10 Key Parameter Control for Deep Silicon Etching

1,The ratio of gas flow rate

SF6 to CF determines the balance between etching and passivation, golden ratio: SF6:CF=3:1 (guaranteed radical concentration > 1016cm-3) Case: A factory fine-tuned the ratio from 2.8:1 to 3.2:1, the etching rate increased from 8um/min to 12um/min, and the sidewall inclination angle was optimized from 88° to 89.5°

2,RF power High frequency source power (13.56MHz) controls plasma density, low frequency bias power (2MHz) regulates ion energy, power coupling formula:

Practical parameters: In the Bosch process, when HF=600W/LF=200W, the aspect ratio reaches 30:1, and the sidewall roughness < 100nm.

3,Temperature gradient:

The wafer temperature needs to be stable at -110°C to -80C (liquid nitrogen cooling), temperature fluctuations ± 2°C will result in an etch rate deviation of >15%. Temperature control scheme:

Electrostatic chuck (ESC) back helium cooling

2. Cavity wall thermoelectric refrigeration (TEC) array

4,Stress adjustment book

The working pressure is controlled at 10-30mTorr, the low pressure (10mTorr) improves anisotropy, and the high pressure (30mTorr) enhances the etch uniformity. Example: A 3D NAND production line achieves a 40:1 aspect ratio at 15mTorr, but after increasing the pressure to 25mTorr, the intra-wafer uniformity is optimized from ±8% to ±3%.

Cyclic timing

Etching/passivation cycles need to be accurate to the millisecond:

Step Time (s) Gas Composition Power (W)

Etch 8-10 SF6 150sccm HF 800

Passivation5-7 C4F8 80sccm LF150

Optimization effect: The cycle is shortened from 15s to 12s, the production capacity is increased by 20%, and the fan wavelength of the sidewall is reduced by 50%.

6,Mask selection

The mask thickness and selection ratio must be met:

Material comparison:

Photoresist: 50:1 selection ratio (for shallow etching only)

Si02: Selection ratio 150:1 (HF pretreatment required)

AL: Selection ratio 200:1 (back cooling is required to prevent peeling)

7,Electrode spacing

The spacing between the upper and lower electrodes is adjusted within the range of 5-10cm, the spacing is reduced by 1cm, and the ionic density is increased by 30%, but the uniformity is deteriorated by 5%. Equilibrium point: When the pitch is 7 cm, the combined score of aspect ratio and uniformity is the best (SEMI standard score >85).

The number of particles per square meter of cavity cleanliness needs to be ≤ 100 (20.3um), exceeding the standard will lead to an increase in the defect rate of microbridge (for every 50 particles, the defect rate is +1.2%), and the maintenance cycle of the equipment will be shortened by 30%.

The end-detection

Optical emission spectroscopy (OES) monitors the signal strength of SiF4 (wavelength 440nm) and triggers termination when the intensity drops to 30% of the peak (error ± 0.5um).

10,Wafer stress

Residual stress needs to be controlled < 200MPa by:

Alternating high/low frequency RF (reducing ion embedding depth)

Post-etching annealing (300°C/N, ambient, 30min)