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==UV resist comparison table==
Comparison of specifications and feature space of the standard UV photoresists available at DTU Nanolab.
'''This section is under construction [[Image:section under construction.jpg|70px]]'''
{| class="wikitable"
= UV resist comparison table =
Comparison of specifications and feature space of UV photoresists.
[[Image:AZ photoresists spectral sensitivity - remake v1.png|400x400px|thumb|Spectral sensitivity of AZ resists represented as optical absorption.]]
During exposure of the resist, the photoinitiator, or photo-active component, reacts with the exposure light, and starts the reaction that makes the resist develop in the developer.
In a positive resist, it makes the resist become soluble in the developer. In a negative resist, usually assisted by thermal energy in the post-exposure bake, it makes the resist ''in''soluble in the developer. The amount of light required to fully develop the resist in the development process, is the exposure dose.
The optimal exposure dose is a function of many parameters, including the type of resist, the resist thickness, and the sensitivity of the resist.
'''Resist sensitivity'''<br>
The resist sensitivity is a measure of how efficiently it reacts to the exposure light. Spectral sensitivity is the sensitivity of the resist as a function of wavelength. It is usually given simply as the range from the wavelength below which absorption in the resist material makes lithography impractical to the wavelength at which the photoinitiator is no longer efficiently activated.
Within the sensitivity range, the optical absorption is commonly used as a measure of sensitivity. A high absorption coefficient signifies a high sensitivity, as the light is absorbed by the photoinitiator. Because of spectral sensitivity, the optimal dose of a given resist type and thickness is also a function of the spectral distribution of the exposure light, i.e. the equipment used for the exposure. Using a combination of experience, calculation and assumptions, it may be possible to estimate the dose for an exposure equipment, if the exposure dose is already known on another equipment.
Due to reflection and refraction at the interface between the resist and the substrate, the optimal dose may also be a function of the type of substrate used. Unless otherwise stated, the exposure doses given below are for standard silicon wafers.
Apart from the already mentioned factors, the optimal dose also depends on the developer chemistry and the parameters used in the development process. Finally, the requirements to the lithographic process in terms of resolution, bias (line broadening), etch selectivity, side wall angle, etc. may narrow down, or widen, the process window. The exposure doses given in the sections below should be used as a starting point for individual fabrication process development.
Manual spin coaters:
* Spin coater: Labspin 02
* Spin coater: Labspin 03
* Spin coater: RCD8
Due to the process of bleaching, where the absorption of the resist changes during exposure, the exposure dose is unfortunately not always constant at different intensities of the exposure light. The exposure time is thus not always a linear function of the exposure intensity.
Spray coater
|
Automatic spin coaters:
*Spin coater: Gamma E-beam & UV
= Calculate exposure time =
Manual spin coaters:
In the maskless aligners, the dose is set directly as a process parameter in the job. In mask aligners, on the other hand, the parameter that is set is the exposure time, i.e. how long the shutter is open during the exposure.
* Spin coater: Labspin 02
* Spin coater: Labspin 03
The exposure dose, ''D'' [J/m<sup>2</sup>], is given by:
* Spin coater: RCD8
|
''D'' = ''I'' x ''t'' ,
Manual spin coaters:
* Spin coater: Labspin 02
where ''I'' [W/m<sup>2</sup>] is the intensity of the exposure light, and ''t'' [s] is the exposure time. As the intensity is specific to the spectral sensitivity of the sensor used to measure the exposure light, and the exposure time is specific to the spectral distribution of the exposure light (cf. spectral sensitivity), this dose is specific to the combination of exposure source and optical sensor.
* Spin coater: Labspin 03
* Spin coater: RCD8
Given an exposure dose, the exposure time, ''t'', is calculated as:
|
Spray coater
''t'' = ''D'' / ''I''
It is important to keep in mind that this exposure time is valid only for a specific combination of exposure source and optical sensor, as well as for a specific development process.
=Exposure dose for mask aligners=
The exposure doses listed below are for ''generic'' good exposure results, and can be a compromise between getting good lines, as well as good dots, in both clear field and dark field exposures. The optimal dose for any given specific project, could be different from the listed values.
