Microlithography. Dale E. Ewbank ul ppt. Microlithography Dale E. Ewbank page 1

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1 Dale E. Ewbank ul ppt 2014 Dale E. Ewbank page 1

2 OUTLINE Masks Optical Lithography Photoresist Sensitivity Processing Exposure Tools Advanced Processes page 2

3 MICROLITHOGRAPHY The production of ultra-small three dimensional relief images based on exposure and subsequent development of a photon sensitive polymer called photoresist. DRAM EXPOSURE LITHO YEAR FEATURE WAVELENGTH TOOL nm 436 nm $0.8 MILLION $ $ $ $ $ $ or 193i $ i double patterning $ ? 16 EUV $150 Micro/Nanolithography page 3

4 Optics Comparison ArF Optics Column 1)Source beam refined through lenses 2)Transmitted through pupil blade 3)Finally through reticle and down to wafer plane EUV Optics System 1)Source Beam passes through spectral purity filter 2)Transmitted through mirrors to Reticle 3)Finally Reflected onto wafer plane [1 ]

5 Corning Museum Of Glass Tropel g-line lens built for GCA steppers page 5

6 MASKS Reduction reticle with pellicle: ASML, Canon, Nikon, others 1X projection with pellicle: Perkin Elmer Micralign page 6

7 TRANSMISSION PROPERTIES OF OPTICAL GLASS page 7

8 Ultra Violet Radiation 193 nm ArF laser 248 nm KrF laser 365 nm i-line 436 nm g-line page 8

9 Optical Lithography Illumination System Source Illumination System Source Condenser Lens Condenser Lens Mask Gap (z) Mask Substrate Objective Lens Proximity/Contact Projection NA = n sinq q Substrate page 9

10 DIFFRACTION Bruce W. Smith page 10

11 FRESNEL DIFFRACTION Proximity/Contact System Resolution Wmin ~ 0.7 ( Z) 1/2 W = space opening width on the mask = exposing wavelength Z = gap (distance) from the mask to the wafer Note that Z cannot go be zero because of resist thickness page 11

12 DIFFRACTION and Imaging Bruce W. Smith page 12

13 RAYLEIGH CRITERIA Projection System Resolution P/2 = k 1 / NA NA = n sinq DOF = +/- k 2 / (n sin 2 q) k 2 = 0.5 g-line i-line KrF KrF ArF ArF ArF k [nm] i 193i NA P/2 [nm] DOF[+/- nm] Micro/Nanolithography page 13

14 Increasing Resolution Using Immersion Imaging E. Abbe 1878 Bruce W. Smith page 14

15 Increasing Resolution Using Immersion Imaging E. Abbe 1878 Bruce W. Smith page 15

16 Increasing Resolution Using Immersion Imaging E. Abbe 1878 Bruce W. Smith page 16

17 Homogeneous Immersion Increasing refractive indices the defocus effect Low index (air) imaging High index imaging Defocus OPD sin 2 q a b The defocus wave aberration is proportional to sin 2 q Higher indices reduce defocus OPD at equivalent NA values Small NA/n is desirable sinq Bruce W. Smith page 17

18 NEGATIVE AND POSITIVE PHOTORESISTS Light Photomask Photoresist Film to be Patterned Substrate Negative Resist Positive Resist Rendered Insoluble Rendered Soluble After Development After Etch and Strip page 18

19 ONE AND TWO COMPONENT RESIST HIERARCHY Positive Resist Chemically Amplified Resist Components Components ONE TWO TWO Acrylates (PMMA) Diazo naphtho quinone Novolac (Shipley 1400) Acid Generator -- Onium salt Polymer backbone -- Polystyrene page 19

20 NOVOLAC RESIN n Binder--Novolac Sensitizer--Diazonaphthoquinone Solubility--organic solvents Image formation--solubility change Chemical resistance--excellent Adhesion--good Developer--alkaline Resolution--to less then 0.25 microns page 20

21 PHOTOACTIVE COMPOUND (PAC) O N 2 C O R Naphthoquinone diazide (PAC) hu R Keto-carbene and Nitrogen O.. + N 2 R R Stable ketene molecule add H 2 O H O C OH Base soluble carboxylic acid page 21

22 SENSITIVITY Wavelength (nm) page 22

23 POSITIVE NOVOLAC PHOTORESIST Novolac Base Matrix Resin % Sensitizer or Photoactive Compound (PAC) % Casting Solvent % Additives - 10% PAC - such as naphthoquinone diazide Solvents - such as EL (Ethyl Lactate) or PGMEA (Propylene Glycol Monomethyl Ether Acetate) Additives - such as adhesion promoters, surfactants, dyes, antioxidants, polymerization inhibitors Unexposed resists dissolves in an alkali developer at a rate of 0.2 nm/sec Partially exposed resists dissolves in an alkali developer at a rate of 200 nm/sec page 23

24 Deep ULTRA VIOLET RADIATION page 24

25 CHEMICALLY AMPLIFIED RESIST Poly(t-butoxycarbonyloxystyrene) with onium salt cationic photoinitiator for acid generation page 25

26 THICKNESS LOG DOSE g Dose to clear The higher the slope, gamma, then the smaller the difference needs to be between exposure in areas to be cleared and areas to leave resist. That is the required aerial image modulation is smaller. page 26

27 MODULATION Mask m(x) Modulation = Imax - Imin Imax + Imin Aerial image I(x) 1 Imax Imin 0 Ideal Actual Wafer page 27

28 PROCESSING Substrate Cleaning Coat track Priming if needed BARC coat and bake Spin Coating Photoresist Soft-Bake (PAB) Top Coat and bake if needed Stepper Exposure Develop track Post Exposure Bake (PEB) Develop Rinse Hard-Bake (PDB) Etching Stripping page 28

