Electroless plating of nickel on silicon for fabrication of high-aspect-ratio microstructures

ELSEVIER

Sensorsand ActuatorsA 56 (1996) 261-266

Electroless plating of nickel on silicon for fabrication of high-aspect-ratio microstructures Shoichi Furukawa

~, M e l u ' a n M e h r e g a n y

Department of Electrical Engineering and Applied Physics. Case WesternReserve University. Cleveland. OH 44106. USA

Received16 May 1995:accepted20 June 1996

Abstract

An electroless plating process for the fabrication of nickel micromechanical structures on a silicon substrate has been investigated. Appropriate chemical roughening of the silicon surface is found to be critical for good adhesion of the nickel to the silicon substrate. The optimum temperature and pH of the elecmless plating solution for smooth nickel films (e.g., R, of 23 A) with practical plating rates (e.g., 15/zm h- ~) are determined. Electroless plating is shown to be a suitable process for the fabrication of micromechanical structures. Keywords: Electrolessplating;Nickel;Highaspect ratio;Chemicalroughening

1. Introduction Surface micromachining is a key technology for the fabrication of microelectromechanical systems (MEMS). To date, surface micromachining has been primarily based on polysilicon as a structural layer and silicon dioxide as a sacrificial layer [ 1 ]. However, polysilicon surface micromachining has certain limitations. First, the thickness of the structural polysilicon layer is typically limited to less than 5/~m because of process technology limitations resulting from slow film deposition rates and pattern delineation problems associated with thicker films. Additionally, polysilicon exhibits ceramic-like properties since it consists of grain and grain-boundury regions. Consequently, the properties of polysilicon are sensitive to deposition and annealing conditions. With the advent of the LIGA process [ 2,3 ], fabrication of high-aspect-ratio (height:width) microstructures has become possible. However, the use of the LIGA technique is limited by its requirement for a synchrotron source used for X-ray lithography to pattern the ndcrostructures. Therefore, the development of optical-lithography-based processes with enhanced height-to-width aspect ratios is being increasingly pursued [4-6]. These so-called molding processes consist of three steps: (i) fabricating the plating mold by lithography; (ii) plating to build up metallic structures on the exposed Present address: Asahi ChemicalIndustry Co., Ltd., 2-1, Samejima, Fuji-City,Shizuoka,416 Japan. 0924-4247/96/$15.00 © 1996ElsevierScienceS.A.All rightsreserved PliS0924-4247 (96) 013 ! 8-0

substrate; and (iii) removal of the sacrificial layer to release the structures. Since the metals may be deposited by plating, which is much faster compared to low-pressure cbemical-vapor deposition (LPCVD), thicker structural films may be obtained. These thicker films allow for stiffer microstructures in the direction normal to the substrate, as well as increased force/ torque in electrostatic microactuators. Additionally, use of metallic structural layers is attractive in some a~ plications, including reflective surfaces for micro-opto-mechanical devices, low contact resistance in relays, metals for magnetic actuators/sensors, microfabricated coils, and electrostatic actuators. Currently plated nickel, copper, or alloys that contain at least one of these metals, are the structural metals commonly in use; chromium [5], deposited oxides [6], polyimide [7], photoresist [8] or titanium [9] are used as sacrificial layers to release the structural materials. A critical part of LIGA and LIGA-like processes is the plating of metals. In general, plating of metals can be performed in two ways: electroplating and electroless plating. Thus far, LIGA and LIGA-like processes have relied on electroplating, whereas electroless plating has not been generally studied for fabricating micromechanical parts. Nonetheless, in the electronics industry, applications of electroless plating are diverse, r~.aging from printed circuits to coating of plastics and ceramics [ 10]. Electroless plating on silicon has also been used in metallization [ 11-13], ohmic contact [ 14,15], and solar-cell applications [ 16]. As a new application, nickel

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electroless plating is also being used to fabricate the etching mask for sub-half-micron lithography [ 17]. This paper studies the fabrication of nickel microstructures by electroless plating on silicon in conjunction with highaspect-ratio photolithography, which provides the plating mold [ 18]. Based on the results from this investigation, a surfaoe-micromachining process has been developed for the fabrication of free-standing nickel microstructures [19]. Since surface smoothness of the plated metal is important for micromechanical devices, our study has two objectives: (i) pretreatment conditions for obtaining smooth nickel surfaces while maintaining good adhesion between the plated nickel and the substrate, and (ii) plating conditions for uniform plating rate and smooth final surface of the nickel.

