5. Lithography. 1. photolithography intro: overall, clean room 2. principle 3. tools 4. pattern transfer 5. resolution 6. next-gen

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1 5. Lithography 1. photolithography intro: overall, clean room 2. principle 3. tools 4. pattern transfer 5. resolution 6. next-gen References: Semiconductor Devices: Physics and Technology. 2 nd Ed. SM Sze. Ch 12 Fundamental of Semiconductor Fabrication. GS May & SM Sze. Ch 4 1

2 Photolithography process 2

3 Clean room: lithography must be carried out in a clean-room environment [A] because the presence of dust particles [B] will lower yields. [A] yield = # good chips / # total chips [B] 1. Pinhole 2. Current constriction 3. S/C or O/C 3

4 "clean" quantified: Figure Particle-size distribution curve for English (---) and metric ( ) classes of clean rooms. 4 ENGLISH System Class 100 = 100 particles/ft 3 (particle size >= 0.5 µm) or particles / m 3 (particle size >= 0.1 µm) cleanliness cost trade-off 1 ft = m 1 ft 3 = m 3 4

5 Clean room & HEPA filter HEPA = high efficiency particulate air. 5

6 Lithography 1. photolithography 2. principle shadow, projection 3. tools 4. pattern transfer 5. resolution 6. next-gen 6

$yield (poor resolution due to diffraction) yield-resolution trade-off CD λg g λ λ (µm) g (µm) CD (µm) 0.4 50 4.5 0.$

7 Shadow Contact: dust particles can be transferred between mask and wafer low yield Proximity: dust not transferred high yield (poor resolution due to diffraction) yield-resolution trade-off CD λg g λ λ (µm) g (µm) CD (µm)

annual-field wafer scan, (b) 1:1 step-and-repeat, (c)

8 Projection Stepper (step-and-repeat system) Image partitioning techniques for projection printing: (a) annual-field wafer scan, (b) 1:1 step-and-repeat, (c) M:1 reduction step-and-repeat, and (d) M:1 reduction step-and-scan. 8

9 1. photolithography 2. principle 3. tools light source mask photoresist 4. pattern transfer 5. resolution 6. next-gen Lithography 9

10 Pre-1990: high-pressure mercury-arc lamp Mercury (Hg) source line λ(nm) G 436 H 405 I 365 source λ(nm) resolution (nm) Hg KrF ArF F Hg 10

11 Light source roadmap CMOS scaling necessitates the development of low-λ light source 11

12 1. photolithography 2. principle 3. tools light source mask photoresist 4. pattern transfer 5. resolution 6. next-gen Lithography ITRS2009. Table LITH5A 12

13 Mask How to make masks? Mask blank electron-beam lithography IC Mask (reticle) [Cr / SiO 2 ] [CAD] info, alignment marks, test structures, etc. alignment marks test structures 13

14 Yield see fig. [B] slide #3 Yield for a 10-mask lithographic process with various defect densities per level. the yield of a lithographic step depends critically on mask cleanliness, or how many fatal defects are presence: In order to improve yield, masks are cleaned / inspected regularly 14

15 1. photolithography 2. principle 3. tools light source mask photoresist 4. pattern transfer 5. resolution 6. next-gen Lithography source: ITRS2011, Table LITH3A 15

It is called photo-resist because: photo it responds to photons resist it resists

16 Photoresist There are two types of resists: positive and negative. The final pattern on wafer is the same as (+) or opposite to ( ) those on mask. It is called photo-resist because: photo it responds to photons resist it resists etching / dopant atoms, protecting the layer underneath Suppliers: AZ, Shipley, Sumitomo photo resist 16

The quality of the resist and the exposure / development process is measured by the steepness of the resist wall, or the contrast ratio: Figure 12.9.

17 The quality of the resist and the exposure / development process is measured by the steepness of the resist wall, or the contrast ratio: Figure Exposure-response curve and cross section of the resist image after development. 1 (a) Positive photoresist; (b) negative photoresist. source: ITRS2011, Table LITH3B 17

18 1. photolithography 2. principle 3. tools 4. pattern transfer 5. resolution 6. next-gen Lithography 18

Pattern transfer transfers the pattern which appears on the photoresist onto the substrate (Si). The transfer process can be subtractive [A], or additive [B] in nature.

19 Pattern transfer transfers the pattern which appears on the photoresist onto the substrate (Si). The transfer process can be subtractive [A], or additive [B] in nature. [A] Subtractive: etching (for oxide removal, interconnect) [B] Additive: metallization (S/D metal, via filling) Lift-Off process Some process (such as doping through photoresist) is neither subtractive nor additive. 19

20 source: ITRS2011, Fig. LITH1 20

21 Lithography 1. photolithography 2. principle 3. tools 4. pattern transfer 5. resolution limit enhancement (PSM, OPC, Immersion) 6. next-gen 21

22 Resolution of photolithography minimum printed line / feature size / resolution (l m ) trigonometry: λ l m = k1 NA NA = nsinθ DOF = k 2 λ ( NA) 2 NA: numerical aperture DOF: depth of focus Q) How to improve resolution (l m )? A) λ, NA Requirements: l m,, DOF 22

$Diffraction diffraction is the limiting factor in printing two$

23 Diffraction diffraction is the limiting factor in printing two closely-spaced lines. Diffraction strong when L ~< λ mask wafer λ L L > λ L λ 23

24 Lithography 1. photolithography 2. principle 3. tools 4. pattern transfer 5. resolution limit enhancement (PSM, OPC, Immersion) 6. next-gen 24

25 source: ITRS2009, Table LITH1 25

Resolution Enhancement Techniques A schematic of an optical lithography system showing the methods that can improve the technique's performance.