All doses are for standard silicon wafers, unless otherwise stated. Development is done using 2.38% TMAH.
==KS Aligner==
The KS Aligner has an i-line notch filter installed. This results in an exposure light peak around 365 nm with a FWHM of 7 nm.
* [[Specific_Process_Knowledge/Lithography/nLOF#Development|Solvent development possible]]
|
AZ 726 MIF (2.38% TMAH)
|
mr-DEV 600 (PGMEA)
|
AZ 726 MIF (2.38% TMAH)
|-
|-
|-style="background:WhiteSmoke; color:black"
! scope=row| Development rinse agent
!rowspan="3"|AZ 5214E<br><span style="color:red">Old German version</span>
| DIW || DIW || DIW || DIW || IPA || DIW
|Long ago
|1.5 µm
|72 mJ/cm<sup>2</sup>
|rowspan="3"|Single puddle, 60 s
|rowspan="3"|Positive process
|-style="background:WhiteSmoke; color:black"
|Long ago
|2.2 µm
|80 mJ/cm<sup>2</sup>
|-style="background:WhiteSmoke; color:black"
|Long ago
|4.2 µm
|160 mJ/cm<sup>2</sup>
|-
|-
! scope=row| Remover
|-
|
|-style="background:LightGrey; color:black"
* Acetone
!AZ 4562<br><span style="color:red">Old German version</span>
* Remover 1165 (NMP)
|Long ago
|
|10 µm
* Acetone
|510 mJ/cm<sup>2</sup>
* Remover 1165 (NMP)
|Multiple puddle, 4 x 60 s
|
|Multiple exposure with 10-15 s pauses is recommended.
* Acetone
* Remover 1165 (NMP)
|
Remover 1165 (NMP)
|
* Cross-linked SU-8 is practically insoluble
* Oxygen plasma ashing can remove cross-linked SU-8
|
* Acetone
* Remover 1165 (NMP)
|-
|-
! scope=row| Comments
| Good adhesion for wet etch
| High selectivity for dry etch
| Negative sidewalls for lift-off
| For processes with resist thickness between 6 µm and 25 µm
|
* High aspect ratio
* Resist thickness 1 µm to hundreds of µm
* Available in cleanroom: 2005, 2035, and 2075. Considering discontinuation of the SU-8 2000 series we need to move to another product SU8 3000 and SU8 XTF series.
* New formulation will be available in cleanroom and tested: 3005 instead of 2005, 3035 instead of 2035, XTF75 instead of 2075 and new 3025.
| TI spray resist is an image reversal resist, similar to AZ 5214E. The process flow will be similar to the process flows for 5214, except for the coating step. The exposure dose and development will depend on the specific process.
|}
|-
<br clear="all" />
|-style="background:WhiteSmoke; color:black"
!rowspan="3"|AZ MiR 701<br><span style="color:red">Old PFOA containing version</span>
|Long ago
|1 µm
|180 mJ/cm<sup>2</sup>
|Single puddle, 60 s
|rowspan="2"|PEB: 60 s at 110°C
|-style="background:WhiteSmoke; color:black"
|Long ago
|2 µm
|200 mJ/cm<sup>2</sup>
|Single puddle, 60 s
|-style="background:WhiteSmoke; color:black"
|Long ago
|4 µm
|400 mJ/cm<sup>2</sup>
|Single puddle, 60 s
|rowspan="1"|PEB: 90 s at 110°C
|-
|-
|-style="background:LightGrey; color:black"
!AZ nLOF 2020
|Long ago
|2 µm
|110 mJ/cm<sup>2</sup>
|Single puddle, 60 s
|PEB: 60 s at 110°C
Side wall angle ~15°
For smaller angle (~5°), develop 30 seconds instead
|}
==Aligner: MA6 - 2==
==Process flow examples==
The Aligner: MA6-2 has an i-line notch filter installed. This results in an exposure light peak around 365 nm with a FWHM of 7 nm.
Comparison of specifications and feature space of the standard UV photoresists available at DTU Nanolab. These are just examples and may contain obsolete information regarding exposure dose, etc.