2. Plating processes Table 1 presents a comparison of electroless plating and electroplating for MEMS fabrication. First, since the driving force for electroplating is electrical potential, a conductive seed layer which is electrically biased by an external power supply to establish the appropriate substrate potential is required. This seed layer must be deposited in advance and then removed subsequent to plating, increasing process complexity. Furthermore, removal of the seed layer by dry or wet etching subsequent to plating may degrade the plated features, and usually an additional layer like Ti or Cr must be deposited to ensure sufficient adhesion to the substrate. Providing an electrical contact to the substrate, as required in electroplating, is often inconvenient and limits processing flexibility. Electroless plating is based on chemical reduction reactions and does not require an external electrical potential. The substrate is simply immersed into a plating solution containing reducing agents and metal ions. Secondly, unlike elecC'oplating, electroless plating may be performed directly on certain non-conductivematerials, such as plastics and ceramics, without the need for a metal seed layer. However, a suitable pretreatment step is required to activate the material surface for electroless deposition. Thirdly, electrolass plating can provide conformal coverage of the substrate topography when all parts of the surface have been catalytically activated and wetted by solution. In electroplating, however, the step coverage is not uniform due Table I Comparisonof electrolessplatingand electroplating

Drivingforce Substrate Stepcoverage Adhesion Controlof rate

Electroless

Electroplating

chemical no externalpower no ~ode metal,non-metal conformal pooron non-metal not simple

electrical needsexternalpower needsanode melal geometrydependent good cunentdensity

to the non-uniformdistribution of the electric potential, which is a function of geometry [20]. Consequently, the region at the top of the steps is thicker than the undersides. Fourthly, in electroplating, the rate of plating and grain size are controlled by the current density. In electroless plating, which is a fully chemical process, there are more parameters that affect the plating rate and deposition charaeteristics. Parameters, such as pH, temperature, and concentration, should be optimized to control the plating process. Finally, adhesion of the plated metal to the substrate for electroless deposits is primarily mechanical in nature [21 ]. In the case of electroless nickel plating on silicon, the adhesion is dependent upon the surface morphology of the silicon surface and the mechanical properties (e.g., stress) of the plated nickel. Chemical roughening provides areas into which the deposited nickel becomes anchored and, therefore, roughening the surface typically improves adhesion. As a result, etching p|ocednres for roughening the silicon surface prior to electroless plating have been studied previously [ 11-16].

3. Fabrication For electroless plating on bare silicon, p-type, 1-10 ~ cm. (100) silicon wafers were used as substrates. For each experiment, the samples were processed as follows: (i) photolithography for fabrication of mold; (ii) pretreatment of bare silicon; (iii) electroless plating; and (iv) characterization. 3.1. Photolithography

Since the electroless plating process described here is intended for the fabrication of high-aspect-ratio micromechanical structures, a compatible photolithography process for the fabrication of the plating mold features is required. An optical lithography process that enables the definition of photoresist features with enhanced height-to-width ratios has been used in conjunction with our electroless plating to selectively deposit nickel over exposed silicon surfaces to fabricate micromechanical devices. This process has been previously outlined [ 18] and documented in detail in Ref. [22]. 3.2. Pretreatment

Prior to plating, it is necessary to treat the silicon surface to be plated. This treatment consists of the following four steps: (i) degrease cleaning of the surface using a weak base solution; (ii) roughening the surface by etching in a solution of HNO3, HF, and deionized (DI) H20; (iii) catalyzation by deposition of Pd in a solution of PdCI2, SnCI2, and HCI; and (iv) removal of the hydrated Sn oxide complex generated on the substrate in a solution of HCI. A rinse in DI water is carried out at the end of each step. The degrease cleaning is required for obtaining uniformly etched surfaces. The surface roughening is needed for removal of native oxide from the surface and to improve the adhesion. Catalyzation by depo-

