1 Photolithography II ( Part 2 ) Chapter 14 : Semiconductor Manufacturing Technology by M. Quirk & J. Serda Saroj Kumar Patra, Department of Electronics and Telecommunication, Norwegian University of Science and Technology ( NTNU )
2 Objectives of this Lecture 1. State and explain the critical aspects of optics for optical lithography. Reflection of Light Refraction of Light Lens Diffraction Numerical Aperture, NA Antireflective Coating 2. Explain resolution, describe its critical parameters, and discuss how it is calculated.
3 Ten steps of Photolithography UV Light HMDS Resist Mask 1-3) Vapor prime 4) Spin coat 5) Soft bake 6) Alignment and Exposure 7) Post-exposure bake (PEB) 8) Develop 9) Hard bake 10) Develop inspect
4 Laws of Reflection The angle of incidence of a light wavefront with a plane mirror is equal to the angle of reflection. Incident light i r Reflected light Law of Reflection: i r Figure 14.11 Quirk & Serda
5 Application of Mirrors Flat mirror Illuminator for a simple aligner Ellipsoidal mirror Mask Important for uniform illumination of the mask Flat mirror Used with permission from Canon USA Figure 14.12 Quirk & Serda
6 Refraction of Light based on two mediums Snell s Law: sin i = n sin r Index of refraction, n = sin i / sin r Speed of light: c = c 0 / n fast medium air (n 1.0) slow medium glass (n 1.5) glass (n 1.5) slow medium air (n 1.0) fast medium Figure 14.13 Quirk & Serda
7 Absolute Index of Refraction for selected materials Material Index of Refraction (n) Air 1.000293 Water 1.33 Fused Silica (Amorphous Quartz) 1.458 Diamond 2.419 Table 14.4 Quirk & Serda
8 Converging Lens with Focal Point 2f f f = focal length F = focal point S = 2f O = origin, center of lens S Object Real F O F image S Figure 14.15 Quirk & Serda
9 Diverging Lens with Focal Point f = focal length F = focal point S = 2f O = origin, center of lens S Object Virtual image F O F S Figure 14.16 Quirk & Serda
10 Optical System of Lenses Masking unit Mirror Condenser lens Condenser lens Fly s eye lens Mirror Flat mirror Collimator lens Optical filter Shutter Mercury lamp Light sensor Reticle Reticle stage (X, Y, ) Lamp monitor Lamp position knob Ellipsoidal mirror Fiber optics X-drive motor Projection optics Optical focus sensor Interferometer mirror -Z drive stage Y-drive motor Vacuum chuck Wafer stage assembly Used with permission from Canon U.S.A., FPA-2000 i1 exposure system Figure 14.14
11 Lens Material 365 nm UV: glass (traditionally) 248 nm DUV: fused silica (less light absorption at DUV wavelengths) 193 nm DUV and 157 nm VUV: calcium fluoride (CaF 2 ) which is more transparent at these wavelengths then fused silica Absorption => loss in exposure power and induces heat in the optics, which leads to refractive index changes and imaging problems
12 Laser-Induced Lens Compaction => reduced image quality Compacted area of lens Figure 14.17 Quirk & Serda
13 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. Diffraction bands Figure 14.18 Quirk & Serda
14 Diffraction in a Reticle Pattern Diffracted light rays Slit Plane light wave Figure 14.19 Quirk & Serda
15 Lens Capturing Diffracted Light Quartz UV Mask Chrome Diffraction patterns 4 4 3 2 1 1 2 3 0 Lens Figure 14.20 Quirk & Serda
16 Numerical Aperture (NA) For a lens, the NA is a measure of how much diffracted light the lens can accept and image by converging the diffracted light to a single point. NA = (n) sin θ m (n) (radius of lens) / (focal length of lens) where, n = index of refraction of the image medium (n 1 for air) θ m = angle between the optical principal axis and the marginal ray at the edge of the lens
17 Effect of Numerical Aperture on Imaging Pinhole masks Lens NA Image results Bad Poor Good Diffracted light Figure 14.21 Quirk & Serda
18 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 Table 14.5 Quirk & Serda
19 Photoresist Reflective Notching Due to Light Reflections UV exposure light Mask Edge diffraction Exposed photoresist Unexposed photoresist Surface reflection Polysilicon Notched photoresist STI Substrate STI Figure 14.22 Quirk & Serda
20 Light Suppression (up to 99 %) with Bottom Antireflective Coating (BARC) UV exposure light Mask Exposed photoresist Polysilicon Unexposed photoresist BARC STI Substrate STI Figure 14.25 Quirk & Serda
21 Incident and Reflected Light Wave Interference in Photoresist Incident wave Reflected wave Photoresist Film Substrate Standing waves cause nonuniform exposure along the thickness of the photoresist film. Figure 14.23 Quirk & Serda
22 Effect of Standing Waves in Photoresist Photograph courtesy of the Willson Research Group, University of Texas at Austin Photo 14.1 Quirk & Serda
23 Antireflective Coating to Prevent Standing Waves Incident wave Antireflective coating 200 2000 Å Photoresist 1 μm Film Substrate The use of antireflective coatings, dyes, and filters can help prevent interference. Figure 14.24 Quirk & Serda
24 BARC Phase-Shift Cancellation of Light (A) Incident light (B) Top surface reflection (C) (D) Photoresist C and D cancel due to phase difference BARC (TiN) Aluminum Figure 14.26 Quirk & Serda
25 Top Antireflective Coating (TARC) Incident light Resist-substrate reflections Incident light Top antireflective coating absorbs substrate reflections. Photoresist Photoresist Substrate reflection Substrate Substrate Figure 14.27 Quirk & Serda
26 Antireflective Coatings (ARC) Organic ARC reduces reflection by absorbing light Inorganic ARC (e.g. TiN) work by phase-shift cancellation Organic ARC easier to remove than inorganic ARC (sometimes left to become part of the device) BARC in general more effective than TARC
27 Optical Lithography Resolution Calculating Resolution Depth of Focus Resolution Versus Depth of Focus Surface Planarity
28 Resolution of Features 0.5 0.25 0.1 1.0 2.0 The dimensions of linewidths and spaces must be equal. As feature sizes decrease, it is more difficult to separate features from each other. Figure 14.28 Quirk & Serda
29 Calculating Resolution for a given, NA and k k = 0.6 Illuminator, R = k NA Mask Lens, NA Wafer R i-line DUV 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 Figure 14.29 Quirk & Serda
30 Depth of Focus (DOF) Lens Center of focus - Depth of focus Photoresist + Film Figure 14.30 Quirk & Serda
31 Resolution Versus Depth of Focus for Varying NA DOF = 2(NA) 2 Illuminator, Mask i-line DUV 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 Figure 14.31 Quirk & Serda
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