NB! Most of the process knowledge about SU-8 is based in research groups
|
|-
|-
|-style="background:WhiteSmoke; color:black"
! scope=row| Mask aligner
!rowspan="2"|AZ 5214E<br><span style="color:red">Old German version</span>
|
|Long ago
* [[media:process_flow_AZ_5214E_positive_MA6_-_2023-02.docx |Process flow AZ 5214E positive MA6]]
|1.5 µm
* [[media:process_flow_AZ_5214E_IR_MA6_-_2023-02.docx |Process flow AZ 5214E image reversal MA6]]
|72 mJ/cm<sup>2</sup>
|
|Single puddle, 60 s
[[media:process_flow_AZ_MiR_701_MA6_-_2023-02.docx|Process flow AZ MiR 701 MA6]]
|rowspan="2"|Positive process
|-style="background:WhiteSmoke; color:black"
|Long ago
|2.2 µm
|90 mJ/cm<sup>2</sup>
|Single puddle, 60 s
|-
|-
|-style="background:LightGrey; color:black"
!rowspan="2"|AZ 5214E Image Reversal<br><span style="color:red">Old German version</span>
|Long ago
|1.5 µm
|22 mJ/cm<sup>2</sup>
|Single puddle, 60 s
|rowspan="2"|Image reversal process.<br>Reversal bake: 120 s at 110°C.<br>Flood exposure: 200 mJ/cm<sup>2</sup>
|-style="background:LightGrey; color:black"
|Long ago
|2.2 µm
|25 mJ/cm<sup>2</sup>
|Single puddle, 60 s
|-
|-
|-style="background:WhiteSmoke; color:black"
!rowspan="3"|AZ MiR 701<br><span style="color:red">Old PFOA containing version</span>
|Long ago
|1.5 µm
|169 mJ/cm<sup>2</sup>
|Single puddle, 60 s
|rowspan="2"|PEB: 60 s at 110°C
|-style="background:WhiteSmoke; color:black"
|Long ago
|2 µm
|200 mJ/cm<sup>2</sup>
|Single puddle, 60 s
|-style="background:WhiteSmoke; color:black"
|Long ago
|4 µm
|280 mJ/cm<sup>2</sup>
|Single puddle, 60 s
|rowspan="1"|PEB: 60 s at 110°C<br>Process adopted from process logs
|-
|-
|-style="background:LightGrey; color:black"
!AZ nLOF 2020
|Long ago
|1.5 µm
|104 mJ/cm<sup>2</sup>
|Single puddle, 30 s
|PEB: 60 s at 110°C<br>Use 60 s development for lift-off
|-
|-
|-style="background:WhiteSmoke; color:black"
!AZ 5214E<br><span style="color:green">New Japanese version</span>
|2023-01-11<br>jehem
|1.5 µm
|70 mJ/cm<sup>2</sup>
|Single puddle, 60 s
|
|
|-
[[media:process_flow_AZ_nLOF_2020_MA6_-_2023-02.docx|Process flow AZ nLOF 2020 MA6]]
|-
|-style="background:LightGrey; color:black"
!AZ 5214E Image Reversal<br><span style="color:green">New Japanese version</span>
|2023-01-11<br>jehem
|2.2 µm
|22 mJ/cm<sup>2</sup>
|Single puddle, 60 s
|Image reversal process.<br>Reversal bake: 60 s at 110°C.<br>Flood exposure: 500 mJ/cm<sup>2</sup>
|-
|-
|-style="background:WhiteSmoke; color:black"
!AZ 4562<br><span style="color:green">New Japanese version</span>
|2021-12-08<br>jehem
|10 µm
|550 mJ/cm<sup>2</sup>
|Multiple puddles, 5 x 60 s
|Priming: HMDS<BR>Rehydration after SB: 1 hour (may not be necessary)<br>Exposure: Multiple exposures with pauses, 5 x (10 s exposure + 10 s pause)<br>Degassing after exposure: 1 hour (may not be necessary)
|-
|-
|-style="background:LightGrey; color:black"
!AZ MiR 701<br><span style="color:green">New PFOA free version</span>
|2021-06-23<br>elkh
|1.5 µm
|150 mJ/cm<sup>2</sup>
|Single puddle, 60 s
|PEB: 60 s at 110°C
|-
|}
=Exposure dose for maskless aligners=
The exposure doses listed below are for ''generic'' good exposure results, and can be a compromise between getting good lines, as well as good dots, in both clear field and dark field exposures. The optimal dose for any given specific project, could be different from the listed values.