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sition of Pd significantly improves plating uniformity. The freshly exposed silicon surface reacts with the Pd ions more readily than the surrounding photoresist; consequently, electroless plating begins on the activated silicon surface. Finaily, the removal of the Sn oxide complex exposes the Pd deposited on the surface to the plating solution. For steps (i) and (iii), pre-mixed solutions are commercially available [ 23 ]. For step (ii), we have chosen a roomtemperature HF, HNO3, and H20 volume mixture of 18%, 44%, and 38%, respectively, based on the results of the surface topology of etched surface. For this mixture, commercially available I-IF (49 wt.%) and HNO3 (70 wt.%) are used. The aforementioned silicon etch used in (ii) and all other pretreatment chemicals are compatible with the photoresist used in our lithography process. 3.3. Electroless plating

The plating solution is composed of a mixture of NiSO4 as the main nickel source, NaH2PO2 as the reducing agent, and CH3COONa as a buffer and a mild complexing agent for nickel [21]. Generally, electroless plated metal contains boron or phosphorus which come from reducers. Since NaH2PO2 is used as a reducer, our nickel structures always incorporate phosphorus. A fresh plating solution is used for each experiment to keep the nickel concentration constant and to avoid contamination from the solution itself, the photoresist, and/or the substrate. 3.4. Characterization

Characterization is carried out both after step (ii) of pretreatment and after electroless plating. The etched depth of the substrate, thickness of plated metal, and metal surface roughness are measured by a surface profiiometer. The etchpit formation of the etched silicon surface and the plated nickel microstructures is inspected by a scanning electron microscope (SEM).

4. Experimental results 4.1. Pretreatment

Etching of the silicon substrate in step (ii) ofpretreatment is a sensitive function of the etching solution temperature, the concentration of the etching solution, and the duration of the etch. Room-temperature etching with the solution described above was found to produce the best results. The silicon surface morphology changes as a function of etch duration, as well as etch depth. Fig. 1 shows the silicon etch depth as a function of the etch duration. After 10 s, hydrogen evolution is observed as a result of silicon dioxide formation [ 24], and then the etch depth increases approximately linearly. Fig. 2 shows SEM photographs of etch-pit formation on the silicon surface for different etch times. The surface density and size

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Etch Dm'attea (ug)

Fig. I. Etchdepthvs. etchduration. of the etch pits increase with etch times; after 30 s of etching, dimples are observed on the silicon surface. This may be explained as follows [ 11 ]: the etching of silicon usually proceeds by anodic oxidation (HNO3), followed by chemical dissolution of the oxide (I-IF). Initially, the anodic oxide is not uniform, resulting in localized etching and etch-pit formarion. However, once uniform oxidation and etching take place with the evolution of hydrogen at the surface, dimples and other rough features start to appear. Electrol~ss plating of nickel was carried out on silicon surfaces etched for 10, 20, and 40 s. For silicon substrates etched for only 10 s, the nickel peeled off the subsWate after several microns of deposition. Although etch pits are not essential for plating, the adhesion of the plated nickel to silicon is dependent on etch-pit density, and therefore on the etch duration. If the silicon surface is not sufficiently rough, nickel peels off. The smoothness of the plated nickel surface depends on the surface morphology of the silicon substrate, as well as plating conditions. For the silicon substrates that were etched for 40 s, the nickel films displayed surface dimpies corresponding to those on the pretreated silicon surfaces. It should be noted that the silicon surface-roughening etch undercuts the photoresist mask; however, this undercut is < 1 btm for the 40 s etch duration. Longer etching times leads to excessive undercut of the photoresist mold due to the isotropic nature of the etch. For silicon substrates that were etched for 20 s, the phetoresist undercut was less than 0.5/.tin and smooth surfaces (no dimples) in 20/~m thick nickel microstructures have been realized. Fig. 3 shows the crosssectional SEM photograph ofelectroless plated nickel on bare silicon for a pretreatment etch of 20 s. 4.2. Plating conditions

The plating rate increases with temperature (Fig. 4) and exhibits a reaction-limited behavior. Surface roughness (Ra) is also dependent on the plating temperature as shown in Fig. 5, and smooth films were obtained at 70°C. Below this temperature, the surface is rougher due to the non-uniformity of the nickel film. At temperatures higher than 70°C, the surface roughness increases due to voids in the deposited

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....

Fig. 3. Cross-sectional SEM photograph of plated nickel on (100) silicon. 5O

50 60 70 80 90 I00 BathTe~eentte,e('C) Fig. 4. Plating rate vs. plating bath temperature.

