Lecture 13 Basic Photolithography

Transcription

1 Lecture 13 Basic Photolithography Chapter 12 Wolf and Tauber 1/64

2 Announcements Homework: Homework 3 is due today, please hand them in at the front. Will be returned one week from Thursday (16 th Nov). 2/64

3 Announcements Term Paper: You are expected to produce a 4-5 page term paper on a selected topic (from a list). Term paper contributes 25% of course grade. You should have all been assigned your first-choice topic. The term paper should be handed in at the start of class on Tuesday 21 st November. Details / regulations are on the course website. The term paper will be returned to you in class on Thursday 30 th November. 3/64

4 Useful Links Berkley: University of Michigan: re%2016%20-%20mar%2009.pdf MIT: cture09.pdf KTH: 9%20Litho.pdf 4/64

5 Lecture 13 Overview of Photolithography Process Mask Fabrication. Photoresists. Photoresist Deposition. Exposure. Development. 5/64

6 Overview of Photolithography Process 6/64

7 Feature Sizes Way of quantifying tolerance in distance of mask from wafer Deep ultraviolet 7/64

8 Patterning Wafers Overall the process of patterning a wafer can be broadly divided into 3 steps: We are interested in the process of wafer exposure. Mask Design Mask Writing Wafer Exposure 8/64

9 Apply The Photolithography Process Apply photoresist. Expose photoresist through a patterned mask or reticle. Develop PR by immersing it in a solvent which preferentially dissolves the PR of higher solubility. Process the exposed part of the wafer. Strip away the remaining photoresist. Inspect pattern. Photoresist PR Substrate Mask Strip Etch Develop Expose 9/64

10 Photo Process Flowchart Photo room Clean Apply HMDS Spin Coat prebake Adhesion Promoter Exposure Develop postbake Plasma descum Process (Etch / Implant/ Lift-off) Strip 10/64

11 Mask Fabrication 11/64

12 Mask Fabrication Starting material for reticle manufacturing is ~80 nm thick film of chromium covered with resist and anti-reflective coating (ARC). Chromium has very good adhesion and opaque properties. Substrate: quartz glass plate. Patterned by direct writing using e- beam or laser usually wet etching of Cr after exposure. 4 or 5 magnification is normal for projection lithography. Pellicle used for dust protection of reticle. 12/64

13 Optical Proximity Correction As we will see later, the wave-nature of light means that we cannot exactly recreate the features on a wafer: Aperture Divergent light source Collimating lens 13/64

14 Optical Proximity Correction Optical Proximity Correction (OPC): Clever mask engineering based on software algorithms can compensate some of this error. This requires sophisticated computer modeling. 14/64

15 OPC Examples 15/64

16 Photoresists 16/64

17 Photoresists Photoresist (PR): An organic compound (often a polymer) with a photoactive component (PAC) whose solubility changes upon exposure to radiation (light). Positive Photoresist: Irradiated regions become more soluble than nonirradiated regions. R O N 2 hn R C + N 2 O More soluble regions Mask Strip Etch Develop Expose 17/64

18 Photoresists Photoresist (PR): An organic compound (often a polymer) with a photoactive component (PAC) whose solubility changes upon exposure to radiation (light). Negative Photoresist: Irradiated regions become less soluble than non-irradiated regions. SU8 Cross-linked polymer hn More soluble regions Mask Strip Etch Develop Expose 18/64

19 Comparison of Photoresists Characteristic Positive Negative Adhesion to Silicon Fair Excellent Relative Cost More expensive Less expensive Developer Base Aqueous Organic Solubility in the developer Exposed region is soluble Minimum Feature 0.5 µm 2 µm Step Coverage Better Lower Wet Chemical Resistance Fair Exposed region is insoluble Excellent 19/64

20 Key Parameters of a Photoresist Resolution: The smallest opening or island structure that can be made under a given set of process conditions (related to contrast). Registration: Overlay accuracy from layer to layer. Sensitivity: The number of photons it takes to cause the chemical response in the PR. A resist is more sensitive if it takes a lower dose to reproduce the mask geometry on the wafer. Shelf life: The time you can reliably store a PR. Etch Resistance: The ability of the resist which remains on the wafer after it is patterned to withstand the process environment that the exposed wafer is subjected to. 20/64

21 Photoresist Deposition 21/64

22 HMDS: Adhesion Promoter HMDS is the common name for hexamethyldisilazane: ((CH 3 ) 3 Si) 2 NH. Common photo-resists do not wet the surface of H-terminated Si/SiO 2 very well. HMDS is applied to the surface to improve wetting before the photoresist is deposited. H H H H O O O O Si O Si O Si O Si O Si Si O O Si Si CH 3 H 3 CSi CH 3 CH 3 H 3 CSi CH 3 Si O Si O Si O Si O CH 3 H 3 CSi O O O O Si Si CH 3 O O CH 3 H 3 CSi Si Si CH 3 22/64

