GaAs: Difference between revisions
Content deleted Content added
No edit summary |
Removed garbage by User:Allwin21 |
||
| Line 1: | Line 1: | ||
#REDIRECT: [[Gallium(III) arsenide]] |
|||
== '''AW software controller''' == |
|||
Introduction |
|||
#REDIRECT [[GaAs]], #REDIRECT [[InP]] and the other compound semiconductor material are used in the production of #REDIRECT [[devices]] such as photovoltaic cells, optoelectronics devices like laser diodes and RF devices like high frequency amplifiers. These processes are used in the formation of GaAs Laser diode, LED, HgCdSb high efficiency solar devices, GaAs HBT, GaAs MESFET and pHEMT. |
|||
Problems with GaAs, InP and other compound semiconductor in alloy processing |
|||
The GaAs, InP and other #REDIRECT [[compound semiconductor]] processes create a region called the ohmic contact between the Metal layer and high doped GaAs layer or InP layer, or other compound semiconductor layer. The current-voltage (I-V) curve of the device needs to be linear and symmetric. Low resistance and stable contacts are critical for performance and reliability. If the I V characteristic is non-linear and asymmetric, the contact can instead be termed a blocking or Schottky contact. |
|||
To get a good ohmic contact, the metal has to flow into the substrate evenly and to a proper depth. The temperature range to flow the metal is very narrow (+/- 8C or less) and low (400C to 450C). Thus, the temperature needs to be controlled precisely. If the temperature is too high, too much metal is flowed into the substrate and it will be too deep. This causes the contact to be Schottky. If too low, not enough metal is flowed. The wafer cannot be re-processed to have more metal flow to increase the depth, because it is now tempered and its properties have changed. In both cases, the wafer is useless and has to be discarded. |
|||
Engineers try to reduce this problem by creating a process recipe with multiple steady state steps, Figure 1. The first step would preheat the chamber and stabilize the temperature control before it ramps up to the alloy process steady-state step. The temperature at this preheat/stabilizer step is low enough that it has no effect on the physical properties of the wafer. When the temperature control ramps up to the alloy process step, the engineers cross their fingers and hope for a good outcome. If the temperature control is stable, then everything is fine. However, if it is not stable, the wafer is junk and has to be thrown away. The reason for this uncertainty in temperature control at the alloy process step is because the temperature control during the stabilizer step was not stable enough or the thermocouple did not have a good contact with the substrate/susceptor. |
|||
Figure 1 |
|||
It is a problem getting good temperature control with a thermocouple that does not have a good and reliable contact with the substrate/susceptor. Too much contact force on the TC and the tip could bend and not make good contact. Too little contact force and the TC will separate away from the susceptor. Also, the TC tip could bounce on the surface of the susceptor. These cases will not give an accurate and true reading of the temperature of the susceptor. All these problems can give the process controller a difficult time controlling the temperature. |
|||
The benefits of using the #REDIRECT [[AW software controller]] for GaAs, InP and other compound semiconductor processes |
|||
A GaAs, InP and other compound semiconductor wafers are very expensive and, as explained earlier, 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 software controller has great temperature control and repeatability, even at low temperatures for the AG610. 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, Figure 2. The first steady-state step preheats the chamber to a low temperature (for example, 150C) to stabilize the temperature control. This is the preheat/stabilizer step. The process then ramps up to another low temperature (for example, 250C) 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. |
|||
Figure 2 |
|||
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. |
|||
Summary |
|||
The AW control software with a new controller can dramatically increase the yield of GaAs, InP and other compound semiconductor wafers. We have had reports from companies having nearly 100% of their wafers saved from being ruined at the alloy process step. |
|||
Revision as of 23:33, 25 August 2007
Redirect to: