Photolithography II ( Part 1 )

Transcription

1 1 Photolithography II ( Part 1 ) Chapter 14 : Semiconductor Manufacturing Technology by M. Quirk & J. Serda Bjørn-Ove Fimland, Department of Electronics and Telecommunication, Norwegian University of Science and Technology ( NTNU )

2 2 Ten steps of Photolithography UV Light HMDS Resist Mask λ λ 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

3 3 Objectives After studying the material in this chapter, you will be able to: 1. Explain the purpose of alignment and exposure in photolithography. 2. Describe the properties of light and exposure sources important for optical lithography. 3. State and explain the critical aspects of optics for optical lithography. 4. Explain resolution, describe its critical parameters, and discuss how it is calculated. 5. Discuss each of the five equipment eras for alignment and exposure. 6. Describe reticles, explain how they are manufactured and discuss their use in microlithography. 7. Discuss the optical enhancement techniques for sub-wavelength lithography. 8. Explain how alignment is achieved in lithography.

4 4 Mask Aligners

5 5 Reticle Pattern Transfer to Resist UV light source Shutter Alignment laser Shutter is closed during focus and alignment and removed during wafer exposure Single field exposure, includes: focus, align, expose, step, and repeat process Reticle (may contain one or more die in the reticle field) 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) Figure 14.1 Quirk & Serda

6 6 Three Functions of Wafer Stepper 1. Focus and align the quartz plate reticle (that has the patterns) to the wafer surface. 2. Reproduce a high-resolution reticle image on the wafer through exposure of photoresist. 3. Produce an adequate quantity of acceptable wafers per unit time to meet production requirements.

7 7 Layout and Dimensions of Reticle Patterns 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 Top view Figure 14.2 Quirk & Serda

8 8 Optical Lithography Light Interference of Light Waves Optical Filters Electromagnetic Spectrum

9 9 Light Wavelength and Frequency λ = v f λ v = velocity of light, m/sec f = frequency in Hertz (cycles per second) l = wavelength, the physical length of one cycle of a frequency, expressed in meters Laser Figure 14.3 Quirk & Serda

10 10 Wave Interference Constructive Destructive Waves in phase A Waves out of phase B A+B Figure 14.4 Quirk & Serda

11 11 Optical Filtration Broadband light Reflected wavelengths Coating 1 (non-reflecting) Coating 2 Secondary reflections (interference) Coating 3 Glass Transmitted wavelength Figure 14.5 Quirk & Serda

12 12 Ultraviolet Spectrum nm Ultraviolet spectrum λ (nm) nm Visible spectrum EUV VUV DUV Mid-UV Violet Blue Green Yellow Orange Red i h g Excimer laser Mercury lamp Yellow light is used as it doesn't affect the photoresist Photolithography light sources Figure 14.6 Quirk & Serda

13 13 Optical Lithography Exposure Sources Mercury Arc Lamp Excimer Laser Spatial Coherence Exposure Control

14 14 Emission Spectrum of Typical High Pressure Mercury Arc lamp Relative Intensity (%) Emission spectrum of high-intensity mercury lamp DUV 248 nm i-line 365 nm h-line 405 nm g-line 436 nm Wavelength (nm) Mercury lamp spectrum used with permission from USHIO Specialty Lighting Products Figure 14.7

15 15 Mercury Arc Lamp Intensity Peaks UV Light Wavelength (nm) Descriptor CD Resolution (µm) 436 g-line h-line i-line Deep UV (DUV) 0.25 Table 14.2 Quirk & Serda Exposure dose for typical i-line resist : 100 mj/cm 2. ( Depending on resist thickness and type ) => For UV light intensity of 10 mw/cm 2, an exposure time of 10 seconds is needed.

16 16 Excessive Resist Absorption of Incident Light Photoresist (after develop) Sloping profile Substrate Figure 14.9 Quirk & Serda Thumb-rule: <20% of UV light must be absorbed in the resist to get good shape of the sidewall

17 17 Spectral Emission Intensity of 248 nm Excimer Laser vs. Mercury Lamp KrF laser 100 Relative Intensity (%) Hg lamp Wavelength (nm) 280 Figure 14.8 Quirk & Serda

18 18 Excimer/exciplex Laser Sources for Semiconductor Photolithography Material Wavelengt h (nm) Max. Output (mj/pulse) Frequency (pulses/sec) Pulse Length (ns) CD Resolution (µm) KrF ArF F Table 14.3 Quirk & Serda ArF (193 nm) laser is used at present (2015) for 32 nm node technology. Excimer = excited dimer (F2) Exciplex = excited complex (KrF, ArF)

19 19 Spatial Coherence Black box illuminator Incoherent light source of a single wavelength Slit Two slits closely spaced Interference patterns Unlike conventional lasers, eximer lasers have LOW amount of spatial coherence Figure Coherent cylindrical wave front Two coherent cylindrical wave fronts

20 20 Lift Off Not in book, but will be used in the lab You can find it in the lab compendium (it s learning) Technique for metallization Apply the metal on top of photoresist

21 21 Lift Off Two methods to add metal Etch the metal using photolithography Apply photoresist on top of metal Lift off Apply photoresist under metal

22 22 Lift Off Photoresist Profile of sidewalls important Undercut is desired If undercut is available Photoresist should be 2x thickness of metal Else Photoresist should be 10x thickness of metal In the lab we will apply ~ nm metal thickness More photoresist undercut

23 23 Lift Off

24 24 Thank You