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Table 2440a. Etchants used in semiconductor manufacturing.
Si is easily oxidized in air during mechanical polishing for EM analysis. In some critical analyses (e.g. Si surface plasmon analysis) in EELS, Si TEM specimens are normally dipped in HF solution (e.g. 10% HF solution) to remove the surface oxide. However, a thin layer of SiOx may still exist due to exposure to air before loading into the TEM.
Table 2440b. HF attack of select metallic materials.
| Material |
Reactivity with Hydrofluoric Acid |
| Aluminum |
Concentrated HF is not an excellent etchant of aluminum. However, dilute solutions of HF can be used as an Al etchant. HNO3-HF mixtures such as 50 mL·L−1 40% HF and 250 mL·L−1 65% HNO3 is recommended as an etchant for aluminum thin films. Exposure to HF causes thick fluorination of aluminum films. The resulting aluminum fluoride film can be 2000 Å thick and greater than 50% fluorine. |
| Copper |
Exposure to HF causes an approximately 100-500 Å thick fluoride film. |
| Molybdenum |
Molybdenum, especially when an anode, is dissolved by anhydrous HF at such a high rate that this reaction can be utilized for preparing molybdenum fluoride films |
| Nickel |
Nickel is recommended as materials for the transport and storage of hydrofluoric acid, since the etch rate at room temperature remains under 2000 Å/min |
| Platinum |
Platinum is not attacked at room temperature in 40% HF or higher concentrations. Platinized titanium has been shown to be a good electrode material in electrolytes which contain HF |
| Titanium |
Hydrofluoric acid strongly attacks Ti. The resulting titanium fluoride creates significant residues. |
Table 2440c. Etch rates of materials using HF-related solutions.
Etched Materials |
Etchant |
Etch rates |
|
Al |
HF (49%) |
6.3 Å/s |
| HF (49%) : H2O = 1 : 10 |
5.3 Å/s |
LiNbO3 |
HNO3 : HF = 2 : 1 |
0.67 nm/s for Z- domains and 0 nm/s for Z+ domains at 49 °C |
Ni |
Hydrofluoric acid |
~3.3 nm/s at room temperature |
Pt |
40% HF or higher concentrations |
~ 0 Å/s |
c-Si, n+ poly Si, poly Si |
HNO3 : HF : H2O = 25 : 1 : 25 |
6 - 12 nm/s with c-Si < n+ poly Si < poly Si |
Undoped poly Si, n+ poly Si, c-Si (100) |
HNO3 : H2O : NH4F = 126 : 60 : 5 |
1.7 - 5.2 nm/s at 20 °C with undoped poly Si < n+ poly Si < c-Si (100) |
Si with n- and p-type dopant concentration below 1017atoms/cm3 |
HF:HNO3:CH3COOH = 1 : 3 : 8 |
113 - 400 Å/s |
Si with n- and p-type dopant concentration above 1018atoms/cm3 |
HF:HNO3:CH3COOH = 1 : 3 : 8 |
17 - 48 nm/s |
Undoped poly Si |
HF : NH4F = 1 : 5 |
0.03 Å/s at 20 °C |
n+ poly Si |
HF : NH4F = 1 : 5 |
0.15 Å/s at 20 °C |
| HF : H2O : H2O2 = 1: 20 : 1 |
0.2 Å/s at 20 °C |
Si (poly) |
HNO3 : H2O : HF |
~0.3 Å/s at room temperature |
Si |
HNO3 (69.5%) : HF (49.25%) = 7 : 3 |
2 µm/s at 25 °C |
| HNO3 (69.5%) : HF (49.25%) = 1: 2 |