23 HMDS: Adhesion Promoter HMDS is applied in vapor phase. But first hydroxyl groups must be removed from the wafer surface. H H H H O O O O Cleaning or oxygen plasma can be used. Si O Si O Si O Si Si O Si O Si O Si Si O Si O Si O Si Si O Si O Si O Si Must be conducted in inert atmosphere. 23/64

24 HMDS: Adhesion Promoter HMDS is applied in vapor phase. In the lab: HMDS-rich atmosphere Industrially: HMDS in beaker Cleaned Wafer HMDS is applied using a bubbler. Hotplate (~100 C) 24/64

25 Photoresist Deposition Photoresist is deposited by spin-coating. Photoresist (in solution) is deposited onto center of wafer. Wafer is rotated and material is spread out by centrifugal force. 25/64

26 Photoresist Deposition 1). Photoresist (in solvent) is deposited in center of the wafer. Wafer is held in place with vacuum chuck. 2). Wafer is rotated slowly (200 rpm) to distribute material. 3). Accelerate the wafer to final speed (~5000 rpm). Spin the wafer at constant speed for 30 60s. Forms a uniform film and evaporates solvent. 26/64

27 Photoresist Deposition Industrially this is done with robotic arms and automated dispensers: Extremely uniform films can be deposited using spin coating (rms roughness ~ Å s) 27/64

28 Film Thickness It can be shown (we won t) that the final film thickness depends on the spinning speed via: Film thickness t 1 ω Angular velocity Actual thickness will also depend on: Concentration. Solvent evaporation rate Viscosity. Local temperature. Local humidity. 28/64

29 Exposure 29/64

30 Photoresist Exposure to UV With our photoresist deposited, we now need to develop it. To do this we expose it to light (typically UV). Our photo-resist molecules absorb these short wavelength photons. Bonds are broken in the polymer. Either the molecules become more soluble in the developer (positive photoresist). Or the molecules cross-link an become insoluble (negative photoresist). 30/64

31 Exposure Techniques Three approaches are typically taken to exposure: Contact Printing Mask Defects Bowing of mask 1:1 Printing Proximity Printing 2-4 μm resolution Mask space ( 25 mm) Mask lens 2-5 X reduction Printing System Magn. Resolution (μm) Use Contact Research Proximity Low Cost Projection Mainstream VLSI Projection Printing 31/64

32 Light Sources Traditionally, mercury vapor lamps were employed for photolithography. 32/64

33 Light Sources Mercury lamp has four main emission lines: E, G, H, I: For projection printing, we ideally want monochromatic light. 33/64

34 Light Sources Nowadays for projection printing, and excimer laser is employed: These are pulsed lasers. Laser Emission Wavelength (nm) Resolution (μm) Max Energy (mj / Pulse) Repetition Rate (pulses / second) KrF ArF F < /64

35 Diffraction Modern lithography tools are limited by the spreading of light (and not their optical elements) Light passing through an aperture of similar dimensions to the wavelength of the incident light (λ~ 100 s nm), will result in diffraction. Divergent light source Collimating lens Aperture Diffraction pattern (Airy disk) 35/64

36 Diffraction The type of diffraction observed depends on the mask-wafer separation. Hard-contact: (almost) no diffraction. Proximity: Near field (Fresnel) diffraction. Projection: Far field or (Fraunhofer) diffraction. 36/64

37 Diffraction The difference between Fresnel (near field) and Fraunhofer (far field) is defined by the dimensionless Fresnel number (F): Where: W is size of the aperture. F = W2 Lλ L is the distance of the screen (wafer) from the aperture. λ is the wavelength of the incident light. We define: F 1 as Fresnel diffraction. F 1 as Fraunhofer diffraction. 37/64

38 Fresnel Diffraction Fresnel (near field) occurs in proximity printing. L Minimum resolvable feature size is: Where: W min = klλ k is an experimental parameter associated with the process conditions. 38/64

39 Example Determine the minimum feature size when exposing a wafer to i-line irradiation, using a mask 25 μm from the surface of the wafer. Assume for this example k = 1. W min = klλ From before we know the wavelength of the i-line is λ i = 365 nm. Work in microns: λ i = μm. W min = W min = 3 μm 39/64

40 Fraunhofer Diffraction Fraunhofer (far field) occurs in projection printing. d = Lens diameter f = Focal length 40/64

41 Fraunhofer Diffraction Diffraction pattern from a single circular opening: f = Focal length λ = Wavelength of incident light d = Lens diameter 41/64

42 Fraunhofer Diffraction We can think of a mask as a diffraction grating: Divergent light source Collimating lens Mask (diffraction grating) Each aperture in mask acts as a point source. 42/64

43 Fraunhofer Diffraction We can think of a mask as a diffraction grating: Divergent light source Collimating lens Mask Focusing lens Each aperture in mask acts as a point source. Photoresist on wafer 43/64

