A GaAs, InP,GaN, GaInP, SiC and other compound semiconductor wafers are very expensive and it is difficult to get a good ohmic contact region. Thus, it is easy to ruin one of these wafers. Therefore, the yield can be very low if adequate precautions are not taken.
The AW control software has great temperature control and repeatability, even at low temperatures for the AW610M. However, operators can upset the software by starting the process at different temperatures, thus making the temperature control not so repeatable and stable under these conditions. To avoid this, a multi-step recipe to process the wafer is used. Below please find the curve for your reference. The first steady-state step preheats the chamber to a low temperature (for example, 150°C) to stabilize the temperature control. This is the preheat/stabilizer step. The process then ramps up to another low temperature (for example, 250°C) steady-state step and the control software checks the stability of the temperature control. This is the stability check step. The temperature is still low enough that it has no effect on the physical properties of the wafer. If the temperature control stability is within user defined limits, then the process control continues to process the wafer. However, if the stability is not within the limits, the process is aborted, thus saving the wafer. The wafer can then be re-processed and, thus, there is a higher yield.
These parameter limits are called PSum1 and PSum2. For this process, set PSum1 to zero (0) and refer to the PSum subsection of “Optimizing a Recipe” of this manual to set PSum2. PSum1 looks at the first steady-state step, which is the preheat/stabilizer step. The temperature control is usually not very stable during this step, until the end. Therefore, checking PSum against PSum1 needs to be disabled by defining it to be zero (0). PSum2 looks at the second steady-state step, which is the stability check step. Here, the temperature control needs to be stable and reliable before the software will allow the process to continue.
Psum is the energy applied on the wafer at the steady stage (including ramp). Repeated process would have repeatable Psum. If Psum getting higher before, it could be:
(1) Lamps are weak, or some lamps are bad. It need more power energy applied on the lamp to get same intensity to heat the wafer.
(2) Quartz tube got dirty and block the light to the wafer. Need more power to maintain the same temperature of the wafer in the process.
(3) Temperature sensor getting bad and reading the temperature value lower than it really is. To maintain the reading temperature same as set point temperature, the higher power would be applied to the lamps.