520.495/580.495 Microfabrication Laboratory
Laboratory Assignment #6
During last week's session, we began the anisotropic etching of the silicon to form the membrane under the piezoresistors. Due to the length of time needed for this step, the etching was completed by the instructor. This week, we'll examine the wafers and try to estimate the thickness of the diaphragms, then clean the wafers and pattern the photoresist needed to open the contact cuts to the piezoresistors.
I. Membrane Thickness Determination
1. Measure the lateral extent of the sloping sidewall (a <111> plane), and from this estimate the depth of the anisotropic etch.
2. Alternatively, for those of you with the alignment "bars" etched into the back of the wafer, use the profilometer to measure the thickness of the thinned portion of the wafer.
II. Cleaning
1. Following the recipe in lab handout 1, clean the wafers using BOTH RCA organic clean and Piranha, rinsing twice in DI water
1. Dip wafers into 50:1 HF for 15 seconds, then rinse in deionized water. Rinse and dry the wafers (drying with N2 gun may be preferable to the spin/rinse/dryer)
2. Load the wafers in the quartz boat and place in the Blue-M oven at 200°C for 30 minutes. (Boat must be placed diagonally on the rack of the oven so that it doesn't capsize! Use narrow green teflon tweezers to grip front bar of the boat.)
III. Photolithography
1. Prepare developer in 1000 ml beaker, 160 ml of KTI 809 developer and 640 ml of water. Prepare DI water rinse in 2000 ml beaker.
2. Spin photoresist on the front (polished) side of the wafer, 4000 rpm for 60 seconds, acceleration and deceleration set at 2 o'clock.
5. Prebake in the Blue-M oven for 15 minutes at 90°.
6. Align wafers to "NC" mask. On those wafers where the vernier alignment marks are visible, align to these, otherwise, align the contact cuts with the diffused piezoresistors.
7. Expose for 15 seconds. Develop for 15 seconds, or until the openings rectangles clear.