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'''Feedback to this page''': '''[mailto:labadviser@danchip.dtu.dk?Subject=Feed%20back%20from%20page%20http://labadviser.danchip.dtu.dk/index.php/Specific_Process_Knowledge/Etch/KOH_Etch click here]'''
'''Feedback to this page''': '''[mailto:labadviser@nanolab.dtu.dk?Subject=Feed%20back%20from%20page%20http://labadviser.nanolab.dtu.dk/index.php/Specific_Process_Knowledge/Etch/KOH_Etch click here]'''


=<span style="background:#FF2800">THIS PAGE IS UNDER CONSTRUCTION</span>[[image:Under_construction.png|200px]]=
'''Unless anything else is stated, everything on this page, text and pictures are made by DTU Nanolab.'''


'''All links to Kemibrug (SDS) and Labmanager Including APV and QC requires login.'''


[[Category: Equipment|Etch Wet KOH etch]]
[[Category: Etch (Wet) bath|KOH etch]]


==KOH etch - ''Anisotropic silicon etch''==
[[index.php?title=Category:Equipment|Etch Wet KOH etch]]
[[index.php?title=Category:Etch (Wet) bath|KOH etch]]
 
==Si etch - ''Anisotropic silicon etch''==


KOH belongs to the family of anisotropic Si-etchants based on aqueous alkaline solutions. The anisotropy stems from the different etch rates in different crystal directions. The {111}-planes are almost inert whereas the etch rates of e.g. {100}- and {110}-planes are several orders of magnitude faster.
KOH belongs to the family of anisotropic Si-etchants based on aqueous alkaline solutions. The anisotropy stems from the different etch rates in different crystal directions. The {111}-planes are almost inert whereas the etch rates of e.g. {100}- and {110}-planes are several orders of magnitude faster.


KOH-etching is a highly versatile and cheap way to realize micro mechanical structures if you can live with the necessary Si<sub>3</sub>N<sub>4</sub>- or SiO<sub>2</sub>-masking materials and the potassium contamination of the surface. '''The latter necessitates in most cases a wet post-clean ([[Specific Process Knowledge/Wafer cleaning/7-up & Piranha|'7-up']] or [[Specific Process Knowledge/Wafer cleaning/RCA|RCA-clean]]) if the wafer is to be processed further.'''
KOH-etching is a highly versatile and cheap way to realize micro mechanical structures if you can live with the necessary Si<sub>3</sub>N<sub>4</sub>- or SiO<sub>2</sub>-masking materials and the potassium contamination of the surface. '''The latter necessitates in most cases a wet post-clean ([[Specific Process Knowledge/Wafer cleaning/7-up & Piranha|'7-up']] or [[Specific Process Knowledge/Wafer cleaning/RCA|RCA-clean]]) if the wafer is to be processed further. we also recommend to rinse the wafers in a 5% HCL solution to remove metal ions from the KOH solution.'''  


At Danchip we use as a standard a 28 wt% KOH. The etch rate - and the selectivity towards a SiO<sub>2</sub>-mask - is depending on the temperature. We normally use T=80 <sup>o</sup>C but may choose to reduce this to e.g. 60 <sup>o</sup>C or 70 <sup>o</sup>C in case of a high-precision timed etch (e.g. defining a thin membrane). In some cases we recommend to saturate the standard 28 wt% KOH with IPA with an etch temperature at T=70 <sup>o</sup>C (reduce evaporation of IPA). One example is for boron etch-stop, where the selectivity towards the boron-doped silicon is improved compared to the standard etch. Etching with IPA added to the KOH solution can be done in KOH fumehood.
At DTU Nanolab we use as a standard a 28 wt% KOH. The etch rate - and the selectivity towards a SiO<sub>2</sub>-mask - is depending on the temperature. We normally use T=80 <sup>o</sup>C but may choose to reduce this to e.g. 60 <sup>o</sup>C or 70 <sup>o</sup>C in case of a high-precision timed etch (e.g. defining a thin membrane). In some cases we recommend to saturate the standard 28 wt% KOH with IPA with an etch temperature at T=70 <sup>o</sup>C (reduce evaporation of IPA). One example is for boron etch-stop, where the selectivity towards the boron-doped silicon is improved compared to the standard etch. Etching with IPA added to the KOH solution (250ml IPA/1000ml KOH) can be done in KOH fumehood.  
   
