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PROC. 29th INTERNATIONAL CONFERENCE ON MICROELECTRONICS (MIEL 2014), BELGRADE, SERBIA, 12-14 MAY, 2014

Glass Micromachining with Sputtered Silicon as a Masking Layer

Ž. Lazić, M. M. Smiljanić and M. Rašljić

Abstract - In this work we present the not so commonly use of RF sputtered silicon as a masking layer for glass wet etching. The main advantages of this technique are low deposition temperature of silicon layer compared to PECVD and LPCVD processes, simplicity to use and low cost. Si layers were sputtered on 2.5×2.5cm2 Pyrex 7740 substrates. The measured thickness of deposited silicon layer was 2.2µm. Silicon layer was patterned using lift-off technique. Etching was done in undilluted HF (49%) with estimated etch rate of 8µm/min. Good quality of the glass surface without pinholes and notch defects on the glass etched edges suggests that using sp-Si layer as a masking material for glass etching is feasible.

I. INTRODUCTION

Micromachined glass is widely used as a building block in an extensive variety of microelectromechanical systems (MEMS). One of the applications include microfluidic devices like microreactors fabricated on a glass substrates which utilize the high glass transparency to monitor fluid flows. As a packaging material, micromachined glass cavities are used as a vacuum sealed reference chambers in pressure sensors and accelerometers 1.

The most common methods used for glass micromachining are mechanical, dry and wet etching. Wet etching using HF is the preffered etching technique and has many advantages over the other techniques owing to high etching rate, great simplicity and low cost. The most important factor affecting the glass wet etching is the choice of masking material. Various masking material have been used depending on specific application like photoresist, metal layer Cr/Au, multilayer of Cr/Au combined with thick photoresist, PECVD amorphous silicon and LPCVD polysilicon and bulk single crystal Si wafer anodically bonded to glass 2.

In this work we present the not so commonly used application of RF sputtered silicon as a masking layer for glass wet etching 3, 4. The main advantages of this technique are low deposition temperature of silicon layer compared to PECVD and LPCVD processes, simplicity to use and low cost.

II. EXPERIMENT

Si layers were sputtered on Pyrex 7740 glass substrates. This type of glass is commonly used in MEMS applications owing to its good bondability to silicon; also its thermal expansion coefficient is near perfectly matched with that of silicon thus minimizing thermal stress.

In order to perform glass etching using sputtered silicon (from this point on: sp-Si) as a masking material, the sp-Si layer must be patterned. Several techniques could be used for patterning sp-Si layer. The most common technique is based on standard photolithographic process: the photoresist is spun on sp-Si layer and patterned using a photomask. Sp-Si is then dry etched to open windows for glass etching. The wet etching of sp-Si in KOH or TMAH water solutions is not possible since most photoresists used are soluble in alkaline etchants.

An alternative wet etching technique could be used: sputtering of silicon layer followed by sputtering of SiO2 layer in the same process chamber. Sputtered oxide layer is then patterned using standard photolithography; now, wet sp-Si TMAH etch could be used since this alkaline solution is highly selective to SiO2. We have tried that technique but it proved to be of no practical use. Due to high mismatch of thermal expansion coefficients of SiO2 and sp-Si layers, cracks appeared in both layers leading to a large number of pinhole defects in etched glass.

Another technique could be performed using a mechanical mask. This mask is placed over glass substrate and the sputtering is done through holes in the mechanical mask. The simple mechanical mask could be fabricated in silicon wafer with through holes made by wet anisotropic etching. Although very simple, this technique is limited by shape geometry (for an example, donut shape is not possible to be made with mechanical mask) and shadowing effects 5.

And finally technique for patterning sp-Si layer that was used in this work is lift-off technique. Reliable and easy to use, this technique proved to be very successful for patterning of sp-Si layer. First, standard photolithography is performed on glass substrate. Then, silicon is sputtered over the the whole substrate covering also the developed photoresist pattern. Finally, photoresist is dissolved in some suitable organic solvent such as aceton lifting-off the sp-Si which was deposited over the top of the photoresist thus opening a window in sp-Si for subsequent glass etching.