All doses are for standard silicon wafers, unless otherwise stated. Development is done using 2.38% TMAH.
==Aligner: Maskless 01==
The Aligner: Maskless 01 has a 365 nm LED light source with a FWHM of 8 nm.
!AZ MiR 701<br><span style="color:green">New PFOA free version</span>
| 2021-08-25<br>jehem
| 1.5 µm
| Fast
| 225 mJ/cm<sup>2</sup>
| -4
| 1.25 µm (due to stitching)<br>Tested using dehydration reducing measures
| PEB: 60s@110°C, Dev: SP60s
|-
|-
|-style="background:LightGrey; color:black"
!AZ 4562<br><span style="color:green">New Japanese version</span>
| 2021-12-07<br>jehem
| 10 µm
| Fast
| 750 mJ/cm<sup>2</sup>
| 0
| ≤5 µm
| Priming: HMDS<BR>Rehydration after SB: 1 hour (may not be necessary)<br>Exposure: Multiple exposures with pauses, 5 x (10 s exposure + 10 s pause)<br>Degassing after exposure: 1 hour (may not be necessary)<br>Development: Multiple puddles, 5 x 60 s
|-
|}
<br>
'''Dehydration reducing measures used for testing AZ MiR 701:'''<br>
The CDA used for the pneumatic autofocus will dehydrate the resist. To reduce this effect, the writehead is parked far away from the write area while setting up the job for at least a few minutes, before starting the exposure.
==Aligner: Maskless 02==
The Aligner: Maskless 01 has a 325 nm laser light source, and a 405 nm laser light source, each with a FWHM of ~1 nm.
!AZ MiR 701<br><span style="color:green">New PFOA free version</span>
| 2021-08-20<br>jehem
| 1.5 µm
| 375
| Optical
| Fast
| 200 mJ/cm<sup>2</sup>
| -6
| 1 µm<br>Tested using dehydration reducing measures
| PEB: 60s@110°C, Dev: SP60s
|-
|-
|-style="background:LightGrey; color:black"
!AZ 4562<br><span style="color:green">New Japanese version</span>
| 2021-12-08<br>jehem
| 10 µm
| 375
| Optical
| Fast
| 750 mJ/cm<sup>2</sup>
| 0
| ≤5 µm
| Priming: HMDS<BR>Rehydration after SB: 1 hour (may not be necessary)<br>Exposure: Multiple exposures with pauses, 5 x (10 s exposure + 10 s pause)<br>Degassing after exposure: 1 hour (may not be necessary)<br>Development: Multiple puddles, 5 x 60 s
|-
|}
|}
<br>
'''Dehydration reducing measures used for testing AZ MiR 701:'''<br>
The CDA used for the pneumatic autofocus will dehydrate the resist. To reduce this effect, the writehead is parked far away from the write area while setting up the job for at least a few minutes, before starting the exposure.
==Aligner: Maskless 03==
'''Other process flows:'''
[[Media:process_flow_chip_on_carrier_-_2023-02.docx|Chip on carrier]]: A procedure for UV lithography on a chip using automatic coater and developer.
[[Image:resistSensitivity_UV.png|400px|thumb|Spectral sensitivity of AZ resists represented as optical absorption coefficient.]]
|
!Date
!Thickness
!Exposure mode
!Dose
!Defoc
!Resolution
!Comments
|-
|-
During exposure of the resist, the photoinitiator, or photo-active component, reacts with the exposure light, and starts the reaction which makes the resist develop in the developer.