   
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<br clear="all" />
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http://labmanager.dtu.dk/function.php?module=Machine&view=view&page_id=169 KOH3 info page in LabManager],
[http://labmanager.dtu.dk/function.php?module=Machine&view=view&mach=376 Si Etch 1: KOH info page in LabManager],
[http://labmanager.danchip.dtu.dk/function.php?module=Machine&view=view&mach=49 KOH2 info page in LabManager],
   
   
[http://labmanager.dtu.dk/function.php?module=Machine&view=view&mach=390 Si Etch 2: KOH info page in LabManager],


[http://labmanager.danchip.dtu.dk/function.php?module=Machine&view=view&mach=197 Fumehood KOH info page in LabManager]
[http://labmanager.dtu.dk/function.php?module=Machine&view=view&mach=407 Si Etch 3: KOH info page in LabManager]


==Process Information==
==Process Information==
[[/V-grooves|Results on V-grooves]]
===Quality Control (QC) for the KOH etching baths.===
{| border="1" cellspacing="2" cellpadding="2" colspan="3"
|bgcolor="#98FB98" |'''Quality Control (QC) for KOH1, KOH2 and KOH3'''
|-
|
*[http://labmanager.danchip.dtu.dk/d4Show.php?id=3203&mach=9 The QC procedure for KOH1]<br>
*[http://labmanager.danchip.dtu.dk/d4Show.php?id=1565&mach=248 The QC procedure for KOH2 and KOH3]<br>
*[http://labmanager.danchip.dtu.dk/view_binary.php?type=data&mach=9 The newest QC data for KOH1]<br>
*[http://labmanager.danchip.dtu.dk/view_binary.php?type=data&mach=49 The newest QC data for KOH2]<br>
*[http://labmanager.danchip.dtu.dk/view_binary.php?type=data&mach=248 The newest QC data for KOH3]<br>
{| {{table}}
| align="center" |
{| border="1" cellspacing="1" cellpadding="2"  align="center" style="width:200px"
! QC Recipe:
! &nbsp;
|-
| Solution
|28 wt% KOH
|-
|Temperature
|80°C
|-
|Time
|90 min
|-
|Substrate
|Si (100)
|-|-
|Masking
|No masking
|-
|}
| align="center" valign="top"|
{| border="3" cellspacing="1" cellpadding="2" align="center" style="width:500px"
!QC limits
!KOH1
!KOH 2&3
|-
|Etch rate in Si(100)
|1.3 ± 0.1 µm/min
|1.29 ± 0.06 µm/min
|-
|Roughness
| not measured
| not measured
|-
|Nonuniformity
|< 3%
|< 3%
|-
|}
|-
|}
|}


<br clear="all" />
*[[/ProcessInfo#QC|QC info for standard KOH baths]]
*[[/ProcessInfo#Mixing KOH|How to mix KOH]]
*[[/ProcessInfo#Backside protection|Backside protection]]
*[[/ProcessInfo#Theory|Crystal orientation dependency]]


==KOH etching baths==
==KOH etching baths==
Key facts for the different etch baths available at Danchip are resumed in the table:
Key facts for the different etch baths available at DTU Nanolab are resumed in the table:
<br clear="all" />
<br clear="all" />


