Concepts and principles of optical lithography

Transcription

1 1/56 2/56 Concepts and principles of optical lithography Francesc Pérez-Murano Institut de Microelectrònica de Barcelona (CNM-IMB, CSIC) 10 cm mà blia 1 cm Gra de sorra Xip 1 mm 100 um Diàmetre cabell humà Micromotors 10 um Circuit integrat 1 um Bacteries Transistor MS 100 nm 10 nm Molècula de DNA Dispositius quàntics 1 nm Distància interatòmica Estructures atomiques 0,1 nm Microelectrònica Nanotecnologia 3/56 4/56 Nanolithographies Summary Part i Concept of optical lithography Nanotechnology, D. M. Tennant. AIP/Springer, New York, 1999 Resists Associated processes Miniaturization Limits optical lithography

2 Bibliography Basic Books Nanoelectronics and Information. Chapter 9. Technology. Rainer Waser. Wiley-V003 Fundamentals of Microfabrication. Chapter 1. Marc Madou. CRC Press Silicon Processing for the VLSI Era. Volume 1-Process Technology. Chapter 12 and 13. S. Wolf and R.N. Tauber. Lattice Press. 1986v Principles of Lithography. Harry J. levinson. SPIE Press Web sites 5/56 Summary Part i Concept of optical lithography Resists Associated processes Miniaturization Limits optical lithography 6/56 7/56 8/56 Summary Fabrication using optical lithography Part i Concept of optical lithography Resists Associated processes Miniaturization Limits optical lithography Exposition Development Etching Initial substrate Resist Mask

3 9/56 10/56 Positive Resist Tone Negative Resist Tone Ultraviolet Light Areas exposed to light become photosoluble. Ultraviolet Light Areas exposed to light become polymerized and sustain the develop chemical Chrome island on glass mask Shadow on Island Window Chrome island on glass mask Exposed area of Island Window Exposed area of Shadow on oxide oxide oxide oxide silicon substrate silicon substrate silicon substrate silicon substrate Positive Lithography Resulting pattern after the resist is developed. Negative Lithography Resulting pattern after the resist is developed. 11/56 12/56 Lithography: Image a mask on a wafer ptical lithography Wavelength of the light sources: Near UV and deep UV ptical source for UV Spectral distribution: Mercury Arc Lamp i-line (365 nm) g-line (435 nm) h-line (405 nm)

4 13/56 14/56 ptical lithography modes UV optical lithography systems Contact Proximity Projection ptical aligners Stepper Light source ptical system Mask Photoresist Sample Gap Exposition 1:1 1:1 5:1 15/56 16/56 Contact printing Proximity printing R=MFS=(d (difraction limited) R= MFS (Minimum feature size) MFS: Minimum feature size d: Resist thickness Light wavelenght K: Experimental parameter (>1) d= 1 m; = 435 nm; R=0.66 m a h

5 Projection printing 17/56 Resolution in projection printing 18/56 MFS=R = k1 λ NA NA: Numerical aperture k1: technology constant ( ) k1= 0,66; = 435 nm; NA= 0.7; R = 0.4 m 19/56 20/56 Step-and scan system (stepper) Ten Basic Steps of Photolithography 1. Surface Preparation 2. Photoresist Application 3. Soft Bake 4. Align & Expose * 5. Develop 6. Hard Bake 7. Inspection 8. Etch 9. Resist Strip 10. Final Inspection * Some processes may include a Post-exposure Bake

6 1. Surface Preparation (HMDS vapor prime) 21/56 HEXAMETHYLSILIZANE (HDMS) Dehydration 22/56 Dehydration bake in enclosed chamber with exhaust Clean and dry wafer surface (hydrophobic) Hexamethyldisilazane (HMDS) Temp ~ C Time ~ 60 sec. HMDS Adhesion promotion by HDMS 23/56 24/56 2. Photoresist Application 2. Photoresist Application Wafer held onto vacuum chuck dispenser Dispense ~5ml of Slow spin ~ 500 rpm Ramp up to ~ rpm Quality measures: time speed thickness uniformity particles & defects vacuum chuck to vacuum pump spindle Resist spinning thickness T depends on: Spin speed Solution concentration Molecular weight (measured by intrinsic viscosity) In the equation for T, K is a calibration constant, C the polymer concentration in grams per 100 ml solution, the intrinsic viscosity, and the number of rotations per minute (rpm) nce the various exponential factors (, and ) have been determined the equation can be used to predict the thickness of the film that can be spun for various molecular weights and solution concentrations of a given polymer and solvent system T Resist Sample Extra resist at the edges

