Hydrofluoric Acid (HF) Etching
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Table 2440a. Etchants used in semiconductor manufacturing.

Film
Etchant
SiO2(Silicon oxide)
Dilute hydrofluoric acid (DHF)
Buffered HF (BHF)
Polysilicon
Alkaline hydroxide + organic
Si3N4(silicon nitride)-selective to SiO2
Boiling phosphoric acid (H3PO4)
Si3N4/SiO2 (non-selective)
Hydrofluoric acid + organic

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
* CH3COOH: Acetic acid.
   Ammonium fluoride: NH4F.
   All Si etching here are isotropic etching.

 

HNO3 : HF : CH3COOH
system HNO3 : HF : H2O
(a)
(b)
Figure 2440. Etch rate [µm/min] of Si surfaces (a) in the system HNO3 : HF : H2O and (b) in the system HNO3 : HF : CH3COOH. [1]

 


 

 

 

 

[1] Joachim Frühauf, Shape and Functional Elements of the Bulk Silicon Microtechnique: A Manual of Wet-Etched Silicon Structures, 2005.

 

 

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