5.XPS spectra for (A) Ni2p3 and (B)Cr2p3 peaks of the Ni-Cr ETFR material.

To identify the chemical states of elements in samples, XPS analysis was performed. Fig.5(A) shows the XPS spectra of Ni2p3/2 on the surface of Ni-Cr ETFR, the main binding energy of peaks are 852.24eV and 855.37eV ,which is close to Ni metal


and Ni+3 [24], so the chemical states is mainly Ni metal and a part of Ni2O3 on the surface. Fig.5(B) shows the XPS spectra of Cr2P3/2 on surface, and the binding energy of peak is in range of (575~576.9)eV. According to the literature, there is mainly Cr2O3 and a part of Cr metal on the surface.

Fig. 6. AFM image of the Ni-Cr ETFR.

Fig.6 displays the AFM image of the Ni-Cr ETFR. the crystal degree of thin-film isn’t high, the crystalloid and amorphous body are mixed, the grain of surface is big and the stacking isn’t uniform, the root-mean-square (RMS) roughness value is about 123.567nm. 4.4 Analysis of influencing factors on the Ni-Cr ETFR performance Evaluation of the ETFR performance include three main factors: the resistance value precision, TCR and resistance stability, they are mainly influenced by the microscopic characteristics of NiCr ETFR which is closely related with experiment condition, such as different sputtering pressure, different power, different substrate temperature and annealing process, therefore we need to further explore a good experimental parameters.

Fig.7 (a)sheet resistance of different line length (b) sheet resistance of 10mm line length.

5.Conclusions The ETFR having a unique advantages is expected to quickly occupy the international market. In view of the sheet resistance types in market is not comprehensive enough, which is not convenient to be used in the practical. In the future the ETFR material may be developed towards the

trend: rich sheet resistance types, improving resistance performance, simplified production technology, and transition from the ordinary type to special type, such as photosensitive, thermal and pressure sensitive ETFR material etc. Our task group committed to the embedded passive component materials research since 2006, already obtained certain result, the optimal deposition parameters of the ETFR material have made some exploration, and portion of sheet resistances have been etched (Fig.7(a)(b)). In the future, we will do more work in this area. Embedded type passive devices are called the fourth revolution of PCB. Over the next few decades, will still popular. The ETFR material production process will be more mature and be applied more widely. Acknowledgments The authors are thankful to the financial support by the National Natural Science Foundation of China (0803011001). References 1. T.p.Ye, “A prospect for the miniaturization development of passive components in the world,”Electronic Component News, No.7(2000), pp.16-20. 2. H.h.Li, Q.y.Chen, J.q.Rong, “A study on the developments of embedded passive components in PCB,” Journal of Northwest University(Online Natural Science Edition), No. 3(2005), pp.1-8. 3. G.Min*.“Embedded Passive Resistors: Challenges and Opportunities For Conducting Polymers, ”Synthetic Metals, Vol.153(2005), pp.49-52. 4. J. Wang and S. Clouser, “Thin Film Embedded Resistor”, Proceedings of the IPC Printed Circuits EXPO, Anaheim, California, April,2001, pp. S08-1. 5.J.Wang,R.Hilburn,S.Clouser,etal.,“Manufacturing Embedded Resistors”, Proceedings of the IPC Annual Meeting, New Orleans, Louisiana, November, 2002, pp. S03-4. 6. S.M.Na, I.S.Park et al, “Electrical and structural properties of Ta–N thin film and Ta/Ta–N multilayer for embedded resistor,”Thin Solid Films,Vol.516 (2008), pp.5465-5469. 7. S.M. Kang, S.G. Yoon et al, “Control of electrical resistivity of TaN thin films by reactive sputtering for embedded passive


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