|-style="background:WhiteSmoke; color:black"
!rowspan="2"| AZ 5214E<br><span style="color:red">Old German version</span>
| 2020-10-01<br>jehem
|rowspan="2"| 1.5 µm
| Fast
| 70 mJ/cm<sup>2</sup>
| 0
| 2 µm (due to stitching)
| Dev: SP60s
|-style="background:WhiteSmoke; color:black"
| 2021-03-23<br>jehem
| Quality
| 65 mJ/cm<sup>2</sup>
| -2
| 1 µm
| Dev: SP60s
|-
|-
In a positive resist, it makes the resist become soluble in the developer. In a negative resist, usually assisted by thermal energy in the post-exposure bake (PEB), it makes the resist ''in''soluble in the developer. The amount of light required to fully develop the resist in the development process, is the exposure dose.
|-style="background:WhiteSmoke; color:black"
!rowspan="2"| AZ MiR 701<br><span style="color:red">Old PFOA containing version</span>
| 2020-02-01<br>jehem
|rowspan="2"| 1.5 µm
| Fast
| 300 mJ/cm<sup>2</sup>
| 0
| 2 µm (due to stitching)<br>Non optimized dehydration reduction
| PEB: 60s@110°C, Dev: SP60s
|-style="background:WhiteSmoke; color:black"
| 2021-08-25<br>jehem
| Quality
| 175 mJ/cm<sup>2</sup>
| -2
| 1 µm<br>Tested using dehydration reducing measures
| PEB: 60s@110°C, Dev: SP60s
|-
|-
The optimal exposure dose is a function of many parameters, including the type of resist, the resist thickness, and the sensitivity of the resist.
|-style="background:WhiteSmoke; color:black"
!rowspan="2"| AZ 5214E image reversal<br><span style="color:red">Old German version</span>
The resist sensitivity is a measure of how efficiently it reacts to the exposure light. Spectral sensitivity is the sensitivity of the resist as a function of wavelength. It is usually given simply as the range from the wavelength below which absorption in the resist material makes lithography impractical to the wavelength at which the photoinitiator is no longer efficiently activated.
Within the sensitivity range, the optical absorption is commonly used as a measure of sensitivity. A high absorption coefficient signifies a high sensitivity, as the light is absorbed by the photoinitiator. Because of spectral sensitivity, the optimal dose of a given resist type and thickness is also a function of the spectral distribution of the exposure light, i.e. the equipment used for the exposure. Using a combination of experience, calculation and assumptions, it may be possible to estimate the dose for an exposure equipment, if the exposure dose is already known on another equipment.
'''AZ nLOF 2020''' has also been tested. It seems to work even at 405nm, but the dose is so high (>10000) that exposure would be slower than Aligner: Maskless 02.
Due to reflection and refraction at the interface between the resist and the substrate, the optimal dose may also be a function of the type of substrate used. Unless otherwise stated, the exposure doses given below are for standard silicon wafers.
Apart from the already mentioned factors, the optimal dose also depends on the developer chemistry and the parameters used in the development process. Finally, the requirements to the lithographic process in terms of resolution, bias (line broadening), etch selectivity, side wall angle, etc. may narrow down, or widen, the process window. The exposure doses given in the sections below should be used as a starting point for individual fabrication process development.
|-
|-
Due to the process of bleaching, where the absorption of the resist changes during exposure, the exposure dose is unfortunately not always constant at different intensities of the exposure light. The exposure time is thus not always a linear function of the exposure intensity.
|-style="background:silver; color:black"
|
!Date
!Thickness
!Laser
!Autofocus
!Exposure mode
!Dose
!Defoc
!Resolution
!Comments
|-
|-
==Calculate exposure time==
|-style="background:WhiteSmoke; color:black"
In the maskless aligners, the dose is set directly as a process parameter in the job. In mask aligners, on the other hand, the parameter that is set is the exposure time, i.e. how long the shutter is open during the exposure.
