Lithography 3 rd lecture: introduction Prof. Yosi Shacham-Diamand Fall 2004 1
List of content Fundamental principles Characteristics parameters Exposure systems 2
Fundamental principles Aerial Image Exposure Development Critical dimension (CD) control 3
Lithography characterization Post exposure characterization Line-width control, photo-resist profile, resolution, process window Registration Process compatibility Etch resistance, thermal stability, adhesion, chemical compatibility, resist removal Manufacturability Cost, safety, defects, stability, shelf life 4
Lithography: information transfer process Mask Design Optical image Latent image in the photo-lithographic material Image development Transfer onto the wafer 5
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Image transfer from the mask to the wafer UV light source Shutter Alignment laser Shutter is closed during focus and alignment and removed during wafer exposure Reticle (may contain one or more die in the reticle field) Single field exposure, includes: focus, align, expose, step, and repeat process 7 Figure 14.1 Projection lens (reduces the size of reticle field for presentation to the wafer surface) Wafer stage controls position of wafer in X, Y, Z, θ
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Typical CMOS masks 1) STI etch 2) P-well implant 3) N-well implant 4) Poly gate etch 5) N + S/D implant 6) P + S/D implant 7) Oxide contact etch 8) Metal etch Resulting layers 4 2 1 5 6 3 7 8 Cross section 9 Top view
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Exposure systems Mask illumination from the backside The light interacts with the mask The electromagnetic wave reaches the lens The lens collects part of the light and forms and image on the wafer There is an information loss due to the finite size of the lens 20
Light an electromagnetic wave phenomena λ = v f λ v = velocity of light, 3 x10 8 m/sec f = frequency in Hertz (cycles per second) λ = wavelength, the physical length of one cycle of a frequency, expressed in meters Laser 21
Constructive Diffraction Destructive A Destructive Waves in phase Waves Waves out of phase out of phase B A+B 22
Optical filtering Broadband light Reflected wavelengths Coating 1 (non-reflecting) Coating 2 Secondary reflections (interference) Coating 3 Glass 23 Transmitted wavelength
UV Spectrum λ (nm) Ultraviolet spectrum Visible spectrum EUV VUV DUV Mid-UV Violet Blue Green YellowOrange Red 4 50 100 150 200 250 300 350 400 450 500 550 600 650 700 13 126 157 193 248 365 405 436 i h g Excimer laser Mercury lamp 24 Photolithography light sources
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High pressure Hg lamp 120 100 Emission spectrum of high-intensity mercury lamp i-line 365 nm Relative Intensity (%) 80 60 40 20 0 DUV 248 nm h-line 405 nm g-line 436 nm 26 200 300 400 500 600 Wavelength (nm)
High pressure Hg lamp UV Light Wavelength (nm) Descriptor CD Resolution (µm) 436 g-line 0.5 405 h-line 0.4 365 i-line 0.35 248 Deep UV (DUV) 0.25 27
Hg lamp vs. Excimer laser 100 KrF laser Relative Intensity (%) 80 60 40 20 Hg lamp 0 210 220 240 260 280 28 Wavelength (nm)
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Resist UV absorption Photoresist (after develop) Sloping profile Substrate 30
Eximer lasers for lithography Material Wavelengt h (nm) Max. Output (mj/pulse) Frequency (pulses/sec) Pulse Length (ns) CD Resolution (µm) KrF 248 300 1500 500 25 0.25 ArF 193 175 300 400 15 0.18 F 2 157 6 10 20 0.15 31
Spatial Coherence Incoherent light source Black box illuminator of a single wavelength Slit Two slits closely spaced Interference patterns Coherent cylindrical wave front Two coherent cylindrical wave fronts 32
Optical lithography Optics Reflection Refraction Lens Diffraction Typical parameters 33
Reflection laws The angle of incidence of a light wavefront with a plane mirror is equal to the angle of reflection. Incident light θ i θ r Reflected light 34 Law of Reflection: θ i = θ r
Reflection lithography Flat mirror Illuminator for a simple aligner Ellipsoidal mirror Mask Flat mirror 35
Refraction Snell s Law: sin θ i = n sin θ r Index of refraction, n = sin θ i / sin θ r fast medium air (n 1.0) slow medium glass (n = 1.5) glass (n = 1.5) θ slow medium air (n 1.0) θ fast medium 36
Refraction index Material Index of Refraction (n) Air 1.000293 Water 1.33 Fused Silica (Amorphous Quartz) 1.458 Diamond 2.419 37
Lens based lithography Illuminator assembly Optical filter Shutter Mercury lamp Masking unit Condenser lens Fly s eye lens Flat mirror Light sensor Mirror Condenser lens Mirror Collimator lens Reticle Reticle stage (X, Y, θ) Lamp monitor Lamp position knob Ellipsoidal mirror Fiber optics X-drive motor Projection optics Optical focus sensor Interferometer mirror θ-z drive stage Y-drive motor Vacuum chuck Wafer stage assembly 38
