Photolithography

PHOTOLITHOGRAPHY PROCESS

1. PHOTOLITHOGRAPHY By Preetha A

2. LITHOGRAPHY Lithography comes from the Greek word “ lithos” which means stones, and “ graphia” , meaning to write. It means quite literally writing on stones. The process of producing patterns on semiconductor crystals for use as integrated circuits. In semiconductor lithography is called as PHOTOLITHOGRAPHY.

3. PHOTOLITHOGRAPHY Photo - litho - graphy can be light-silicon wafer-printing. Photolithography is a printing method in which light plays an essential role. It can produce very small patterns which may be down to few tons of nanometers in size. It provides precise control of the shape and size of the objects.

4. PHOTOLITHOGRAPHY

5. STEPS INVOLVED IN PHOTOLITHOGRAPHY Surface cleaning Spin coating with photoresist Soft baking Alignment of mask Exposure Development Hard baking Plasma Etch-Or Add Layer Post process cleaning Final Inspection

6. STEP 1: SURFACE CLEANING Organic or inorganic contaminations may present on the wafer surface can be cleaned using a solution containing hydrogen peroxide. Solutions containing trichloroethylene, acetone or methanol can also be used. Contaminants may be photoresist residue from previous photolithography, atmospheric dust, bacteria, films from other sources etc.

7. STEP:2 SPIN COATING WITH PHOTORESIST Spin coating is a process of depositing uniform thin films onto flat substrates. It is intensively used in photolithography to deposit layers of photoresist about 1 micrometer thick. A amount of coating material is applied on the center of the substrate, which is either spinning at low speed.

8. STEP: 3 SOFT BAKING The purpose of this is to clean the wafer by removing excess solvents. Typical soft-bake temperature is around 90°C for ten to twenty minutes.

9. STEP 4: ALIGNMENT OF MASK One of the most important steps in the photolithography process is mask alignment. The mask is aligned with the wafer, so that the pattern can be transferred onto the wafer surface. Each mask after the first one must be aligned to the previous pattern. Once the mask has been accurately aligned with the pattern on the wafer's surface, the photoresist is exposed through the pattern on the mask with a high intensity ultraviolet light

10. STEP 5: EXPOSURE Three primary exposure methods are: Contact Proximity P rojection

11. EXPOSURE METHODS In contact printing, the resist-coated silicon wafer is brought into physical contact with the glass photomask. The wafer is held on a vacuum chuck, and the whole assembly rises until the wafer and mask contact each other. The photoresist is exposed with UV light while the wafer is in contact position with the mask. V ery high resolution is possible in contact printing. Proximity Printing exposure method is similar to contact printing except that a small gap, 10 to 25 microns wide, is maintained between the wafer and the mask during exposure. This gap minimizes mask damage. Approximately 2- to 4-micron resolution is possible with proximity printing. Projection Printing avoids mask damage entirely. An image of the patterns on the mask is projected onto the resist-coated wafer, which is many centimeters away. In order to achieve high resolution, only a small portion of the mask is imaged. This small image field is scanned or stepped over the surface of the wafer.

12. STEP 6 & 7: DEVELOPMENT AND HARD BAKING The non-exposed areas of the photoresist can be removed by developing the wafers in either acid or a base solution, followed by a rinse to stop further development and also remove any residual chemicals. This baking step cleans any unwanted particles left over from the solution and increases adhesion of the resist and hardens it, especially at the newly created edges of the resist film. Temperature should be higher than soft baking.

13. STEP 8: PLASMA ETCH-OR ADD LAYER Plasma etching is a form of plasma processing used to fabricate integrated circuits In this process plasma will generate volatile etch product at room temperature from the chemical reactions between the elements. Selective removal of upper layer of wafer through windows in photoresist.

14. STEP: 9 POST PROCESS CLEANING Plasma etching with O2 takes place that is ashing process takes place. Simple solvents are used for Positive photoresists are aceton, trichloroethylene (TCE) and for negative photoresists are methyl ethyl ketone, methyl isobutyl ketone.

15. STEP 10: FINAL INSPECTION Photoresists can be completely removed. Quality issues like step height, defects, particles, critical dimensions can be checked.

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