Lithography. 3 rd. lecture: introduction. Prof. Yosi Shacham-Diamand. Fall 2004

Transcription

1 Lithography 3 rd lecture: introduction Prof. Yosi Shacham-Diamand Fall

2 List of content Fundamental principles Characteristics parameters Exposure systems 2

3 Fundamental principles Aerial Image Exposure Development Critical dimension (CD) control 3

4 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

5 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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7 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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9 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 Cross section 9 Top view

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20 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

21 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

22 Constructive Diffraction Destructive A Destructive Waves in phase Waves Waves out of phase out of phase B A+B 22

23 Optical filtering Broadband light Reflected wavelengths Coating 1 (non-reflecting) Coating 2 Secondary reflections (interference) Coating 3 Glass 23 Transmitted wavelength

24 UV Spectrum λ (nm) Ultraviolet spectrum Visible spectrum EUV VUV DUV Mid-UV Violet Blue Green YellowOrange Red i h g Excimer laser Mercury lamp 24 Photolithography light sources

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26 High pressure Hg lamp Emission spectrum of high-intensity mercury lamp i-line 365 nm Relative Intensity (%) DUV 248 nm h-line 405 nm g-line 436 nm Wavelength (nm)

27 High pressure Hg lamp UV Light Wavelength (nm) Descriptor CD Resolution (µm) 436 g-line h-line i-line Deep UV (DUV)

28 Hg lamp vs. Excimer laser 100 KrF laser Relative Intensity (%) Hg lamp Wavelength (nm)

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30 Resist UV absorption Photoresist (after develop) Sloping profile Substrate 30

31 Eximer lasers for lithography Material Wavelengt h (nm) Max. Output (mj/pulse) Frequency (pulses/sec) Pulse Length (ns) CD Resolution (µm) KrF ArF F

32 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

33 Optical lithography Optics Reflection Refraction Lens Diffraction Typical parameters 33

34 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

35 Reflection lithography Flat mirror Illuminator for a simple aligner Ellipsoidal mirror Mask Flat mirror 35

36 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

37 Refraction index Material Index of Refraction (n) Air Water 1.33 Fused Silica (Amorphous Quartz) Diamond

38 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

39 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

40 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

41 Laser-Induced Lens Compaction Compacted area of lens 41

42 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

43 Diffraction in a Reticle Pattern Diffracted light rays Slit Plane light wave 43

44 44 Bragg s condition for constructive diffraction

45 Lens Capturing Diffracted Light Quartz UV Mask Chrome Diffraction patterns Lens 45

46 Effect of Numerical Aperture on Imaging Pinhole masks Lens NA Image results Bad Exposure light Poor Good 46 Diffracted light

47 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

48 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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54 Typical NA Values for Photolithography Tools Type of Equipment Scanning Projection Aligner with mirrors (1970s technology) NA Value 0.25 Step-and-Repeat Step-and-Scan

55 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 57 Definition of line-width (L) and line-spacing (S)

58 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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61 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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63 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

64 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 65 Standing waves in the resist

66 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.

67 BARC - Bottom Antireflective Coating UV exposure light Mask 67 Exposed photoresist Polysilicon STI Unexposed photoresist BARC Substrate STI

68 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

69 Top Antireflective Coating Incident light Resist-substrate reflections Incident light Top antireflective coating absorbs substrate reflections. Photoresist Photoresist Substrate reflection Substrate Substrate 69

70 Optical Lithography Resolution Calculating Resolution Depth of Focus Resolution Versus Depth of Focus Surface Planarity 70

71 Resolution of Features The dimensions of linewidths and spaces must be equal. As feature sizes decrease, it is more difficult to separate features from each other

72 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 nm 365 nm nm 193 nm nm 193 nm nm 72

73 Depth of Focus (DOF) Lens Center of focus - Depth of focus Photoresist + Film 73

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76 Example to focal plane problems due to a non-planar surface 76

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80 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 nm 901 nm 365 nm nm 507 nm 193 nm nm 476 nm 193 nm nm 268 nm Lens, NA Center of focus - Depth of focus Photoresist Wafer DOF + Film 80

81 New approaches super resolution imaging 81