《Microelectronics Process》MEMS LAB SESSION 2

MEMS LAB SESSION 2 Undercut Silicon Nitride using KOH Etching OVERVIEW OF LAB SESSION: This lab session utilizes potassium hydroxide(koh) wet etching to undercut and release the silicon nitride cantilevers and fixed-fixed beams. In Step 2. 1, KOH etching is used to anisotropically etch the silicon, and undercut the silicon nitride. The wafers undergo a series of solvent cleans and then are allowed to dry in the air in Step 2.2 LAB OBJECTIVES: This lab session has the following primary objectives Introduction to wet, anisotropic KOH etching Complete and release silicon nitride beams Complete electron-beam evaporation of chromium and gold Initial instruction on the following major pieces of lab equipment KOH Wet Station Acidhood 2 Wet station Electron-Beam Evaporative Deposition System Before the beginning of this lab, make sure to read the corresponding Standard-Operating Procedures(SOP)for these major pieces of equipment. The SOP for each of the equipments can be accessed at the following site http://www-mtl.mitedu/mtlhome/3mfab/sop.html LAB PROCEDURES MEMS Lab Session 2 has 2 major ster 2.1 RELEASE CANTILEVER 2.1.1 Tool: KOH Wet Station 2. 1.2 The silicon nitride beams are undercut using a solution of potassium hydroxide (KOH). This strong-base solution preferentially etches the and planes relative to the , in single-crystal silicon, ther eby creating an an Pour 3 liters of DI water into the quartzware. Use the scale to measure out a sufficient amount of KoH pellets to create a 20% by weight solution Set the water bath temperature to 85C, which will give a KOH bath temperature of about 80C The etch rate is exponentially dependent on temperature, and therefore maintaining good temperature uniformity in the bath is important 2.2 EVAPORATED SOLVENT DRY 2.2.1 Tool: KOH Wet station 2.2.2 Due to the small scale of microfabricated devices surface forces become considerable compared to body forces. To try to avoid any stiction of the released Page 1 of 3

MEMS LAB SESSION 2 Undercut Silicon Nitride using KOH Etching OVERVIEW OF LAB SESSION: This lab session utilizes potassium hydroxide (KOH) wet etching to undercut and release the silicon nitride cantilevers and fixed-fixed beams. In Step 2.1, KOH etching is used to anisotropically etch the silicon, and undercut the silicon nitride. The wafers undergo a series of solvent cleans and then are allowed to dry in the air in Step 2.2. LAB OBJECTIVES: This lab session has the following primary objectives: ¾ Introduction to wet, anisotropic KOH etching. ¾ Complete and release silicon nitride beams. ¾ Complete electron-beam evaporation of chromium and gold. ¾ Initial instruction on the following major pieces of lab equipment: KOH Wet Station Acidhood 2 Wet Station Electron-Beam Evaporative Deposition System Before the beginning of this lab, make sure to read the corresponding Standard-Operating￾Procedures (SOP) for these major pieces of equipment. The SOP for each of the equipments can be accessed at the following site: http://www-mtl.mit.edu/mtlhome/3Mfab/sop.html LAB PROCEDURES: MEMS Lab Session 2 has 2 major steps: 2.1 RELEASE CANTILEVER 2.1.1 Tool: KOH Wet Station 2.1.2 The silicon nitride beams are undercut using a solution of potassium hydroxide (KOH). This strong-base solution preferentially etches the and planes, relative to the , in single-crystal silicon, thereby creating an anisotropic etch. Pour 3 liters of DI water into the quartzware. Use the scale to measure out a sufficient amount of KOH pellets to create a 20% by weight solution. Set the water bath temperature to 85o C, which will give a KOH bath temperature of about 80 o C. The etch rate is exponentially dependent on temperature, and therefore maintaining good temperature uniformity in the bath is important. 2.2 EVAPORATED SOLVENT DRY 2.2.1 Tool: KOH Wet Station 2.2.2 Due to the small scale of microfabricated devices, surface forces become considerable compared to body forces. To try to avoid any stiction of the released Page 1 of 3

cantilevers to the sloped sidewall, the wet wafers will be gently sprayed with isopropyl acohol. The alcohol will then be allowed to dry in the air PRE-LAB QUESTIONS 1. What chemical is used to etch the silicon? Is this an isotropic or anisotropic etch? 2. Given that the density of water is l gram/ml, how much KOH should be mixed with 3 liters of de-ionized water to create a 20% by weight solution? 3. Referring to Figure 2.1, what is the KOh etch rate in the direction for the given process parameters? 4. Using the mask shown below,( Figure 2.2), sketch how the cross-sections A-A and B-B would look after Step 2.1 silicon etch rotes in [um/h)for various KOH concentrations and etch temperatures as calculated from Eq [A-1]by setting E。=0.595 ev and k。=2480μmh·(mol)-423 Temperature ['cI %KoH"203040506070°80°90100 10 1.493.26.713.32524682140233 151.56347.014.026.54986147245 20 157347.11402674986148246 1533369136259478 144239 144316.51282444579135225 35 132295911.82234172124206 1172.55.310.51993664110184 45050 101224.69017.1315595158 08418387.51422646 79131 066143059112213662104 0.50 22448415274778 Reference: H. Seidel et al., ""Anisotropic Etching of Crystalline Silicon in Alkaline Solutions, J Electrochem.Soc,137(11)3612-32,1990 Figure 2.1 2 of 3

cantilevers to the sloped sidewall, the wet wafers will be gently sprayed with isopropyl acohol. The alcohol will then be allowed to dry in the air. PRE-LAB QUESTIONS: 1. What chemical is used to etch the silicon? Is this an isotropic or anisotropic etch? 2. Given that the density of water is 1 gram/ml, how much KOH should be mixed with 3 liters of de-ionized water to create a 20% by weight solution? 3. Referring to Figure 2.1, what is the KOH etch rate in the direction for the given process parameters? 4. Using the mask shown below, (Figure 2.2), sketch how the cross-sections A-A and B-B would look after Step 2.1. Reference: H. Seidel et al., “Anisotropic Etching of Crystalline Silicon in Alkaline Solutions”, J. Electrochem. Soc., 137(11):3612-32, 1990. Figure 2.1 Page 2 of 3

B B Figure 2.2 3 of 3

A A B B Figure 2.2 Page 3 of 3