MICROSTRUCTURING OF METALLIC LAYERS FOR SENSOR APPLICATIONS Vladimír KOLAŘÍK, Stanislav KRÁTKÝ, Michal URBÁNEK, Milan MATĚJKA, Jana CHLUMSKÁ, Miroslav HORÁČEK, Institute of Scientific Instruments of the ASCR, v. v. i., Královopolská 147, 612 64 Brno, Czech Republic, EU, kolariq@isibrno.cz Abstract This contribution deals with a patterning of thin metallic layers using the masking technique by electron beam lithography. It is mainly concentrated on procedures to prepare finger structure in thin Gold layer on electrically isolated Silicon wafer. Both positive and negative tone resists are used for patterning. The thin layer is structured by the wet etching or by the lift off technique. The prepared structures are intended to be used as a conductivity sensor for a variety of sensor applications. Patterning of the thin layer is performed by the e beam writer with shaped rectangular beam BS600 by direct writing (without the glass photo mask). Besides the main technology process based on the direct write e beam lithography, other auxiliary issues are also discussed such as stitching and overlay precision of the process, throughput of this approach, issues of the thin layer adhesion on the substrate, inter operation control and measurement techniques. Keywords: Gold etching, lift-off, electron beam lithography, interdigitated sensor. 1. INTRODUCTION Development of interdigitated sensors has received considerable attention in last years when these structures have been widely used in telecomunication, biotechnology, chemical sensors, acoustic sensors, micromechanical aplication and humidity sensors [1, 2]. Each of these aplication requires appropriately designed structures with properties enabling intended purpose. The main goal of this contribution focuses on interdigitated structures fabrication using shaped E-beam writer and additional technologies to it (etching, plasma etching, metal sputtering) [3, 4, 5]. 2. DESIGN OF INTERDIGITATED SENSORS It was intended that sensors will be used for biological application. Due to this requirement, interdigitated electrodes had to be fabricated from gold. Silicon wafer used as a substrate had to be electrically isolated which provides silicone dioxide layer of 130 nm thick. Sensor application required chemically resistant layer all over the sensor apart from interdigitated electrodes area and bonding pads (design of sensors is in figure 1). As the resistant layer electron resist SU-8 was used [6].
3,18 0,005 1,2 2,2 1,14 0,005 0,5 0,005 0,5 3,5 Fig. 1 Design of interdigitated sensors (mm scale). 3. WET ETCHED GOLD INTERDIGATATED SENSORS Wet etching of gold over the resist mask was the first testing approach to prepare interdigitated sensors. The gold layer of thickness 100 nm thick gold layer was sputtered on silicon wafer (covered with silicone dioxide). Then negative resist SU-8 (etching mask) was spin-coated onto the wafer. Desired pattern was exposed by e-beam writer BS600. After developing process gold layer was etched by standard Sigma-Aldrich gold etchant [7]. Final etched electrodes are shown in figure 2. Fig. 2 Wet etched gold interdigitated electrodes. Figure 2 shows that electrode edges are not very uniform. Non-homogenous etching causes different size of electrodes on different substrate position (gold etchant remains for several seconds on some areas on substrate during the cleaning process and gold is etched further). Optical inspection (CLSM) was carried out in this phase of microstructuring to determine the yield of this approach. The yield was 63% (63 of 100 sensors), which was acceptable. In order to spin-coat second resist layer of SU-8 the resist mask was removed in oxygen plasma after optical inspection and subsequently the same procedure as in the first exposition was repeated (precision of second exposition is sufficient figure 3). Throughput of this approach (from preparing substrate to optical inspection before laser cutting) was about five days for one wafer (plus three days for preparing data for e-beam writer). Complete sensor prepared for laser cutting is shown in figure 3.
desired position 5 μm shifted a) b) Fig. 3 Sensor covered with chemically resistant layer (a), overlay precision of second exposition (b). Main issues of unsuccessful process repeating were as follows: impossibility to control the etching process (it runs too fast to precisely determine the right etching time), weak adhesion of gold layer on silicon dioxide in following procedures (cleaning after laser cutting, bonding, ). 4. INTERDIGITATED SENSORS PREPARED BY LIFT-OFF TECHNIQUE Lift-off is an additive technique as opposed to subtracting technique like etching. It is based on sacrificing layer, in which is fabricated desired pattern. There are several lift-off techniques, which are different in number of layers and their composition. We used lift-off technique, which takes advantage of two resist layer differing in their molecular weight. The bottom layer has lower molecular weight than the top layer. Because of that the bottom layer dissolves faster than the top layer and it creates undercut effect, which helps to avoid coverage of side walls by sputtered metal [6, 8]. Lift-off process steps are shown in figure 4. a) b) c) d) e) f) Fig. 4 Lift-off process steps, a) deposition of first resist layer, b) deposition of second resist layer, c) patterning sacrificial layers (exposition, developing), d) deposition of metal, e) dissolving of the sacrificial layers, f) final pattern. Positive resist PMMA [9] was used for both sacrificial layers. Bottom layer had molecular weight of 350k and it was 14% wt solution in anisole. The bottom layer was baked on hot plate at 150 C for 30 minutes. The thickness of the layer was 760 nm. The top layer had molecular weight of 950k and it was 6% wt solution in anisole. The top layer was baked on hot plate at 175 C for 30 minutes. Total thickness of both resist layers was 1050 nm. After resist baking the exposition was done (dose of 14 µc.cm -2 ). We used acetone based developer for chemical developing. Some undesirable issues as cracks on electrodes (because of stress in such a thick resist layer) occurred after wet developing. Also acetone based developer is aggressive. Due to this there is a small technological window in developing process. If developing time is too short, there can be some resist residue on the silicon dioxide surface and if developing time is too long, there can be cracks on electrodes or electrodes can be totally shifted and they can overlap each other.