29 SUBSTRATE CLEAN AND PRIME Cleaning is done with a high pressure (2000 psi) water scrub A dehydration bake is typically done on a hot plate at 250 C for 1 min. (Wafers are clean and dry just after removing from oxide growth furnace) HMDS (hexa-methyl-di-silizane), - Adhesion promoter or primer: Are commonly applied as a liquid or vapor. HMDS attaches to remaining OH molecules releasing ammonia gas and creating an organic-like surface improving adhesion Too much HMDS is detrimental to sensitivity and adhesion. page 29

30 HMDS PRIME page 30

31 SPIN COATING Most spin coating is performed at spin speeds from 3000 to 6000 RPM for 20 to 60 seconds, producing coating uniformities to +/- 100 Å page 31

32 SOFT-BAKE The main purpose is to reduce the solvents from a level of 20-30% down to 4-7%. Baking in a convection oven about 20 minutes is equivalent to hot plate baking for about 1 minute. Forced Air Oven Exhaust Photoresist wafer Hot Plate Fan 90 TO 100 C page 32

33 EXPOSURE E = I t where E is exposure dose in mj/cm 2 I is irradiance in mw/cm 2 t is exposure time in seconds Humidity should be 40-44% because exposed PAC requires water to convert to carboxylic acid page 33

34 POST EXPOSURE BAKE Post exposure bake increases speed of resist Standing wave effects Post exposure bake is require for chemically amplified and image reversal resists (100 to 115 C for 1 min.) Micro/Nanolithography page 34

35 Relative Exposure Positive Resist Processing d= m / (2 n) 1.2 Relative exposure versus resist thickness Thickness, d Wavelength, Refractive index, n Number of waves, m Resist thickness [nm] page 35

36 DEVELOP AND RINSE Develop is done in an alkali solution such as NaOH or KOH (Metal Containing Developers) Trace quantities of these metals can cause transistor threshold voltage shifts. Metal Ion Free Developers are available (TMAH) Developer Concentration and Temperature of Developer are the most important parameters to control. page 36

37 ETCHING Isotropic Etching - etches at equal rate in all directions Oxide wet etched Photoresist Wet Chemical Etching - is isotropic Anisotropic Etching - etches faster vertically than horizontally Plasma Etching (Dry Etch or Reactive Ion Etching, RIE) is either isotropic or anisotropic depending on ion energy and chemistry of etch. page 37

38 STRIPPING Oxygen Plasma Ashing Plasma Damage is Possible to sensitive gate oxide layers Hot Sulfuric Acid and Hydrogen Peroxide If underlying layers are not etched by these chemicals Organic Solvents are available page 38

39 ASML 5500/200 NA = 0.48 to 0.60 variable = 0.35 to 0.85 variable With Variable Kohler, or Variable Annular illumination Resolution = K1 /NA = ~ 350 nm for NA=0.6, =0.85 Depth of Focus = k 2 /(NA) 2 = > 1.0 µm for NA = 0.6 i-line Stepper = 365 nm 22 x 27 mm Field Size page 39

40 STEPPER UV-Light Source Illumination Diffuser Filter Condenser Lens, NA C Reticle Alignment Motors Fiducial Marks Reticle Thru Lens Alignment Detector Aperture Blades Alignment Microscope and TV Zeeman Two Frequency Laser Interferometer 5X Reduction Lens, NA O Auto Focus Stage Motors Reference Mirror X Y Baseline Auto Leveling X-Y Stage page 40

41 STEP AND SCAN page 41

42 ADVANCED PROCESSES Tri-layer Process Bi-layer Process Lift-off Process Reversal Process Dyed Resists Anti-reflective Coatings Contrast Enhancement Materials Chemically Amplified Resists Silylation Process page 42

43 TRI-LAYER Film to be Etched Substrate with Topology Coat with Planarizing Layer Coat with Barrier Layer Coat with Photoresist Image Photoresist Etch Barrier Layer Reactive Ion Etch Planarizing Layer Etch Film page 43

44 SILYLATION Film to be Etched Polymerized Areas Coat with Planarizing Layer of Photoresist SiO2, 10% in Photoresist Expose Desired Pattern causing exposed areas to become polymerized Soak in HMDS Vapor Note: HMDS is incorporated into the non polymerized areas only Reactive Ion Etch in Oxygen coverts silicon into SiO2, Only polymerized areas are etched. Silicon containing areas form and in-situ mask After Reactive Ion Etch Etch Film page 44

45 LIFT-OFF 1. Create a reverse slope or undercut resist edge profile 2. Deposit film by evaporation 3. Chemically strip photoresist and lift off film, leaving film in desired pattern Substrate Photoresist Film page 45

46 REVERSAL PROCESS No Extra Processing Normal Process After Development Photoresist Substrate Coat Expose Reversal Process Post Exposure Reversal Bake Flood Expose After Development page 46

47 REFERENCES Science and Technology, Second Edition, edited by Kazuaki Suzuki and Bruce W. Smith, CRC Press, Introduction to Micro/Nanolithography, Second Edition, Edited by Larry F. Thompson, C.Grant Willson and Murrae J. Bowden, ACS Professional Reference Book, American Chemical Society, Washington, DC Micro/Nanolithography, David Elliott, McGraw Hill Book Company, IC/hbo_ic.pdf, accessed 03/13/ accessed 06/10/ accessed 03/14/2011. Micro/Nanolithography page 47