Lithography techniques,types

Lithography is the process of transferring patterns of geometric shapes in a mask to a radiation sensitive material called resist,which cover the surface of semiconductor wafer.

1. LITHOGRAPHY TECHNIQUES PRESENTED BY ANJANI S PONDICHERRY UNIVERSITY S1 M-TECH ECE 20304001

2. The term Lithography comes from the Greek word, lithos, means "stone“ and graphein, means "to write ”. Lithography is the transfer of geometric shapes on mask to smooth surface. First introduced by German author Alois Senefelder at 1771-1834 . Important lithographic company of 19th century was Currier & Ives, 1852. In the 20th and 21st century, it becomes an important technique with unique dramatic capabilities in the Art field . INTRODUCTION

3. Lithography is the most complicated, expensive and critical process of modern IC manufacturing. Lithography transforms complex circuit diagrams in to pattern which are define on the wafer in a succession of exposure and processing steps . Typically 8-25 lithography steps and several hundred processing steps between exposure are required to fabricate a packed IC.

4. PHOTOLITHOGRAPHY ELECTRONE BEAM LITHOGRAPHY X-RAY LITHOGRAPHY INTERFERENCE LITHOGRAPHY SCANNING PROBE LITHOGRAPHY CHARGED PARTICLE LITHOGRAPHY NANOIMPRINT LITHOGRAPHY TYPES OF LITHOGRAPHY

5. Photolithography, also called optical lithography or UV lithography. P rocess used in  micro fabrication  to pattern parts on a thin film or the bulk of a substrate (also called a wafer ). Uses light to transfer a geometric pattern from a  photo mask to a   photosensitive chemical photoresist on the substrate . A series of chemical treatments, etches  the exposure pattern into the material or enables deposition of a new material in the desired pattern upon the material underneath the photoresist. 40 to 50% total wafer process time . PHOTOLITHOGRAPHY

6. A CMOS wafer may go through the photolithographic cycle as many as 50 times. fig: Photolithographic process

7. Surface cleaning Spin coating with photoresist Soft baking Mask alignment Exposure Development P ost baking Plasma Etch-Or Add Layer Post process cleaning Final Inspection STEPS USED IN PHOTOLITHOGRAPHY

8. Typical contaminants that must be removed prior to photoresist coating D ust from scribing or cleaving (minimized by laser scribing) photoresist residue from previous photolithography (minimized by performing oxygen plasma ashing) Atmospheric dust (minimized by good clean room practice) B acteria (minimized by good DI water system) SURFACE CLEANING

9. Wafer is held on a spinner chuck by vacuum and resist is coated to uniform thickness by spin coating. Typically 3000 - 6000 rpm for 15-30 seconds. Resist thickness is set by: primarily resist viscosity secondarily spinner rotational speed. Most resist thicknesses are 1-2 μm for commercial Si processes. 2. SPIN COATING WITH PHOTORESIST

10. Resist thickness is given by t =square of( kp)/root of(w1), where, k = spinner constant, typically 80-100 p = resist solids content in percent w = spinner rotational speed in rpm/100

11. RESIST COATING-STAGES

12. To ensure reproducible processing by removal of excess solvent from the resist. The silicon wafer coated with photoresist is heated at 75-85°C for 45 sec. Improve adhesion by reducing stress. The thickness of the resist is usually decreased by 25 %. 3 . SOFT BAKING

13. Fig :SOFT BAKING

14. The coated wafer is placed in an apparatus called mask aligner in very close proximity to a photo mask. Photo mask has photographic emulsion on one side. The pattern has clear and opaque areas. A highly collimated UV light is turned on. The areas of wafer that are not covered by the opaque areas of the photo mask are exposed to UV. 4 . MASK ALIGNMENT AND EXPOSURE

15. Master patterns which are transferred to wafers . Types : Photographic emulsion, Fe2O3 , Cr on glass Cr on quartz glass Dimensions : 4”x4”x0.060 ” for 3 inch wafers 5”x5”x0.060 ” for 4 inch wafers PHOTOMASK

16. Fig: mask alignment and exposure

17. The exposed wafer is then put in a developer solution. The developer solution will remove the exposed or unexposed regions of photoresist. Removal of regions depend on the type of photoresist used [positive or negative resist]. 5 . DEVELOPMENT

18. Negative Photoresist Becomes insoluble after exposure When developed, the unexposed parts dissolved. Cheaper positive Photoresist Becomes soluble after exposure When developed, the exposed parts dissolved. Better resolution PHOTORESIST

19. Fig: development

20. Post bake in an oven at a temperature about 150℃ for 30 to 60 minutes. Used to stabilise and harden the developed photoresist . Post bake removes any remaining traces of the coating solvent or developer . Introduces some stress into the photoresist . Some shrinkage. 6 . POST BAKE Fig: post bake

21. To remove unwanted regions, where are not protected by resist. Two main Etching Methods : Wet Chemical Etching – Difficult to control – Cheapest Dry Etching – Ion Beam – Plasma 7 .PLASMA ETCH-OR ADD LAYER

22. For etching wafers are immersed in or sprayed with hydrofluoric acid(HF) solution. HF will etch the sio2 layer. HF will not attack the underlying silicon. Etch rate = The rate at which etching process occurs. Etching is a isotropic process.

23. Fig: Etching process

24. Plasma etching with O2 (ashing) . Simple solvents are generally sufficient for non-post baked photoresists : The remaining photoresist is finally removed or stripped off with a mixture of sio2 and hydrogen peroxide. A step of washing and drying complete the required window in the oxide layer. Overall photolithographic process 8 . POST PROCESS CLEANING & FINAL INSPECTION

25. Overall photolithographic process

26. Easy to produce pattern of smaller dimensions. High resolution compared to proximity lithography. Low production cost. Improved production ADVANTAGES OF PHOTOLITHOGRAPHY

27. Specialized technique for creating extremely fine pattern(∽50nm). T he preferred method for producing the stamps used for  Nano-imprint lithography. Utilizes an accelerated electron beam focusing on an electron-sensitive resist . Basic process design is same as photolithography. ELECTRON BEAM LIRHOGRAPHY

28. Fig: electron beam lithography

29. ADVANTAGES OF EBL Print complex pattern directly on wafers. Eliminates the diffraction problem. High resolution. Flexible technique . DISADVANTAGES OF EBL Slower than optical lithography. Expensive and complicated Forward scattering Backward scattering

30. X-ray lithography (XRL) is an advanced version of optical lithography in which shorter wavelengths are used. Process used in electronic industry to selectively remove parts of a thin film. It uses X-rays to transfer a geometric pattern from a mask to a light-sensitive chemical photoresist. X-ray resists . X-ray mask. X-RAY LITHOGRAPHY

31. Fig:x-ray lithography

32. ADVANTAGES OF XRL Less diffraction effect. High resolution. Large area(large depth of focus). Excellent resist profiles DISADVANTAGES OF XRL Distortion in absorber. Cannot be focused through lens .