Defining the Characteristic Feature of the Mental Mask on the Wafer

Introduction and background

        The main accomplishment for the previous lab session was defining the characteristic feature of the mental mask on the wafer. I defined the place of growing metal in clean room by using photolithography technology. In the offline session, our TAs applied the metal mask on the wafer by using metal evaporation and lift-off technology. Currently, the wafer was covered by a thin metal layer (see Figure 1). The purpose of this lab session was to finish defining the metal mask by etching down the unnecessary GaAs, which was not covered by the metal mask.

[pic 1]

Figure 1: Top View and Side View of Wafer after Metal Evaporation [1]

Methods and materials

                First of all, the same as each time entering the clean room, I equipped myself with safe garment: smocks, shoe covers, rubber gloves and hat. I also grabbed a small slant sweeter with myself. The TAs served for this lab session were Mary. Before we started, Mary told me that I might have to do multiple times of etching since the target etching depth was about 1000 Angstroms, which was hard to achieve by just one – time etching. The acidic solution using for etching was a mixture of 1 part of H2SO4 (sulfuric acid), 8 parts of H2O2 and 320 parts of deionized water.

My wafer was baked on the hot plate for 5 minutes by our TA before I came to the clean room. The first thing for this lab session was to measure the metal height by using the profilometer. I used my tweezer to transfer my wafer onto the stage of the machine. Before the measuring process, I tried to make the top corners of my wafer under the edge of the stage to avoid scratching and contaminating my wafer. After adjusting the position of my wafer by using the stage position knob, I located my wafer under the stylus, which was slightly in front of the light beam. I also moved the stage front and back to make sure that my wafer did not hit the stylus. Our TA Mary helped me to do the measurement. First, she pressed the up button and watched the screen. When the stylus appeared on the screen, she released the button. Then she used the stage position knob to place the four different sized emitter regions under the stylus scanning range. After doing this, he pressed the START/STOP button to begin the measurement. When the scan was finished, the result showed on the screen. I recorded the height of metal mask (1500 angstroms) and then opened the cover after the scan and put my wafer on the container for the etching process.

Since I need to use strong acidic solution, before the etching process, I dressed myself with extra safety equipment: safety goggles, rubber gloves, face shield, arm covers, and plastic apron. Our TA Mary set up the timer for three minutes and fifty-five seconds. I used my tweezer to hold the wafer and emerged it into the solution, and then gently swirled it around the beaker. After that, I emerged my wafer into the water to wash it and my tweezer. I used the nitrogen gun to blow dry my wafer and tweezer. At this moment, Mary helped me measure the etching depth on the profilometer (400 angstroms). Since the first etching depth (1100 angstroms) was close to our target (1000 angstroms), we did not need to do an extra one.