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!colspan="2" border="none" style="background:silver; color:black;" align="center"|Equipment  
!colspan="2" border="none" style="background:silver; color:black;" align="center"|Equipment  
|style="background:WhiteSmoke; color:black"|<b>KOH3</b>
|style="background:WhiteSmoke; color:black"|<b>Si Etch 01: KOH</b>
|style="background:WhiteSmoke; color:black"|<b>KOH2</b>
|style="background:WhiteSmoke; color:black"|<b>Si Etch 02: KOH</b>
|style="background:WhiteSmoke; color:black"|<b>KOH1</b>
|style="background:WhiteSmoke; color:black"|<b>Si Etch 03: KOH</b>
|style="background:WhiteSmoke; color:black"|<b>KOH Fumehood</b>
|-
|-
!style="background:silver; color:black;" align="center" width="60" rowspan="2"|Purpose  
!style="background:Silver; color:black;" align="center" width="60" rowspan="2"|Purpose  
|style="background:LightGrey; color:black"|Wet etch of Silicon
|style="background:LightGrey; color:black"|
|style="background:WhiteSmoke; color:black"|
*Etch of Silicon in 28 wt% KOH
*Etch of Silicon in 28 wt% KOH
|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
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|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
*Etch of Silicon in 28 wt% KOH  
*Etch of Silicon in 28 wt% KOH  
The bath is dedicated wafer with electroplated Nickel or otherwise dirty wafers
|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
*Etch of Silicon in user mixed KOH
*Etch of Silicon in 28 wt% KOH
The bath is dedicated wafers with metal or otherwise dirty wafers
|-
|-
|style="background:LightGrey; color:black"|Link to safety APV and KBA
|style="background:LightGrey; color:black"|Link to safety APV and SDS
|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
*:[http://labmanager.danchip.dtu.dk/d4Show.php?id=1906&mach=248 see APV here]
*:[http://labmanager.danchip.dtu.dk/d4Show.php?id=4964&mach=376 see APV here]
*:[http://kemibrug.dk/KBA/CAS/106882/?show_KBA=1&portaldesign=1 see KBA here]
*:[http://kemibrug.dk/KBA/CAS/106882/?show_KBA=1&portaldesign=1 see SDS here]
|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
*:[http://labmanager.danchip.dtu.dk/d4Show.php?id=1906&mach=49 see APV here]
*:[http://labmanager.danchip.dtu.dk/d4Show.php?id=4964&mach=376 see APV here]
*:[http://kemibrug.dk/KBA/CAS/106882/?show_KBA=1&portaldesign=1 see KBA here]
*:[http://kemibrug.dk/KBA/CAS/106882/?show_KBA=1&portaldesign=1 see SDS here]
|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
*:[http://labmanager.danchip.dtu.dk/d4Show.php?id=1906&mach=49 see APV here]
*:[http://labmanager.danchip.dtu.dk/d4Show.php?id=4897&mach=407 see APV here]
*:[http://kemibrug.dk/KBA/CAS/106882/?show_KBA=1&portaldesign=1 see KBA here]
*:[http://kemibrug.dk/KBA/CAS/106882/?show_KBA=1&portaldesign=1 see SDS here]
|style="background:WhiteSmoke; color:black"|
*:[http://labmanager.danchip.dtu.dk/d4Show.php?id=2479&mach=197 see APV here]
*:[http://kemibrug.dk/KBA/CAS/106882/?show_KBA=1&portaldesign=1 see KBA here]
|-
|-
!style="background:silver; color:black" align="center" valign="center" rowspan="5"|Performance
!style="background:silver; color:black" align="center" valign="center" rowspan="7"|Performance
|style="background:LightGrey; color:black"|Etch rates in crystalline silicon (100)
|style="background:LightGrey; color:black"|Etch rates in crystalline silicon (100)
|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
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|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
*0.4 µm/min (60 °C)
*0.4 µm/min (60 °C)
*0.7 µm/min (70 °C)
*1.3 µm/min (80 °C)
*1.3 µm/min (80 °C)
Etch rates might vary due to contamination and poor controlled concentration of the KOH solution
|-
|style="background:LightGrey; color:black"|Etch rates in crystalline silicon (110)
|style="background:WhiteSmoke; color:black"|
*2.5 µm/min (80 °C)
|style="background:WhiteSmoke; color:black"|
*2.5 µm/min (80 °C)
|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
*Somewhat lower than in the dedicated baths. Approximately 1 µm/min @ 80 °C in 28 wt%
*2.5 µm/min (80 °C)
|-
|-