Ž. Lazić, M. M. Smiljanić and M. Rašljić are with the Centre of Microelectronic Technologies, Institute of Chemistry, Technology and Metallurgy, University of Belgrade, Njegoševa 12, 11000 Belgrade, Serbia, E-mail: zlazic@nanosys.ihtm.bg.ac.rs

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It is well known that unlike evaporation, sputtering process in not directed and thus to a certain extent isotropically coats the substrate and photoresist surface as well as the photoresist sidewalls. In some cases if the sidewall coverage is pronounced this makes the lift-off impossible. Therefore, for the purpose of a regular lift-off process, it is recommended that the photoresist film thickness should be significantly higher than the thickness of sp-Si layer.

To perform lift-off technique on sp-Si we used Microchemicals® AZ1518 positive tone photoresist with nominal thicknesses 1.5÷2.5µm 6. For the purpose of a fast and reproducible lift-off manufacturer recommends image reversal or negative tone photoresist. These types of photoresists develop patterns with negative sidewalls which reduces or even prevents sputter deposition on the resist sidewalls. This makes lift-off process more reproducible as compared to the use of positive resists which develop positive pattern sidewalls. But depending on the resolution required, the positive tone photoresist could be also used as well.

Based on our previous experiments, in order to have small number of pinhole defects in the etched glass, we expected the sp-Si layer thickness to be around 2m. Regarding the nominal photoresist thickness, AZ1518 photoresist seemed to be inadequate for lift-off. In order to use AZ1518 resist for the purpose of lift-off technique, the only solution was to increase the thickness by double layer resist coating.

According to the resist manufacturer, double coating can only be realized with high-viscosity resists with low solvent concentration. Since AZ1518 resist contain high solvent concentration, this could cause a dissolution of the existing resist layer. To prevent the dissolution of the first layer it is important to increase the resist softbake between two coating steps and to spin-coat immediately after the second resist layer dispensing with high spinner acceleration.

Pyrex glass substrates with dimensions of 2.5×2.5cm2 and thickness of 1.7mm were first cleaned in piranha solution (H2SO4:H2O2 at 2:1) at 120C for 20min and then they were Di rinsed and N2 gun dried. Then, double layer resist coating was performed. The fist resist layer was coated with standard spin speed of 4000rpm and then oven softbaked at 100C for 30min. The softbake time was increased twice to allow the solvent enough time to evaporate. The second resist layer was dispensed at the spin speed of 500rpm and then accelerated to 4000rpm. Finally, the second layer was oven softbaked for 15min at 100C. The final measured thickness of the double layer photoresist was 3.6÷3.9m.

Then a standard photolithography was performed using photomask with square patterns with dimensions of 1.5×1.5mm2 separated by 1mm. Photoresist was exposed on EVG620 Double Sided Mask Aligner for 8sec and developed for 50sec in Microchemicals® AZ726 MIF developer.

The photoactive component of AZ positive photoresist family during UV exposure release nitrogen. Developed positive photoresist pattern are still partly photoactive and can be exposed by UV radiation from the plasma during spattering process and to release nitrogen additionally. In order to ensure that all of the nitrogen has been released before sputtering, it is manufacturer recommendation to flood exposure without mask the developed resist pattern at a dose at least twice as high as the exposure dose with mask. This step was neccessary to prevent the formation of SiNx compound layer which occured in our previous experiments. This layer is hardly removed and requires additional technological step. So, glass substrates with developed photoresist were additionally flood exposed for 20sec.

The sputtering was performed on Perkin Elmer Sputtering System Model 2400 using 8 silicon target. The sputtering chamber was evacuated at 2×10-6 Torr pressure and the process was carried out at 10mTorr Ar pressure with 500W sputtering RF power. The measured thicknes of deposited silicon layer was 2.2µm.

