Optical Lithography
Overview
Optical lithography (also known as photolithography) uses a light source to expose a photosensitive compound (photoresist). The photoresist is then developed, with part of the photoresist removed depending on the exposure mask and the photoresist polarity.
The optical lithography process consists of several steps, each of which may need to be revisited depending on the required results:
- Process flow: An itial sketch of the overall goal of the lithographic process is made. Substrate size, materials, and end pattern goals all must be considered.
- Photomask design: A layout file is created, typically in GDS, CIF, or DXF formats, based on the patterns needed.
- Photomask fabrication: Submit to BNC staff via iLab.
- Substrate cleaning and surface preparation: Clean and prepare substrate for photoresist coating.
- Photoresist coating: Spin coating of photoresist on cleaned substrate.
- Photoresist softbake: Hotplate bake to remove solvent remaining after spin coating.
- Exposure: Use mask aligner to expose substrate for predetermined period of time.
- Development: Remove unexposed photoresist.
- Post-development processing: Additive or subtractive manufacturing processing
- Photoresist removal: Remove remaining photoresist.
Exposure
Mercury Vapor (Hg) Lamp
Exposure on all aligners at Birck (the Suss MA6 Mask Aligner and both Suss MJB3 UV400 Mask Aligner aligners) is done via a mercury vapor lamp (Ushio USH-350DS for all aligners). A typical emission spectra is seen below (via a great overview from Zeiss):
The glass used in the aligners filters out all light below 350 nm, and resists are not sensitive to light above 500 nm (yellow light filters filter light below 500 nm). Without any special UV filters added to the aligners, this gives a so-called "broadband" exposure covering three spectral lines in the NUV:
- g-line: 436 nm
- h-line: 405 nm
- i-line: 365 nm
In general, the resists used will be sensitive to all three lines, with the specific proportion depending on the resist (see chart below). Sensitivity will also depend on the substrate used, as reflection percentage will depend on the material.
Both the MJB3 and MA6 are measure and set weekly on at 405 nm (h-line), with the MA6 at 14 mW/cm2 and both MJB3 machines at 10 mW/cm2. Intensity is adjusted if the measured value is more than the uncertainty in the power meter.
The 365 nm line intensity is a fixed percentage of the h-line power, and thus its intensity will not change over time for a properly maintained aligner. However, the ratio of 365 nm power to 405 nm intensity will depend on the specific aligner used. All the aligners use the same 350 W lamp, meaning this difference is due to the particular aligner optics. The 436 nm line (g-line) is currently unable to be measured, but should be similarly fixed per aligner used. In general, unless the power of all three lines is known, and the absorption spectrum of the resist is known, it will not be possible to calculate an exposure time from published values.
The intensity of each line for each tool in the cleanroom is listed below. These values will be updated if a significant shift in the ratios is observed.
Note that for g-line sensitive resists, a direct comparison between aligners cannot be made at this time due to the g-line power being unknown.
Please comment below or edit this page if you've found a good ratio between different machines.
| Aligner | h-line, 405 nm | i-line, 365 nm Intensity (mW/cm2) |
|---|---|---|
| MJB3_1 | 10.0 | 3.7 |
| MJB3_2 | 10.0 | 7.4 |
| MA6 | 14.0 | 7.0 |
| 557 MJBL | 10.0 | 4.9 |
| 557 MJBR | 10.0 | 6.9 |
Thus, exposure time on a particular aligner can be varied to give more or less dose depending on the needs of a particular photoresist. Proper exposure time will depend on:
- Mask aligner
- Substrate
- Photoresist formulation
- Photoresist thickness
- Softbake time and temperature (if remaining solvent is not fully removed)
- Developer chemical
- Developer concentration
- Development time
Contact Types
Three types of contact between the mask and substrate are available: Soft, Hard, and Vacuum.
- Soft contact (for large features): Uses vacuum pressure to pull the substrate towards the chuck. The chuck is then pressed against the photomask with mechanical pressure.
- Hard contact (most common): Mechanical pressure presses the substrate into contact with the photomask, and nitrogen is applied to further press the mask against the photomask.
- Vacuum contact (not typically used): Vacuum contact is hard contact, but a seal between the substrate and photomask allows the volume between the mask and substrate to be pumped free of air. This helps further solidify the contact between mask and substrate, and also helps to eliminate diffraction effects.
Soft contact results in minimal damage to the photomask, and is preferable for most (≥2 μm) feature sizes. Hard contact should be used for features in the 1 μm range. Vacuum contact (HP mode on MJB3) allows the highest resolution, but a chuck with a vacuum seal must be used to allow for vacuum between the substrate and mask. Vacuum contact is typically not used at BNC, features below 1 μm are preferentially exposed via electron beam lithography.