~3.3 µm/s |
| HNO3 (69.5%) : HF (49.25%) = 21 : 4 |
103 - 230 nm/s at 25 °C |
| HNO3 (40%) + HF (60%) |
16.7 µm/s |
| HNO3 (50%) + HF (50%) |
3.3 µm/s |
| HNO3 (50%) + H2O (50%) |
167 µm/s |
| HNO3 : HF : CH3COOH = 75 : 8 : 17 |
83 nm/s at 25 °C |
| HNO3 : HF : CH3COOH = 5 : 3 : 3 |
0.83 - 1.25 µm/s at 25 °C |
| HNO3 : HF : CH3COOH = 40 : 1 : 15 |
2.5 nm/s for {111} and 3.3 nm/s for {100} at 25 °C |
| HNO3 : HF : CH3COOH = 3 : 1 : 10 |
50 nm/s at 25 °C, sensitive to type and density of dopants |
| HNO3 : HF : CH3COOH = 27 : 27 : 46 |
417 nm/s at 25 °C |
| HNO3 : HF : CH3COOH = 5: 3 : 3 |
~1.3 µm/s |
| HNO3 : H2O : NH4F = 24 : 12 : 1 |
2.5 nm/s, isotropic etching, photoresist-compatible |
| HNO3 : HF : CH3COOH = 15 : 2 : 5 |
83 nm/s, planarizing etch |
| HF:HNO3:CH3COOH = 1 : 3 : 10 |
8.3 Å/s with light n-substrate |
| HF : KMnO4 = 20 : 1 |
5 nm/s |
| HNO3 (66%) : HF (34%) |
0.83 µm/s |
| HF:CrO3 (1:1) |
58 nm/s |
| HF:KrCr3O3 = 2 : 1 |
25 nm/s with ultrasound agitation |
| HF:CrO5 = 2 : 1 |
8.3 nm/s with ultrasound agitation |
| HF (48%) : H2O = 1 : 4 |
0.008 Å/s at 25 °C |
| HF (48%) : NH4F (40%) = 1 : 7 |
0.007 Å/s at 25 °C |
| HNO3 : HF : H2O = 10 : 6 : 40 |
~0.33 µm/s |
c-Si |
HF (48%) |
0.005 Å/s at 25 °C |
a-Si |
0.012 Å/s at 25 °C |
Borosilicate SiO2 |
HF (49%) |
167 nm/s |
Thermal SiO2 |
HF (0.49%) |
0.33 Å/s |
TEOS |
HF (40%) or HF (0.49%) |
2.5 Å/s |
TEOS |
HF (0.49%) |
2.65 Å/s |
SiO2 |
NH4F (40%) : HF (49%) = 6 : 1 |
26.7 Å/s at room temperature |
| HF (1%) |
~ 1.7 nm/s |
Etch rate goes down linearly with decreasing HF concentration |
| HF (49%) |
> 16.7 nm/s |
| HF : H2O = 200 : 1 |
0.33 Å/s at 30 °C |
Etch rate goes down linearly with decreasing temperature and with decreasing HF concentration |
| HF : H2O = 200 : 1 |
0.18 Å/s at 22 °C |
| HF : H2O = 1 : 20 |
0.03 Å/s at 22 °C |
| HF : H2O = 3 : 20 |
0.17 Å/s at 22 °C |
| HF : H2O = 9 : 50 |
0.25 Å/s at 22 °C |
| HF : H2O = 27 : 100 |
0.37 Å/s at 22 °C |
| HF : H2O = 9 : 25 |
0.55 Å/s at 22 °C |
| HF : H2O = 11 : 20 |
0.83 Å/s at 22 °C |
| HF : H2O = 100 : 1 |
0.83 Å/s at 30 °C |
| HF : H2O = 100 : 1 |
0.5 Å/s at 22 °C |
| HF : H2O = 100 : 1 |
1.3 Å/s at 40 °C |
| CH3COOH : NH4F = 8:1 |
~0.28 Å/s at room temperature |
| HF : NH4F = 8:1 |
~0.17 Å/s at room temperature |
LPCVD Si3N4 |
HF (40%) |
3.3 Å/s at 25 °C (Selectivity of Si3N4 : SiO2 : Si = 1 : >100 : 0.1) |
Si3N4 |
HF (49%) |
0.67 Å/s |
| HF : H2O = 10 : 1 |
4.7 Å/s at 390 °C |
Etch rate goes down linearly with increasing temperature |
| HF : H2O = 10 : 1 |
7.3 Å/s at 360 °C |
| HF : H2O = 10 : 1 |
16.7 Å/s at 320 °C |
| HF : H2O = 10 : 1 |
35 Å/s at 285 °C |
| HF : H2O = 10 : 1 |
68 Å/s at 250 °C |
Ti |
HF (49%) : H2O2 (30%) : H2O = 1 : 1 : 20 |
15 nm/s at room temperature |
[1] Joachim Frühauf, Shape and Functional Elements of the Bulk Silicon Microtechnique: A Manual of Wet-Etched Silicon Structures, 2005.
