DSE

The key to successful DSE is the cooler flow rate. The Helium gas flow is critical for a good DSE process, the recommended value is between 2 - 4 sccm. If it's too high, it means too much He gas in the chamber, which will affect the reaction rate; if it's too low, there is not enough He gas for cooling the sample, the sample will overheat.

To maintain a proper gas flow, you should adjust the He pressure point, heavier sample (a large sample  on a carrier wafer) usually requires higher pressure, lighter sample (thin wafer) requires low pressure. Here I found that 4000 in the standard recipe is good for most of the applications.

The backside cleanliness of a wafer is EXTREMELY important, make sure clean the backside with Acetone. if there are some debris on the backside, it will create a small gap between the wafer and the chuck, causing abnormal gas flow. 

Thermal conductivity is also EXTREMELY important, FOMBLIN oil is used to dissipate heat from samples. before applying the FOMBLIN oil, put on an extra pair of gloves, use the Q-tip to apply a thin layer of oil on the back side of the sample, then remove the pair of gloves. this is to ensure that you don't get oil onto the carrier wafer, it will damage the chamber.

If you run a standard recipe (ZL_4inch DSE standard), the etch rate is around 1um per cycle. Based on my measurement, 20 cycles yielded 18um trench. Very close.