Silicon Valley Microelectronics offers a variety of broadband, I-line, 248nm, E-beam, positive, negative, wet and dry resists, etc. SVM acknowledges how much the photoresist deposition method varies depending on each customer’s specifications so we are ready to help customers meet all of their unique requirements.
Some common uses for these wafers are bumping, MEMS, TSV, and CMP/etch/metrology/clean e-test applications, as well as custom short loop design wafer patterning work. Test patterns are also available.
- Substrate preparation. The substrate is cleaned, then goes through a dehydration bake and/or an adhesion promoter is added in order to prepare the wafer for the rest of the process.
- Photoresist spin coating. A thin, uniform coating made from a mixture of photoresists and solvents is deposited via spin coating. The film is deposited either while the wafer is either spinning (dynamic dispense) or not spinning (static dispense).
- Prebake. After spin coating, 20-40% of the film is still solvent. The addition of a prebake dries the photoresist and stabilizes the film. This step reduces thickness, changes the properties of film, improves adhesion, and makes it less susceptible to particle contamination. Prebake usually takes place on a 100°C hot plate at about 1 minute/μm (1 minute for each micron of film coating).
- Exposure. This changes the solubility of the film in order to pattern the wafer by exposing it to light. There are 3 forms of exposure:
- Contact – Mask is in contact with photoresist.
- Proximity – Mask is ~15 – 20μm above photoresist.
- Projection – Mirrors project a slit of light from the mask to the wafer surface. To determine the correct exposure does, engineers measure the light intensity, slit size, and spin speed of the substrate. There are two versions of projection expsoure:
- Scanning – A computer scans the wafer surface, and a light moves around to expose correct area.
- Step-and-repeat – Expose the wafer to light in small sections.
- Post-exposure bake. This step is generally for wafers with high resolution (<1 micron). In most other circumstances, the substrate goes straight from exposure to development.
- Development. The wafer goes through a chemical rinse to expose the etching left by the mask. This an important step, as it determines the quality of the photoresist.
- Strip photoresist. Chemical removal or etching gets rid of all remaining photoresist to produce the final pattern on the wafer.
SVM Sample Photoresist Methods:
This is a portion of SVM’s photoresist capabilities, for more information on photoresist, lithography or wafer patterning, or to request a quote, please CONTACT SVM today.
I-line photoresist is a general purpose single layer resist film that exposes wafers to a 355nm – 375nm wavelength of UV radiation and uses step-and-repeat for deposition. This film has excellent adhesion and plating characteristics for precise control of exposure which makes it great for use in MEMs and bumping processes.
193nm, 248nm Deep-Ultraviolet (DUV)
248nm deep-ultraviolet (DUV) lithography starts with krypton fluoride (KrF) discharging electrical energy from light exposure to create a laser. The KrF laser moves through a filter to create a specific wavelength for the target wafer’s particular pattern. Engineers measure the wavelength and energy of the beam to make sure everything is matches the user’s specifications. Next, the beam moves to a chamber where the beam is amplified to produce 10 to 20 times more energy. It is checked again to make sure it’s stable and the wafer is in focus then the beam is stretched to target wavelength (248nm or 193nm) then shot to wafer.
Thick photoresist is deposited via spin coating at low rotations per minute to reduce the amount of resist lost during the spin stage. Using a 150mm wafer, the resulting film can be between 20µm – 70µm depending on the spin rate. For most thick film applications, the substrate spins between 500RPM and 3000RPM, with the film thickness reducing as spin speed reduces.
Spin-on is a process where an ink-like liquid is placed on the middle of a substrate and then spun at a high speed (600rpm – 10,000rpm) to create a uniform film across a wafer. This process uses centrifugal force and surface tension in order to create an even coating across the wafers. The total time and spin speed for this process varies based on wafer diameter and desired thickness of the spin-on coating.
Steps in spin-on applications:
1. Deposit an ink on the substrate that contains target molecules (red) in a liquid solvent (grey).
2. Spin the substrate at high speeds, throwing most of the ink off the substrate.
3. Send air through system to evaporate and dry any remaining solvent, leaving a thin film.
4. Dry the substrate completely, leaving an even layer film on the substrate.