!AZ 5214E<br><span style="color:green">New Japanese version</span>
| 2023-01-06<br>taran
| 1.5 µm
| 405
| Pneumatic
| Quality
| 70 mJ/cm<sup>2</sup>
| -2
| 1 µm
| Dev: SP60s
|-style="background:WhiteSmoke; color:black"
|-
|-
The exposure dose, ''D'' [J/m<sup>2</sup>], in terms if exposure light intensity ''I'' [W/m<sup>2</sup>] and exposure time ''t'' [s], is given by:
|-style="background:LightGrey; color:black"
!AZ 5214E image reversal<br><span style="color:green">New Japanese version</span>
!AZ MiR 701<br><span style="color:green">New PFOA free version</span>
| 2021-08-25<br>jehem
| 1.5 µm
| 405
| Pneumatic
| Quality
| 175 mJ/cm<sup>2</sup>
| -2
| 1 µm<br>Tested using dehydration reducing measures
| PEB: 60s@110°C, Dev: SP60s
|-
|-
Since the intensity is specific to the spectral sensitivity of the sensor used to measure the exposure light, and the exposure time is specific to the spectral distribution of the exposure light (cf. spectral sensitivity), this dose is specific to the combination of exposure source and optical sensor.
|-style="background:LightGrey; color:black"
!AZ 4562<br><span style="color:green">New Japanese version</span>
| 2021-12-08<br>jehem
| 10 µm
| 405
| Pneumatic
| Quality
| 550 mJ/cm<sup>2</sup>
| 1
| ≤5 µm
| Priming: HMDS<BR>Rehydration after SB: 1 hour (may not be necessary)<br>Exposure: Multiple exposures with pauses, 5 x (10 s exposure + 10 s pause)<br>Degassing after exposure: 1 hour (may not be necessary)<br>Development: Multiple puddles, 5 x 60 s
|-
|}
Given an exposure dose, the exposure time, ''t'', is calculated as:
<br>
'''Dehydration reducing measures used for testing AZ MiR 701:'''<br>
The CDA used for the pneumatic autofocus will dehydrate the resist. To reduce this effect, the writehead is parked far away from the write area while setting up the job for at least a few minutes, before starting the exposure.
=Exposure dose when using AZ 351B developer (NaOH)=
<math>t = D \sdot I^{-1}</math>
The exposure doses listed below are for ''generic'' good exposure results, and can be a compromise between getting good lines, as well as good dots, in both clear field and dark field exposures. The optimal dose for any given specific project, could be different from the listed values.
All doses are for standard silicon wafers, unless otherwise stated. Development is done using AZ 351B developer (NaOH).
It is important to keep in mind that this exposure time is valid ''only'' for a specific combination of exposure source and optical sensor, as well as for a specific development process.
==KS Aligner (351B)==
==Exposure dose for mask aligners==
The KS Aligner has an i-line notch filter installed. This results in an exposure light peak around 365 nm with a FWHM of 7 nm.
[[Specific Process Knowledge/Lithography/Resist/UVresist/exposureDoseMaskAligners|Information about the exposure dose for mask aligners can be found here.]]
[[Specific Process Knowledge/Lithography/Resist/UVresist/exposureDoseMasklessAligners|Information about the exposure dose for maskless aligners can be found here.]]
|-
==Exposure dose when using AZ 351B developer (NaOH)==
|-style="background:silver; color:black"
[[Specific Process Knowledge/Lithography/Resist/UVresist/exposureDoseNaOH|Information about the exposure dose when using AZ 351B developer can be found here.]]
|
!Date
!Thickness
!Dose
!Development]
!Comments
|-
|-
|-style="background:WhiteSmoke; color:black"
!rowspan="3"|AZ 5214E
|Long ago
|1.5 µm
|70mJ/cm<sup>2</sup>
|60 s
|rowspan="3"|Positive process
|-style="background:WhiteSmoke; color:black"
|Long ago
|2.2 µm
|72 mJ/cm<sup>2</sup>
|70 s
|-style="background:WhiteSmoke; color:black"
|Long ago
|4.2 µm
|160 mJ/cm<sup>2</sup>
|180 s
|-
|-
|-style="background:LightGrey; color:black"
!rowspan="2"|AZ 5214E
|Long ago
|1.5 µm
|30 mJ/cm<sup>2</sup>
|60 s
|rowspan="2"|Image reversal process.