|style="background:LightGrey; color:black"|Etch rates in Thermal SiO2  
|style="background:LightGrey; color:black"|Etch rates in Thermal SiO2  
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*Theoretical values:
*Theoretical values:
*1.2 nm/min (60 °C)  
*1.2 nm/min (60 °C)  
*6 nm/min (80 °C)
*7.5 nm/min (80 °C)
|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
*Theoretical values:
*Theoretical values:
*1.2 nm/min (60 °C)  
*1.2 nm/min (60 °C)  
*6 nm/min (80 °C)
*7.5 nm/min (80 °C)
|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
*Theoretical values:
*1.2 nm/min (60 °C)
*7.5 nm/min (80 °C)
|-
|-
|style="background:LightGrey; color:black"|Etch rates in SiN
|style="background:LightGrey; color:black"|Etch rates in other oxides
|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
*0.02 nm/min (80 °C)<sup>{{fn|1}}</sup>
.
|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
yannickseis@nbi.ku nov. 2017 @80 °C:
*BPSG from PECVD4: 311nm in about 3 min
*Waveguide oxide from PECVD4: 320nm etched in 26 min
*TEOS oxide from furnace: 300nm etched in 11 min
jemafh@nilt 2019-Marts:
*Standard from PECVD3: selectivity 1:100 to Si
|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
.
|-
|style="background:LightGrey; color:black"|Etch rates in PECVD SiN
|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|See etchrates for PECVD SiN [https://labadviser.nanolab.dtu.dk/index.php?title=Specific_Process_Knowledge/Thin_film_deposition/Deposition_of_Silicon_Nitride/Deposition_of_Silicon_Nitride_using_PECVD/PECVD3:_Low_stress_nitride_testing#DOE_made_to_find_a_good_QC_nitride_recipe_with_low_stress_and_low_KOH_etch_rate_(by_Berit_Herstrøm_@_DTU_Nanolab_2016_Marts) here]
|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
|-
|-
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|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
*May be high due to contamination and poor controlled concentration of the KOH solution
*May be high due to contamination and poor controlled concentration of the KOH solution
|style="background:WhiteSmoke; color:black"|
*Typical worse than KOH2 and KOH3
|-
|-
|style="background:LightGrey; color:black"|Anisotropy
|style="background:LightGrey; color:black"|Anisotropy
|style="background:WhiteSmoke; color:black"|
*The etch rate is very dependent on the crystal orientation of the silicon.
|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
*The etch rate is very dependent on the crystal orientation of the silicon.
*The etch rate is very dependent on the crystal orientation of the silicon.
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|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
*Mixing ratios giving 28 wt% KOH solutions
*Mixing ratios giving 28 wt% KOH solutions
KOH:H<sub>2</sub>O - 500 g : 1000 ml, when using pills
KOH:H<sub>2</sub>O - 1000 ml: 1200 ml, when using premixed 50% KOH solution
KOH:H<sub>2</sub>O - 1000 ml: 1200 ml, when using premixed 50% KOH solution
|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
*Mixing ratios giving 28 wt% KOH solutions
*Mixing ratios giving 28 wt% KOH solutions
KOH:H<sub>2</sub>O - 500 g : 1000 ml, when using pills
KOH:H<sub>2</sub>O - 1000 ml: 1200 ml, when using premixed 50% KOH solution
KOH:H<sub>2</sub>O - 1000 ml: 1200 ml, when using premixed 50% KOH solution
|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
*Mixing ratios giving 28 wt% KOH solutions
*Mixing ratios giving 28 wt% KOH solutions
KOH:H<sub>2</sub>O - 500 g : 1000 ml, when using pills
KOH:H<sub>2</sub>O - 1000 ml: 1200 ml, when using premixed 50% KOH solution
KOH:H<sub>2</sub>O - 1000 ml: 1200 ml, when using premixed 50% KOH solution
|style="background:WhiteSmoke; color:black"|
*Custom made
|-
|-
|style="background:LightGrey; color:black"|Temperature
|style="background:LightGrey; color:black"|Temperature
|style="background:WhiteSmoke; color:black"|
*Max 80 °C (standard etch)
|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
*Max 80 °C (standard etch)
*Max 80 °C (standard etch)
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!style="background:silver; color:black" align="center" valign="center" rowspan="4"|Substrates
