Important Note:

HSQ is sensitive to the time between coating and exposure. It is best to expose immmediately after coating samples.

Develop process:

There are many ways to develop resists in general and HSQ also has many ways it can be developed. Some example develop processes are 1) lower contrast, faster, 2) high contrast, mid-dose, 3) highest contrast, high dose. The higher the contrast the process, the higher the dose is needed, and thus the write time increases.


1) lower contast, faster

1. after spin coat, hot plate bake at 250 C for 2 min
2. EBL expose, base dose ~1000 uC/cm2
3. develop 70 sec, with 2.3% TMAH (MF-319) at room temperature
4. rinse with gently flowing DI water from tap for ~60 sec (it is important to use the flowing DI water to reduce residue)



2) high contrast, mid-dose

1. after spin coat, either no hot plate bake, or 80C hot plate bake for 4 min
2. EBL expose, base dose ~1500 uC/cm2
3. develop 30 sec, with 25% TMAH at room temp (~21 C)
4. rinse with gently flowing DI water from tap for ~60 sec (it is important to use the flowing DI water to reduce residue)


3) highest contrast, high dose

1. after spin coat, either no hot plate bake, or hot plate bake at 80C for 4min
2. EBL expose, base dose ~2000 uC/cm2
3. develop 30 sec, with 25% TMAH at 40-50 C
4. rinse with gently flowing DI water from tap for ~60 sec (it is important to use the flowing DI water to reduce residue)


Comments:

1. Increasing contrast increases dose which in turn increases write time. This is a trade off.
2. Room temperature developing is simpler and does not require a hot plate.
3. Improving contrast helps with thicker HSQ (e.g. 250 nm thickness or higher) to prevent resist exposure in backscattered areas adjacent to large features, in between small gaps like those found in optical gratings, or dense small patterns like 10 nm dots on 40 nm pitch.
4. Higher contrast process also has a wider process window which allows for a wider margin of error or variation in dose to print a nanometer scale feature.
5. Increasing contrast also gives straighter sidewalls in resist.


Removing HSQ:

After exposing HSQ it turns into silicon dioxide. Therefore, HSQ can be removed with the same chemical that removes silicon dioxide which is HF or BOE. The removal rate of thermal silicon dioxide with 5:1 BOE is approximately 100 nm/min, while for unannealed PECVD oxide the removal rate is ~490 nm/min [1]. But with HSQ that has been exposed by EBL, or baked at temperatures at 250 C or lower, it has been observed that the removal rate of HSQ is much faster, possibly as fast as removing 100 nm in a few seconds. Also, please consider that BOE will etch any other oxide which is present on your sample.

[1] Kirt R. Williams, Kishan Gupta, Matthew Wasilik, "Etch Rates for Micromachining Processing - Part II", Journal of Microelectromechanical Systems, Vol. 12, No. 6, December 2003