5o

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60 70 80 90 100 BathTemperetu~('C) Fig. 5. Surface roughness vs. plating bath temperature.

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BathpH

Fig. 6. Plating rate vs. pH of the plating bath.

F..g. 2. SEM photographs of etched silicon surfaces for different etch durations: (a) 5 s; (b) tO s; (c) 20 s; (d) 40 s.

film, which are caused by the hydrogen released as one of the reaction by-products. The pH of the plating solution influences both the plating rate and surface roughness o f t b e nickel deposit (Figs. 6 and 7). Below pH 5, the plating rate may be impractically slow for thick-film plating applications in mieromechanics. Also, the surface roughness increases in this regime because of void generation due to hydrogen evolution from excess acid during plating. On the other hand, operation at pH 7 exhibits spontaneous plating. Therefore, it is desirable to maintain the

S. Furukawa. M. MehreganyI Senst,rs and ActuatorsA 56 (1996) 261-266

~

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5. Coneluslons

2

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4 5 6 7 PlatingBathpH Fig. 7. Surface roughnessvs. pH of the plating bath. plating solution at pH 5 in order to obtain the proper plating rate and to enhance the smoothness of the nickel surface.

Nickel films up to 20/~m thick have been selectively deposited on exposed silicon surfaces in an electroless plating process. Roughening of the silicon surface is found to be critical for good adhesion o f the plated nickel to the silicon substrate. The most suitable pretreated surface for electroles~ plating had many small (e.g., less than 1 p~n) etch pits and provided smooth nickel surfaces after plating. Optimum plating conditions for smooth surfaces and practical deposition rates are determined to be solution temperatures o f 70-85°C at pH 5. Our results clearly show that electroless plating is suitable for the fabrication o f micromechanical stn.~ctures on silicon.

4.3. P l a t e d nickel microstructures

Fig. 8 demonstrates the utility o f the plating process described in this paper in conjunction with the high-aspectratio photolithography described in Ref. [22]. Fig. 8(a) shows the photoresist mold with 8 / z m lines and spaces in 15/.~m thick photoresist. Fig. 8 ( b ) shows the electroless plated nickel lines after the photoresist is removed. Note the smoothness o f the plated nickel surface.

Acknowledgements The authors would like to thank Shuvo Roy for reviewing this paper, Professors C.C. Liu and U. Landau for initial technical discussions on plating, and Mr Roger Vojinov, o f McGean-Rohco, Inc., for his technical support in electroless plating. This work was supported by DARPA under ConU'act No. FQ8761-9301675.

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Fig. 8. (a) SEM photographof a panem of lines and spacesin 15/tm thick photoresist.The smaller lines and spacesate 7 and 9/zm wide, respectively. (b) SEM photographof a patternof lines and spacesin 14 p.mthick nickel. The smaller lines and spaces are 10 and 6 btm wide, respectively.

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Biographies Mehran Mehregany received his B.S. degree in electrical engineering from the University o f Missouri in 1984 and his S.M. and Ph.D. in electrical engineering from Massachusetts Institute of Technology in 1986 and 1990, respectively. From 1986 to 1990 he was a consultant to the Robotic Systems Research Department at AT and T Bell Laboratories, where he pioneered research in silicon micromechanical devices. In 1990, he joined the Department o f Electrical Engineering and Applied Physics at Case Western Reserve University as an assistant professor. He was awarded the Nord Assistant Professorship in 1991, and was promoted to associate professor with tenure in July 1994. He is Editor-in-Chief o f the JournalofMicromechanics andMicroengineering. His research interests include microelectromechanical systems ( M E M S ) and integrated circuit (IC) technologies, micromachining and microfabrication technologies, materials and modeling issues related to M E M S and IC technologies, and silicon carbide semiconductor technology. Shoichi Furukawa received his B.S. in the Department o f Applied Chemistry and his M,S. in the Department o f Materials Science and Technology, Kyushu University, Japan, in 1982 and 1984, respectively. In 1984, he joined the Central Laboratory, Asashi Chemical Industry. He worked on meterial research for electronic devices. From 1992 to 1994, he was with the Department o f Electrical Engineering and Applied Physics at Case Western Reserve University as a visiting scientist, where he developed the nickel mieromachining technology. Currently his research includes photofabrication of microdevices.