!style="background:silver; color:black" align="center" valign="center" rowspan="4"|Substrates
|style="background:LightGrey; color:black"|Batch size
|style="background:LightGrey; color:black"|Batch size
|style="background:WhiteSmoke; color:black"|
*1-25 wafers at a time
|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
*1-25 wafers at a time  
*1-25 wafers at a time  
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|style="background:LightGrey; color:black"|Size of substrate
|style="background:LightGrey; color:black"|Size of substrate
|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
*4” wafers
*4”-6" wafers
*6” wafers
|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
*4” wafers
*4”-6" wafers
|style="background:WhiteSmoke; color:black"|
*4” wafers
|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
*2” wafers
*2” wafers
*4” wafers
*4” wafers
*6” wafers
*Small pieces
*Small pieces
|-
|-
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*Silicon oxide  
*Silicon oxide  
*Silicon (oxy)nitride
*Silicon (oxy)nitride
|style="background:WhiteSmoke; color:black"|
*Silicon
*Silicon oxide
*Silicon (oxy)nitride
*Nickel
*Nickel vanadium alloys
*Titanium
*Chromium
*Gold
|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
*All except for Polymers
*All except for Polymers
|-
|-
|style="background:LightGrey; color:black"|Masking material
|style="background:LightGrey; color:black"|Masking material
|style="background:WhiteSmoke; color:black"|
*Stoichiometric Si3N4
*Silicon rich nitride SiN
*PECVD Si3N4
*Thermal SiO2
|style="background:WhiteSmoke; color:black"|
|style="background:WhiteSmoke; color:black"|
*Stoichiometric Si3N4  
*Stoichiometric Si3N4  
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<sup>{{fn|1}}</sup> Measured by Eric Jensen from DTU-Nanotech, October 2013.
<sup>{{fn|1}}</sup> Measured by Eric Jensen from DTU-Nanotech, October 2013.
<br clear="all" />
<br clear="all" />
===Definition of structures===
Due to the almost inert (111)-planes it is possible by KOH etching to realize high aspect ratio structures in sigle crytalline silicon using the (111)-planes as sidewalls. In Si(100) these sidewalls are inclined - 54.7<sup>o</sup> with respect to the (100) surface - whereas in Si(110) the sidewalls are vertical (see figures below).
<gallery caption="Anisotropic wet silicon etch: dependency on crystal orientation" widths="380px" heights="150px" perrow="2">
Image:KOH_Anisotropy.jpg|Etched profile when etching Si(100).
Image:KOH_Anisotropy(110).jpg|Etched profile when etching Si(110).
</gallery>
<br clear="all"/>
For Si(100), the relation between the width of the bottom of the etched groove (W<sub>b</sub>) and the width of the opening (W<sub>o</sub>) at the wafer surface in a groove etched to the depth l is given by:
<math>W_b = W_o - 2lcot(54.7^o) = W_o - \sqrt{2} l</math>
<br clear="all" />
===Definition of <110> alignment structures===
The etch rate dependence on the crystallographic planes can be used to determine the <110> crystal directions with high precision (better than +/- 0.05 <sup>o</sup>). A fast method for doing this, using the symmetric under-etching behavior around but not at the <110>-directions, was described by Vangbo and Bäcklund in J. Micromech. Microeng.'''6''' (1996), 279-284. High-precision control of the <110>-direction during alignment can be necessary in order to control the dimensions of KOH-etched structures (e.g. precise control of V-groove dimensions). A dedicated mask (MASK NAME) has been designed for this purpose.
<br clear="all" />
===Etch rates: Empirical formula (Seidl et al)===
The following empirical formula can be used for concentrations in the range of 10-60 wt%:
R = k<sub>0</sub> [H<sub>2</sub>O]<sup>4</sup> [KOH]<sup>0.25</sup> e<sup>-E<sub>a</sub>/kT</sup>,
where k<sub>0</sub> = 2480 µm/hr (mol/l)<sup>-4.25</sup>, E<sub>a</sub> = 0.595 eV for Si(100)
and  k<sub>0</sub> = 4500 µm/hr (mol/l)<sup>-4.25</sup>, E<sub>a</sub> = 0.60 eV for Si(110)
Retrieved from "https://labadviser.nanolab.dtu.dk//index.php?title=Specific_Process_Knowledge/Etch/KOH_Etch"