Converging lens 2f f f = focal length F = focal point S = 2f O = origin, center of lens S Object Real F O F image S 39
Diverging lens S Object Virtua l image f = focal length F = focal point S = 2f O = origin, center of lens F O F S 40
Laser-Induced Lens Compaction Compacted area of lens 41
Interference Pattern from Light Diffraction at Small Opening Light travels in straight lines. Diffraction occurs when light hits edges of objects. Diffraction bands, or interference patterns, occur when light waves pass through narrow slits. 42 Diffraction bands
Diffraction in a Reticle Pattern Diffracted light rays Slit Plane light wave 43
44 Bragg s condition for constructive diffraction
Lens Capturing Diffracted Light Quartz UV Mask Chrome Diffraction patterns 4 4 3 2 1 1 2 3 0 Lens 45
Effect of Numerical Aperture on Imaging Pinhole masks Lens NA Image results Bad Exposure light Poor Good 46 Diffracted light
Image formation - example Coherent illumination Quartz plate with chromium pattern with a periodicity (pitch) of p. θn Bragg s law p sin(θ n ) = nλ 47
The lens Lens with diameter D & distance f from the focal plane NA = nsin( α) D 2 f 48 Mask Focal plane
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Typical NA Values for Photolithography Tools Type of Equipment Scanning Projection Aligner with mirrors (1970s technology) NA Value 0.25 Step-and-Repeat 0.60 0.68 Step-and-Scan 0.60 0.68 54
What determines the resolution? The resolution is proportional to the wavelength, λ The resolution is proportional to the numerical aperture, NA; larger lens collects more information. Re solution = k 1 λ NA 55
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57 Definition of line-width (L) and line-spacing (S)
Image contrast Intensity, I 1 I max I min 0 Image contrast = I I max max - I + I min min 58
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The limitations of the optical contrast as a figure of merit It represents only an image of a simple pattern with lines and spaces Not practical for large or complex features Too sensitive to the minimum intensity than the real image Low correlation to the lithography quality. 61
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Photoresist Reflective Notching Due to Light Reflections UV exposure light Mask Edge diffraction Exposed photoresist Polysilicon Unexposed photoresist Notched photoresist Surface reflection STI Substrate STI 63
Incident and Reflected Light Wave Interference in Photoresist Incident wave Reflected wave Photoresist Film Substrate 64 Standing waves cause nonuniform exposure along the thickness of the photoresist film.
65 Standing waves in the resist
Solution Anti Reflective Coating (ARC) Incident wave Antireflective coating Photoresist Film Substrate 66 The use of antireflective coatings, dyes, and filters can help prevent interference.
BARC - Bottom Antireflective Coating UV exposure light Mask 67 Exposed photoresist Polysilicon STI Unexposed photoresist BARC Substrate STI
BARC Phase-Shift Cancellation of Light (A) Incident light (B) Top surface reflection (C) (D) Photoresist BARC (TiN) Aluminum C and D cancel due to phase difference 68
Top Antireflective Coating Incident light Resist-substrate reflections Incident light Top antireflective coating absorbs substrate reflections. Photoresist Photoresist Substrate reflection Substrate Substrate 69
Optical Lithography Resolution Calculating Resolution Depth of Focus Resolution Versus Depth of Focus Surface Planarity 70
Resolution of Features 0.5 0.25 0.1 1.0 The dimensions of linewidths and spaces must be equal. As feature sizes decrease, it is more difficult to separate features from each other. 71 2.0
Calculating Resolution for a given λ, NA and k k = 0.6 Illuminator, λ R = k λ NA Mask Lens, NA Wafer R i-line DUV λ ΝΑ R 365 nm 0.45 486 nm 365 nm 0.60 365 nm 193 nm 0.45 257 nm 193 nm 0.60 193 nm 72
Depth of Focus (DOF) Lens Center of focus - Depth of focus Photoresist + Film 73
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Example to focal plane problems due to a non-planar surface 76
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Resolution Versus Depth of Focus for Varying NA DOF = λ 2(NA) 2 Illuminator, λ Mask i-line DUV λ ΝΑ R DOF 365 nm 0.45 486 nm 901 nm 365 nm 0.60 365 nm 507 nm 193 nm 0.45 257 nm 476 nm 193 nm 0.60 193 nm 268 nm Lens, NA Center of focus - Depth of focus Photoresist Wafer DOF + Film 80
New approaches super resolution imaging 81