26 Resolution Enhancement Techniques A schematic of an optical lithography system showing the methods that can improve the technique's performance. Key: λ is the wavelength of the illumination source; OAIis offaxis illumination; IIL is imaging interferometric illumination; OPC is optical proximity correction; and PSM is phase-shift masks. Link 26

27 Phase-Shift Mask (PSM) diffraction intensity d = λ/2(n-1) Figure The principle of phase-shift technology. (a) Conventional technology; (b) phase-shift technology. 9 27

28 28

Optical Proximity Correction (OPC) a purely mathematical process

application system for CD scanning electron microscopy (CD-SEM),

Proteus customers to seamlessly obtain a large sampling of

29 Optical Proximity Correction (OPC) a purely mathematical process The automated algorithmic link between DesignGauge, the application system for CD scanning electron microscopy (CD-SEM), and Proteus OPC for pre-processing OPC model building data allows Proteus customers to seamlessly obtain a large sampling of metrology data to account for process variations across the entire process window. 29

30 Immersion λ l m = k1 NA NA = nsinθ use n > 1 30

31 source: ITRS2011, Table LITH2 31

$High-Index Lenses Push Immersion Beyond 32 nm Aaron Hand -- Semiconductor International, 4/1/2006 water's refractive index (n) = 1.$

32 High-Index Lenses Push Immersion Beyond 32 nm Aaron Hand -- Semiconductor International, 4/1/2006 water's refractive index (n) = 1.44 high-index fluids n~1.65 photoresist n = 1.7 lens (n=1.56) becomes the limiter: calcium fluoride (CaF2) higher-index lens materials: lutetium aluminum garnet (LuAG), n =

33 33

34 Lithography 1. photolithography 2. principle 3. tools 4. pattern transfer 5. resolution 6. next-gen EUV, XRL, EBL, IBL, NIL 34

35 source: ITRS2011, Fig. LITH3A source: ITRS2011, Fig. LITH3B 35

36 EUV: extreme-ultra violet lithography In an laser produced plasma (LPP)-based system, EUV light is produced by bombarding a sliver of tin with a high-power laser. The light is then gathered by specially engineered EUV mirrors, which then focus an EUV beam in the EUV scanner to produce microchip patterns. ITRS2009: Table LITH5C 36

37 XRL: X-ray lithography 37

38 EBL: electron-beam lithography Schematic of an electron-beam machine. Direct-write (serial) Figure Schematic of positive and negative resists used in electron-beam lithography. 38

39 EBL: parallel Basic SCALPEL principle of operation showing contrast generation by differentiating more- or less-scattered electrons. Scattering with angular limitation projection electron beam lithography system R SiN x nm small scattering angle L Cr/W 30-60nm large scattering angle L 100 kev R 39

40 IBL: ion-beam lithography IBL: Trajectories of 60 kev H* ions traveling through PMMA into Au, Si, and PMMA. 18 EBL: Proximity effect: (a) Simulated trajectories of 100 electrons in PMMA for a 20-keV electron beam. 15 (b) Dose distribution for forward scattering and backscattering at the resist-substrate interface. 40

41 NIL: Nano-imprint lithography ITRS2009: Table LITH5D 41

42 42

43 photolithography process, principle, tools resolution limits Conclusions resolution-enhancement techniques PSM, OPC, Immersion, DP, future lithographic systems EUV, XRL, ML2 (EBL, IBL), NIL,... from ITRS2009:... To continue as the dominant technique for leading-edge critical layer lithography, resolution enhancement techniques (RETs) such as off-axis illumination (OAI), phase shifting masks (PSMs), and optical proximity corrections (OPCs) are being used with imaging systems at the 193 nm wavelength. In addition to RETs, lenses with increasing numerical apertures and decreasing aberrations will be required to extend the life of optical lithography. Liquid immersion imaging with a fluid between the final lens element and the wafer is also being used to extend optical lithography. 43

Section 2: Lithography. Jaeger Chapter 2 Litho Reader. The lithographic process

Section 2: Lithography. Jaeger Chapter 2 Litho Reader. The lithographic process Section 2: Lithography Jaeger Chapter 2 Litho Reader The lithographic process Photolithographic Process (a) (b) (c) (d) (e) (f) (g) Substrate covered with silicon dioxide barrier layer Positive photoresist