Fully loaded carrier with twelve 2” wafers in situ
Written by Mark Dineen PhD, Principal Applications Engineer, OIPT
Dr Mike Cooke detailed in the Autumn 2008 issue of ProcessNews, how a sample experiences heating when it is plasma processed. Mike also explained how ‘helium backside cooling’ can be used to control the sample temperature.
This is especially important when using a photoresist mask on samples. Photoresist is the easiest material to use to define the complicated patterns that create devices from flat semiconductor wafers. However there is a disadvantage when using photoresist as it has relatively low upper temperature limits (typically 130ºC).
When photoresist is used as a mask for plasma processing the vital questions are:
• How hot is the sample getting?
• How can it be controlled?
As you can imagine life gets harder when trying to increase the number of wafers you are try to cool at one time!
Why etch more than one wafer?
HBLED devices are commonly on 2” wafers and to significantly reduce costs one must process as many of these in one run as possible. If performing Patterned Sapphire Substrate (PSS) processing, photoresist is used as a mask and to achieve a decent etch rate on the sapphire, cooling is required.The best way to approach this problem is to start with the basics; in this case how hot is the electrode getting? The electrode is actively cooled but if one is relying on the electrode to provide the wafer cooling source, this must stay as cold as possible.
Without the He cooling, the temperature of the sample placed on the electrode continues to rise over time, eventually reaching over 200ºC! However, the electrode temperature is below 50°C so it is hoped that this can be used to cool the sample during etching.
The next obstacle to overcome is how to make sure that there is a good measure of He cooling medium behind each wafer for a multiple wafer batch. The solution is to load the wafers in to a carrier which clamps each wafer, and then clamp this carrier in the system. The carrier is specially designed to allow He to flow to the back of every wafer ensuring each one is kept cool.
Wafer temperature is kept well below 90°C even after one hour of etching. The photoresist integrity is maintained at this temperature. Keeping samples cool is only part of the story. The samples must now be etched with good uniformity and selectivity to the photoresist, to obtain the best feature possible.
Summary
Etching large numbers of wafers with a photoresist mask requires good temperature control of each wafer, and this requires an understanding of how to transfer the heat from the plasma away from the samples to the cooled electrode. Helium backside cooling is the key, and understanding how to enable this for every wafer ensures success.
For more information on Oxford Instruments Plasma Technology processes and process techniques click here