III. RESULTS AND DISCUSSION

Fig.1 shows the optical photo of the part of the

square resist pattern after Si sputtering. The surface of the resist was wrinkled and very rough. The softening point of AZ1518 photoresist starts from 100÷110C. It is possible that glass surface attained such a high temperatures during sputtering process and combined with built-in stress in sp-Si layer a highly rough resist surface could result.

Fig. 1. Optical photo of the photoresist surface after Si sputtering: probably, the rough resist surface is the consequence of the resist thermal softening and/or built-in stress in sp-Si layer (magnification 100×).

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Fig. 2 shows that positive sidewalls of a positive resist are really covered with sputtered material.

Fig. 2. Optical photo of 50×50m2 pattern of dice-markers with visible positive sidewalls covered with sputtered silicon typical for positive tone photoresists (magnification 400×).

Fig. 3. Optical photo of the substrate surface after the lift-off was performed Negative sidewalls of sp-Si layer with fence-like structures are clearly visible (magnification 400×).

Glass substrates were then immersed for 10min in aceton and the photoresist was dissolved lifting the sp-Si

layer atop of it. Fig.3 shows the substrate surface and negative sidewall of sp-Si layer with fence-like structures. Since the resist sidewalls were covered during sputtering, lift-off starts irregularly along the sidewalls thus producing fence-like structures of the sputtered material.

If exposed and developed photoresist AZ1518 is subjected to temperatures higher than 140÷150C then it starts to thermally cross-link and become practically insoluble in aceton. Since in our experiments photoresist was disolved in aceton completelly for 10min that proves that the temperature developed during sputtering process was much lower then 140÷150C.

Glass substrates were etched to a depth of 100m. Etching was done in undilluted HF (49%) with an estimated etch rate of 8µm/min. Fig.4a, b, c shows the etching progress after 2, 7, and 12min, respectively. Fig.4d shows the glass surface after sp-Si was removed by TMAH etch at 80C for 8min. As can be seen from Fig.4d and in great detail in Fig.5a, b the glass surface is relatively free of defects. There are a few larger pinholes defects which are probably atributed to particles dropped on the glass substrates in the sputter process chamber. Smaller pinholes are atributed to defects in a sputter layer itself which are generated during sputtering of the sp-Si layer.

What is more important is that the notch defects on the mask edges were eliminated. Thus residual stresses, especially tensile stress in the masking sp-Si layer, which are responsible for pinholes and notch defects on the glass etched edges were minimized. Such a low residual stress in the sp-Si layer proves that the sputtering process parameters were optimal 7-9.

a) b)

c) d) Fig. 4. Glass etching progress in HF(49%): a) after 2min; b) after 7min; c) after 12min; d) after sp-Si removal (magnification 100×).

Glass surface

Sp-Si layer

Negative sidewall with fences

Positive sidewall

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a)

b)

Fig. 5. a) Optical photo of etched glass cavity 1.5×1.5mm2 after 100µm glass etching in 49% HF (50× magnification) and sp-Si removal; b) enlarged detal of the etched edge wihout notch defects (100× magnification).

IV. CONCLUSION

A new masking technique for glass etching is presented. Sp-Si layer was RF sputter-deposited on the glass surface

and than patterned using lift-off technique. Rectangular windows were formed in the sp-Si layer and the exposed glass surface was etched in HF(49%) etchant. Very small number of pinholes defects was observed and the notch defects were completely eliminated. Hence the presented results prove that using sp-Si layer as a masking material for glass etching is simple, low cost and feasible.

ACKNOWLEDGEMENT

This publication is a result of joint work of Institute of Chemistry, technology and Metallurgy (ICTM), Serbia and Austrian Center for Medical Innovation and Technology (ACMIT), Austria

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