Part 5-1: Lithography

Transcription

1 Part 5-1: Lithography Yao-Joe Yang 1 Pattern Transfer (Patterning) Types of lithography systems: Optical X-ray electron beam writer (non-traditional, no masks) Two-dimensional pattern transfer: limited tolerance for non-planar topography on wafer 2-D pattern transfer imposes a constraint on process design The position control of the lithography systems need 10 times better than the achieved electronic resolution level. semiconductor industry strongly depends on precision machine design. 2

2 Optical Lithography Optical Lithography Photo-masks Interface between designers and devices Designers layout design (mask) on computers (2D patterns) photo-masks Photo-masks: opaque patterns (chromium) on transparent glass (fused silica) 3 Masks 4

3 Photoresist (PR) Optical resists: photosensitive polymers PR is coated on the whole substrate surface using spin-coater Positive PR: can be removed by KOH or NaOH solution if it is exposed most popular for IC process Negative PR: can be removed by specific solution if it is not exposed not good for feature size < 3 um Photoresist 5 Photoresist Spin Coating PR suck back PR dispenser nozzle Wafer Chuck Spindle To vacuum pump 6

4 Photoresist Spin Coating PR suck back PR dispenser nozzle Wafer Chuck Spindle To vacuum pump 7 Coater / HMDS HMDS Spin coater 8

5 Coating of Photoresists Spinning coating Final thickness of photoresist is a function of rotating speed. Thickness 1 ω Relationship has been calibrated and formed a look up table 9 Photoresist Baking Pre-bake (soft-bake) After PR coating, before exposure Removing residual solvent and increasing adhesion C for 1 min (hot plate) ~ 30 min (oven) Post-exposure bake After exposure, before developing Hard-bake After PR developing Further reduce the solvent concentration Increase resistance to etchant and ions. I.e., selectivity C for 2 min (hot plate) ~ 30 min (oven) 10

6 Photoresist Removing After exposure, PR is usually removed in NaOH or KOH based solution PR stripping After etching, we need to remove PR. Organic stripper (phenol-based) Oxidizing-type stripper (H 2 O 2 ) Oxide plasma 11 Aligners Alignment and exposure systems A microscopic system which: Align the photo-mask and the substrate Expose PR Also called: printers 12

7 Classification of aligners by exposure methods Contact printer High resolution ( 1µm) Mask deterioration Proximity printer µm gap --> longer mask life Diffraction effect --> 2-4 µm resolution Projection printer Image of mask usually reduced Scanning or stepping of small field (~ 1cm) VLSI standard (0.25 µm possible with deep- UV source) 13 Alignment and Exposure Tools Contact printer Proximity printer Projection printer Stepper 14

8 Contact Printer Simple equipment Use before mid-70s Resolution: capable for sub-micron Direct mask-wafer contact, limited mask lifetime Particles 15 Contact Printer Light Source Lenses Mask Photoresist Wafer 16

9 Contact Printing UV Light Mask PR N-Silicon 17 Proximity Printer ~ 10 µm from wafer surface No direct contact Longer mask lifetime Resolution: > 3 µm 18

10 Proximity Printer Light Source Lenses Mask Photoresist Wafer ~10 µm 19 Proximity Printing ~10 µm UV Light Mask PR N-Silicon 20

11 Projection Printer Works like an overhead projector Mask to wafer, 1:1 Resolution to about 1 µm 21 Scanning Projection System Light Source Slit Lens Synchronized mask and wafer movement Mask Lens Photoresist Wafer 22

12 Stepper Most popular used photolithography tool in the advanced IC fabs Reduction of image gives high resolution 0.25 µm and beyond Very expensive 23 Step-&-Repeat Alignment/Exposure Light Source Projection Lens Reticle Projection Lens Wafer Wafer Stage 24

13 Step&Repeat Alignment System Light Source Reference Mark Interferometer Laser Reticle Stage Alignment Laser Reticle Projection Lens Y Wafer X Wafer Stage Interferometer Mirror Set 25 Lithography Karl Suss Contact Aligner stepper 26

14 Exposure Light Source Short wavelength High intensity Stable High-pressure mercury lamp Excimer laser 27 Photolithography Light Sources Name Wavelength (nm) Application feature size (µm) G-line Mercury Lamp H-line 405 I-line to 0.25 XeF 351 XeCl 308 Excimer Laser KrF (DUV) to 0.15 ArF to 0.13 Fluorine Laser F to