1. Equipment and technological processes for manufacturing GaAs MMICs LITHOGRAPHY ONE TALK 3 1

2. Equipment and technological processes for manufacturing GaAs MMICs LITHOGRAPHY ONE

3. Equipment and technological processes for manufacturing GaAs MMICs LITHOGRAPHY ONE

4. Overview of the Photolithography Process Surface Preparation Coating (Spin Casting) Pre-Bake (Soft Bake) Alignment Exposure Development Post-Bake (Hard Bake) Processing Using the Photoresist as a Masking Film Stripping Post Processing Cleaning (Ashing) Equipment and technological processes for manufacturing GaAs MMICs LITHOGRAPHY ONE

5. Cleaning Typical contaminants that must be removed prior to photoresist coating: atmospheric dust (minimized by good clean room practice) abrasive particles photoresist residue from previous photolithography (minimized by performing oxygen plasma ashing) bacteria (minimized by good DI water system) films from other sources: – solvent residue – H 2 O residue – photoresist or developer residue – oil – silicone Equipment and technological processes for manufacturing GaAs MMICs LITHOGRAPHY ONE

6. Wafer Priming Adhesion promoters are used to assist resist coating. Resist adhesion factors are-:  moisture content on surface  wetting characteristics of resist  type of primer  delay in exposure and prebake  resist chemistry  surface smoothness  stress from coating process Most common for GaAs use are 1,1,1,3,3,3-hexamethyldisilazane (HMDS) or monazoline C or trichlorobenzene Equipment and technological processes for manufacturing GaAs MMICs LITHOGRAPHY ONE

7. Equipment and technological processes for manufacturing GaAs MMICs LITHOGRAPHY ONE

8. Equipment and technological processes for manufacturing GaAs MMICs LITHOGRAPHY ONE

9. Soft bake Used to evaporate the coating solvent and to densify the resist after spin coating. Typical thermal cycles: – 90-100°C for 20 min. in a convection oven – 75-85°C for 45 sec. on a hot plate Hot plate heating the resist is usually faster, more controllable, and does not trap solvent like convection oven baking. The thickness of the resist is usually decreased by 25 % during prebake for both positive and negative resists Equipment and technological processes for manufacturing GaAs MMICs LITHOGRAPHY ONE

10. Equipment and technological processes for manufacturing GaAs MMICs LITHOGRAPHY ONE

11. Masks For simple contact, proximity, and projection systems, the mask is the same size and scale as the printed wafer pattern. i.e. the reproduction ratio is 1:1. Projection systems give the ability to change the reproduction ratio. Going to 10:1 reduction allows larger size patterns on the mask, which is more robust to mask defects. 5:1 is common now Mask size can get unwieldy for large wafers. Most wafers contain an array of the same pattern, so only one cell of the array is needed on the mask – so use a stepper Equipment and technological processes for manufacturing GaAs MMICs LITHOGRAPHY ONE

12. Postbake (Hard Bake) Used to stabilize and harden the developed photoresist prior to processing steps that the resist will mask. Main parameter is the plastic flow or glass transition temperature. Postbake removes any remaining traces of the coating solvent or developer. This eliminates the solvent burst effects in vacuum processing. Postbake introduces some stress into the photoresist. Some shrinkage of the photoresist may occur. Longer or hotter postbake makes resist removal much more difficult. Equipment and technological processes for manufacturing GaAs MMICs LITHOGRAPHY ONE

13. Equipment and technological processes for manufacturing GaAs MMICs LITHOGRAPHY ONE

14. Equipment and technological processes for manufacturing GaAs MMICs LITHOGRAPHY ONE

15. Equipment and technological processes for manufacturing GaAs MMICs LITHOGRAPHY ONE

16. After process strip Resist is usually stripped these days in a plasma etch system An O 2 plasma is created which generates many O radicals – these efficiently oxidise the resist and produce CO, CO 2 and other gaseous products which are pumped away Some systems use RF plasmas – others use microwaves Equipment and technological processes for manufacturing GaAs MMICs LITHOGRAPHY ONE

17. For GaAs The main lithography tools for MMIC production are:- 1)Proximity contact aligners – non critical layers 2)Optical steppers – medium critical layers 3)Electron beam systems – critical layers (next talk) Equipment and technological processes for manufacturing GaAs MMICs LITHOGRAPHY ONE

18. Proximity contact layers – modern example Suss Microtec MA6 – common in GaAs fabs, Other major vendor for GaAs is EVG Equipment and technological processes for manufacturing GaAs MMICs LITHOGRAPHY ONE

19. Proximity system - schematic 1Mask holder 2Mask 3Wafer/substrate 4Chuck 5Chuck stage 6 Wedge compensation pistons (3x) 7Pneumatic brake 8 Adjustable contact force piston with pressure controller 9Precise ball-bearing guide 10High precision Z-drive 11,12,13Manual alignment x-y- Ɵ MA6 stage with main functions shown Equipment and technological processes for manufacturing GaAs MMICs LITHOGRAPHY ONE

20. Proximity system The proximity system offers non contact exposure at small mask-wafer gaps. The key to uniform exposure results is a constant exposure gap over the entire surface. Three reference balls with precisely matching diameters are placed near the edges of a wafer or substrate; together with the compensation system, they ensure that the mask-to-substrate gap is defined with high accuracy. Equipment and technological processes for manufacturing GaAs MMICs LITHOGRAPHY ONE

21. Wavelength350- 450nm 280- 350nm 240- 260nm Hg lamp 1 kW  Hg lamp 300W  HgXe lamp 500W  Resolution proximity gap 100um 7um<7um Resolution proximity gap 20um 2.5um2um<2um Proximity system –optics and performance Equipment and technological processes for manufacturing GaAs MMICs LITHOGRAPHY ONE

22. Steppers -1 Outline of how a stepper covers an entire wafer Steppers, as the name suggest, expose one circuit at once and then jump to the next one All steppers have reduction optics so the features written on the mask are usually 5x bigger than the final features will be on the wafer. This means the structures on the reticule can be written with greater precision. Equipment and technological processes for manufacturing GaAs MMICs LITHOGRAPHY ONE

23. Steppers -2 Light source, condenser lens, reticule and projection lens The main optics of a stepper is shown below In practice a stepper will have up to 20 lenses in it Equipment and technological processes for manufacturing GaAs MMICs LITHOGRAPHY ONE

24. Steppers -3 The light source is one of the major definer of the smallest feature that can be written using a stepper For GaAs MMIC applications the most common wavelengths used are I-line (365nm) or 248nm Light sources for steppers Equipment and technological processes for manufacturing GaAs MMICs LITHOGRAPHY ONE

25. Steppers -4 X-Y stage and optical interferometer arrangement of typical stepper Alignment for steppers needs to very accurate if the reticle is to be produced in the correct position all over the wafer. Typical laser interferometer alignment systems like that shown are typical and are capable of giving a positional accuracy of <10nm Equipment and technological processes for manufacturing GaAs MMICs LITHOGRAPHY ONE

26. Steppers -5 There are only really 3 suppliers of steppers worldwide ASML in Europe and Canon and Nikon in Japan. LensField sizeOverlay NAResolutionX & Y 2 point global alignment 0.4 to 0.63<150nm22 x 28mm<25nm Typical data for steppers used in GaAs fabs Equipment and technological processes for manufacturing GaAs MMICs LITHOGRAPHY ONE