! Cleanroom humidity warning !
Control of the relative humidity (RH) in photolithography zones is extremely critical. Stable and reproducible photolithography is expected within 38% to 48% RH range.
- In case of low RH (< 38%), the resist sensitivity and development rate decreases. It is then recommended to increase the recommended exposure doses.
- In case of high RH (> 48%), the resist adhesion decreases. It is then recommended to do an additional bake (>10 minutes @ 150°C) before loading the wafers in the HMDS or coating equipment.
Wafer surface preparation
Usually adhesion of photoresist on inorganic materials is poor resulting in losses of fine structures after development. To solve the issue, silicon wafers are generally treated using the HMDS vapor prime treatment before spincoating the photoresist. Details about the HMDS process and control can be found here: link
Assuming wafers with a clean surface free of organic contamination, the best adhesion will be obtained with the surface preparation recommended in the table:
|Surface material (larger area)||Vapor HMDS||Plasma O2||Thermal dehydratation|
|SiO2, fused silica, SiN, Si3N4||√√||√||√|
|Float glass, pyrex||√||√√||√|
|Metals: Al, Au, Pt, Ti||…||√||√√|
|Metals: Ag, Cu, Cr, Fe||…||X||√√|
|III/V semiconductors (GaN, GaAs)||…||X||√√|
Legend: √√ Strongly recommended / √ Alternative process / … Not effective / X May affect or destroy underlaying material
The AZ nLOF 2020 spincurve is shown below, as well as process details for both automatic and manual coating.
Available thicknesses: 1.4 um / 1.6 um / 2 um / 2.4 um / 3 um
- nLOF_2020_Std_NoDehydrate_x_y (for HMDS-treated wafers)
- nLOF_2020_GlassOnly_Dehydrate_x_y (for transparent wafers)
- nLOF_2020_Quartz_Dehydrate_x_y (for quartz, stacked glass-silicon wafer or SOI wafers. Softbake temperature is 118°C)
|Recipe name||Spin speed [rpm]||Softbake time [mm:ss]||PR thickness [µm]|
Spin time is 40s. Softbake is performed at 50µm proximity. Softbake temperature is 110° for all databases except for “Quartz” (118°C).
- Find the spin-coating speed “XXXX” [RPM] matching your target thickness from the AZ nLOF 2020 spincurve.
- When coating on wafers, use the STD_”XXXX” recipe, which includes a 500 RPM spreading step and 40 seconds of main coating step.
- When coating on small chips, use the CHIP_”XXXX” recipe, which includes 40 seconds of main coating step and a short acceleration at the end to reduce edge bead effects.
- Softbake temperature: 105°C
- Softbake time: 60” + 15”/um
The following table lists the recommend dose “to clear” for AZ nLOF 2020 coated on silicon wafers. It is recommended to perform a contrast curve / exposure matrix calibration for wafers other than silicon.
|Illumination:||Broadband*||i-line (355-365 nm)||h-line (375 nm)|
|Equipment:||MABA6, MA6 Gen3 (no filter)||VPG 200, MA6 Gen3 (filter), MJB4||MLA 150|
|PR thickness [µm]||Dose [mJ/cm2]+||Dose [mJ/cm2]++||Dose [mJ/cm2]+++|
|1.4 — 3||>180||>90||>150|
* Mercury Lamp, Mask Aligner with UV400 configuration & no filter / + Based on intensity readings from Süss optometer broadband CCD / ++ Based on intensity readings from Süss optometer i-line CCD / +++ Based on MLA150 internal dose measurements
The recommended developer for AZ nLOF 2020 is AZ 726 MIF (or MF CD 26), an organic solution based upon TMAH.
After development, it is mandatory for wafers to go through an additional rinsing step with DI water to avoid backside contamination and damage on equipments (chuck in etcher) in further processing steps. The water baths of the following wet benches can be used free of charge (5 min. billing delay after login):
! Important! AZ nLOF is an image reversal resist. A post-exposure bake (PEB) step is required in order to complete the photoreaction. This PEB step is part of the standard development recipes on the EVG 150 system. PEB is performed at 110°C for 75 seconds with a 50µm proximity gap.
Overdevelopment time is necessary to produce a negative resist profile (undercut) for optimal liftoff. Three different development recipes are available for nLOF 2020 on the EVG 150 for each specific thickness, allowing to control the undercut (UC):
- Dev_nLOF_XumX_max: Large undercut, optimal for lift-off applications.
- Dev_nLOF_XumX_mid: Small undercut, for layouts containing nLOF lines / arrays of lower critical dimension.
- Dev_nLOF_XumX_min: Negative profile with minimum undercut, optimal for etching applications. Not recommended for lift-off.
Standard development recipes: Database file name: nLOF_2020_std_2_0
|Recipe name||PR thickness [µm]||*_min contact time [s]||*_mid contact time [s]||*_max contact time [s]|
!Important! AZ nLOF is an image reversal resist. A post-exposure bake (PEB) step is required in order to complete the photoreaction.
- PEB temperature: 105°C
- PEB time: 75”
- Recommended developer: AZ 726 MIF (no dilution needed)
- Development time: 15″/um (minimum) + 40″ (undercut)
- Rinse: H2O 1min