After chemical developing we removed resist residues from silicon dioxide surface in oxygen plasma [10]. Afterwards thin gold layer (approximately 100 nm) was sputtered onto the wafer [4]. Sacrificial resist layers were dissolved in trichlormethane for a several seconds and undesired gold layer was washed away. However there were some areas between electrodes still covered by gold (figure 5a). Due to the cracks in resist layer some electrodes were connected by very thin strip of gold (figure 5b) resulting in short-circuit of the whole sensor, which makes the sensor unusable. We tried to clean wafer in trichlormethane bath in ultrasonic cleaner to remove remaining gold between electrodes. Most of gold (even gold on desired places like electrodes, bonding pads, ) was washed out after few seconds in ultrasonic cleaner. a) b) Fig. 5 a) areas between electrodes covered by gold, b) short-circuited electrodes. Throughput of this approach was same as in first approach (etching) about five days (data for e-beam writer were the same). The biggest problem of this approach was adhesion of gold to silicon dioxide. To improve gold adhesion we sputtered thin layer of chromium (approximately 10 nm) onto the silicon dioxide before gold sputtering. The adhesion differed significantly. After washing wafer in trichlormethane we can remove easily remaining gold between electrodes in ultrasonic cleaner without damaging of sensor pattern. However problem with short-circuited electrodes remains same as problem with small technological window of acetone based developer. It was necessary to completely change resist deposition parameters (resist layers thickness, baking time, less aggressive developer) to avoid these problems. Again we used PMMA for both sacrificial layers, but in different concentration. The bottom layer was 9% wt in anisole (molecular weight 350k). The bottom layer was baked on hot plate at 100 C for 10 minutes, then at 125 C for 10 minutes and finally at 150 C for 10 minutes. The thickness of this layer was approximately 500 nm. The top layer was same as in the first case. However baking process was different. The top layer was baked on hot plate at 100, 125, 150 and 160 C, each step took 10 minutes. Total thickness of both layers was approximately 800 nm. Thereafter sensors pattern was exposed (dose of 20 μc.cm -2 ). We used pentyl acetate for developing process. This developer is not aggressive as acetone based one, therefore developing took longer time. Total developing time was 40 minutes. Optical inspection confirmed that there were no cracks on electrodes after developing process. Necessary oxygen plasma treatment was carried out after chemical developing. Between the developing process and metal deposition it is necessary to let the substrate dry out for several hours at room temperature. Afterwards thin layer of chromium was sputtered onto the wafer (as layer improving adhesion) and then the gold was sputtered. Next we used trichlormethane bath in ultrasonic cleaner to remove sacrificial layers with redundant gold. There were no gold residues between electrodes and gold pattern withstood the cleaning process without damage. The optical inspection determined yield of 87,5% (105 of 120 sensors) for this approach.
After the cleaning process we deposited SU-8 resist and resistant layer pattern was exposed onto the wafer. Then the chemical developing was carried out and the whole process was finished. Completed sensors are shown in figure 6. Throughput of this approach was 6 days. Fig. 6 Sensor covered by chemical resistant layer prepared by lift-off technique. 5. RESULTS We successfully tested two approaches of gold interdigitated sensors fabrication, wet etching and lift-off technique. According to our results lif-off technique is more suitable for this process. Final process consists of these steps: 1) deposition of first sacrificial resist layer - PMMA 350k, 9% wt solution in anisole, 2) baking on hot plate at 100, 125 and 150 C, each step for 10 minutes, thickness of layer is about 500 nm, 3) deposition of second sacrificial resist layer - PMMA 950k, 6% wt solution in anisole, 4) baking on hot plate at 100, 125, 150 and 160 C, each step for 10 minutes, thickness of second layer is about 300 nm, 5) pattern exposition by e-beam writer BS600, dose of 20 μc.cm -2, 6) chemical developing with pentyl acetate for 40 minutes, oxygen plasma treatment (100 W, 1 minute), 7) drying out the substrate for several hours, 8) chromium deposition (10 nm), gold deposition (100 nm), 9) removal of sacrificial layers in trichlormethane bath in ultrasonic cleaner. 6. CONCLUSION We created deep precise microstructures in double resist layer and consequently we fabricated metallic layers microstructures for sensor application. As we enhanced thin gold layer adhesion we improved cleaning process. ACKNOWLEDGEMENT This work was partially supported by the EC and MEYS CR (project No. CZ.1.05/2.1.00/01.0017 ALISI), the TACR project No. TE 01020233 and by the institutional support RVO: 68081731.
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