7 25/56 26/56 3. Soft Bake 4. Alignment and Exposure Partial evaporation of solvents Improves adhesion Improves uniformity Improves etch resistance Improves linewidth control ptimizes light absorbance characteristics of Transfers the mask image to the resistcoated wafer Activates photosensitive components of Quality measures: linewidth resolution overlay accuracy particles & defects UV Light Source Mask Resist 27/56 28/56 4. Alignment and Exposure 4. Alignment and Exposure Alignment errors (many different types) Mask aligner equipment Double sided alignment especially important in micromachines

8 5. Develop 29/56 CLEAN RM TRAINING 6. Hard Bake Semester II 30/17 Soluble areas of are dissolved by developer chemical Visible patterns appear on wafer windows islands Quality measures: line resolution uniformity particles & defects developer dispenser Evaporate remaining Improve adhesion Higher temperature than soft bake vacuum chuck to vacuum pump spindle CLEAN RM TRAINING Semester II 31/17 CLEAN RM TRAINING Semester II 32/17 7. Development Inspection 8. Plasma Etch-r Add Layer ptical or SEM metrology Quality issues: particles defects critical dimensions linewidth resolution overlay accuracy Selective removal of upper layer of wafer through windows in : subtractive Two basic methods: wet acid etch dry plasma etch Quality measures: defects and particles step height selectivity critical dimensions Adding materials (additive) Two main techniques: Sputtering evaporation CF 4 Plasma

9 CLEAN RM TRAINING Semester II 33/17 CLEAN RM TRAINING Semester II 34/17 9. Photoresist Removal (strip) 10. Final Inspection No need for following etch process Two common methods: wet acid strip dry plasma strip Followed by wet clean to remove remaining resist and strip byproducts 2 Photoresist has been completely removed Pattern on wafer matches mask pattern (positive resist) Quality issues: defects particles step height critical dimensions Plasma 35/56 36/56 Summary Positive Resist Tone Part i Concept of optical lithography Resists Associated processes Miniaturization Limits optical lithography Chrome island on glass mask Exposed area of Ultraviolet Light oxide silicon substrate Positive Lithography Shadow on Areas exposed to light become photosoluble. oxide silicon substrate Island Resulting pattern after the resist is developed. Window

10 Photoresists 37/56 Photoresist profiles 38/56 Photoresist profiles vercut (LIFT-FF) Vertical Undercut Dose : High Developer: Low Dose : Medium Developer: Moderate Dose : Low Developer: Dominant 39/56 40/56 Photoresist Material Parameters (requirements) Positive tone : DQN ptical Mechanical/Chemical Process related Resolution Photosensitivity Refractive Index Viscosity Adhesion Etch resistance Thermal stability Cleanliness (particle count) Metal Content Shelf life Toxicity Stability to process variations Resin (N) / sensitizer(dq) N: phenolic Novolak resin: low molecular weight polymer. Forms the resists films properties. It dissolves in presence of water. DQ (Photoactive siazoquinone ester) Photosensitive, insoluble in aqueous solution. Prevents the resin to be dissolved Upon exposure to light, the dizaoquinones photochemically decompose

11 41/56 42/56 Example: AZ 1500 Photoresists Positive tone : PMMA PMMA: poly(methylmethacrylate) Chain scission under DUV exposition Also suitable for electron-beam lithography 43/56 44/56 Example: nano-pmma Negative tone Resin: Cyclic Synthetic rubber (non radiaton sensitive, strongly soluble in the solvent) PAC is a bis-arylazide. Upon exposure, it dissociates into nitrene and N 2. The nitrene reacts with the rubber molecules so that a cross linking between resin molecules occurs, becoming unsoluble.

12 45/56 46/56 Example: AZ-N4035 Epoxy based negative Negative s become insoluble in developing solutions when exposed to optical radiation H 3C CH CH CH CH C CH 3 H 3C C CH 3 H 3C C CH 3 H 3C C CH 3 CH n exposure the PAG generates a strong acid Protons attack oxygen on some epoxides CH CH CH CH Crosslinking occurs during PEB resulting in an insoluble very dense polymer network SU-8 is a commercial name for a fixed formulation. Any variation of this formulation becomes a very similar resist, but as it is not exactly SU-8, the variations are called epoxy based resists. 47/56 Example: SU-8