Reversal bake: 100s at 110°C.<br>Flood exposure after reversal bake: 210 mJ/cm<sup>2</sup>
|-style="background:LightGrey; color:black"
|Long ago
|2.2 µm
|35 mJ/cm<sup>2</sup>
|70 s
|-
|-
|-style="background:WhiteSmoke; color:black"
!AZ 4562
|Long ago
|10 µm
|320 mJ/cm<sup>2</sup>
|5 minutes
|Multiple exposure with 10-15 s pauses is recommended.
|-
|}
Latest revision as of 10:45, 12 January 2026
UV resist comparison table
Comparison of specifications and feature space of the standard UV photoresists available at DTU Nanolab.
For processes with resist thickness between 6 µm and 25 µm
High aspect ratio
Resist thickness 1 µm to hundreds of µm
Available in cleanroom: 2005, 2035, and 2075. Considering discontinuation of the SU-8 2000 series we need to move to another product SU8 3000 and SU8 XTF series.
New formulation will be available in cleanroom and tested: 3005 instead of 2005, 3035 instead of 2035, XTF75 instead of 2075 and new 3025.
TI spray resist is an image reversal resist, similar to AZ 5214E. The process flow will be similar to the process flows for 5214, except for the coating step. The exposure dose and development will depend on the specific process.
Process flow examples
Comparison of specifications and feature space of the standard UV photoresists available at DTU Nanolab. These are just examples and may contain obsolete information regarding exposure dose, etc.
NB! Most of the process knowledge about SU-8 is based in research groups
Other process flows:Chip on carrier: A procedure for UV lithography on a chip using automatic coater and developer.
Exposure dose
Spectral sensitivity of AZ resists represented as optical absorption coefficient.
During exposure of the resist, the photoinitiator, or photo-active component, reacts with the exposure light, and starts the reaction which makes the resist develop in the developer.
In a positive resist, it makes the resist become soluble in the developer. In a negative resist, usually assisted by thermal energy in the post-exposure bake (PEB), it makes the resist insoluble in the developer. The amount of light required to fully develop the resist in the development process, is the exposure dose.
The optimal exposure dose is a function of many parameters, including the type of resist, the resist thickness, and the sensitivity of the resist.
Resist sensitivity
The resist sensitivity is a measure of how efficiently it reacts to the exposure light. Spectral sensitivity is the sensitivity of the resist as a function of wavelength. It is usually given simply as the range from the wavelength below which absorption in the resist material makes lithography impractical to the wavelength at which the photoinitiator is no longer efficiently activated.
Within the sensitivity range, the optical absorption is commonly used as a measure of sensitivity. A high absorption coefficient signifies a high sensitivity, as the light is absorbed by the photoinitiator. Because of spectral sensitivity, the optimal dose of a given resist type and thickness is also a function of the spectral distribution of the exposure light, i.e. the equipment used for the exposure. Using a combination of experience, calculation and assumptions, it may be possible to estimate the dose for an exposure equipment, if the exposure dose is already known on another equipment.
Due to reflection and refraction at the interface between the resist and the substrate, the optimal dose may also be a function of the type of substrate used. Unless otherwise stated, the exposure doses given below are for standard silicon wafers.
Apart from the already mentioned factors, the optimal dose also depends on the developer chemistry and the parameters used in the development process. Finally, the requirements to the lithographic process in terms of resolution, bias (line broadening), etch selectivity, side wall angle, etc. may narrow down, or widen, the process window. The exposure doses given in the sections below should be used as a starting point for individual fabrication process development.
Due to the process of bleaching, where the absorption of the resist changes during exposure, the exposure dose is unfortunately not always constant at different intensities of the exposure light. The exposure time is thus not always a linear function of the exposure intensity.
Calculate exposure time
In the maskless aligners, the dose is set directly as a process parameter in the job. In mask aligners, on the other hand, the parameter that is set is the exposure time, i.e. how long the shutter is open during the exposure.
The exposure dose, D [J/m2], in terms if exposure light intensity I [W/m2] and exposure time t [s], is given by:
Since the intensity is specific to the spectral sensitivity of the sensor used to measure the exposure light, and the exposure time is specific to the spectral distribution of the exposure light (cf. spectral sensitivity), this dose is specific to the combination of exposure source and optical sensor.
Given an exposure dose, the exposure time, t, is calculated as:
It is important to keep in mind that this exposure time is valid only for a specific combination of exposure source and optical sensor, as well as for a specific development process.