44 Fraunhofer Diffraction The resolution in Fraunhofer diffraction is defined by the Rayleigh criterion. Rayleigh Criterion: when the peak of one projection lands on the first zero of the other. 44/64

45 Fraunhofer Diffraction The resolution in Fraunhofer diffraction is defined by the Rayleigh criterion. Rayleigh Criterion: when the peak of one projection lands on the first zero of the other: 45/64

46 Fraunhofer Diffraction The resolution in Fraunhofer diffraction is quantified by the Rayleigh Criterion (R): f = Focal length d = Lens diameter R = k 1 λ f d k 1 is an experimental parameter associated with the system and resist (0.6 <k 1 < 0.8). The parameter dτf is sometimes called NA (numerical aperture): NA = d f = nsinα R = k 1λ NA n = index of refraction (1 in air). α = maximum half angle of incident light: α = Maximum half-angle 46/64

47 Modulation Transfer Function The modulation transfer function (MTF) quantifies how much modulation of light we achieve on the wafer: Photoresist on wafer Divergent light source Intensity at Mask Collimating lens Mask Intensity on wafer Focusing lens 1 1 I Max I Min 0 0 Position Position 47/64

48 Modulation Transfer Function Intensity at Mask 1 Intensity on wafer 1 I Max I Min 0 Position 0 Position We define MTF: MTF = I max I min I max + I min MTF is defined between 0 (small features) and 1 (large features). Generally, MTF needs to be > 0.5 for the resist to resolve features. 48/64

49 MTF vs Feature Size MTF MTF = I max I min I max + I min 49/64

50 Depth Focus: δ Depth of focus defines distance along straight optical path that wafer can be moved but keep the image in focus Photoresist on wafer Divergent light source Collimating lens Mask Focusing lens δ Basically, δ defines how accurate we need to be when positioning the lens at a distance from the wafer. 50/64

51 Depth Focus: δ Recall the resolution (Raleigh Criteria) was defined as: R = k 1 λ f d = k 1λ NA The depth of focus is defined as: d f Photoresist on wafer δ = k 2λ NA 2 = k 2λ f d 2 Focusing lens δ k 2 is another experimental parameter associated with the system and resist (k 2 ~0.5). 51/64

52 Example Consider a setup where the focal length is 5 the diameter of the lens. Assume k 1 = k 2 = 0.5. f = 5d Use F 2 laser: λ = 157 nm, NA = d 5d = 0.2 d Photoresist on wafer R = = nm Focusing lens δ δ = = 1.96 μm 52/64

53 Standing Waves Standing waves a problem, in particular when exposing on reflective layers such as metals. 53/64

54 Standing Waves Suppressed by antireflective coating (ARC) prior to resist spinning 54/64

55 Development 55/64

56 Development We now wish to dissolve the regions of the photoresist which have higher solubility. Exposed regions (positive photoresist). Masked regions (negative photoresist). Three strategies: Immersion Developing Spray Developing Puddle Technique Wafers develop solution ph and # of lots processed are monitored Developer fresh developer with each batch fixed amount of developer dispensed and rinsed 56/64

57 Development We now wish to dissolve the regions of the photoresist which have higher solubility. Exposed regions (positive photoresist). Masked regions (negative photoresist). We need the following: Original thickness of positive resist should not be measurably reduced. Development time should be short. Minimum pattern distortion (negative resists tend to swell). 57/64

58 Prebake & Postbake Evaporates resist solvent. Improves adhesion. Anneals out stress in PR. Increases etch resistance (postbake). Bake ovens: Convection/ hot air. Infrared (IR). Hot-plate. Temperature: Prebake: C. Postbake: 120 C. Clean Exposure Process Apply HMDS Adhesion Promoter (Etch / Implant/ Lift-off) Develop Strip Spin Coat postbake Photo room Plasma descum 58/64 prebake

59 Plasma descum and Strip Not all the resist is removed during development. Plasma descum or ashing is used to remove the residue. Plasmas are also used to strip the PR. Clean Exposure Process Apply HMDS Adhesion Promoter (Etch / Implant/ Lift-off) Barrel or downstream etchers are used. O 2 gas is used. Develop Strip Spin Coat postbake Photo room prebake Plasma descum Since the photoresist is organic we make use of elemental oxygen free radicals in the plasma. C-H Photoresist(s) + O(g) CO 2 (g) + H 2 O(g) 59/64

60 Downstream Plasma Plasma Ashing: Downstream Etcher Charged particles (ions & electrons) stay in plasma. Free radicals flow to wafer. Free radicals etch the wafer isotropically No surface damage and heating due to ion bombardment. R E A C T A N T G A S Electronics & Power supply (RF Microwave) Plasma Free radicals Wafer is downstream of the plasma Wafer 60/64

61 Resist Contrast Resist Contrast quantifies the ability of the resist to distinguish light/dark in the aerial image. To evaluate it we plot the developed thickness (i.e. thickness remaining after development) as a function of dose (Q). Dose (mjcm -2 ) = Intensity (mw/cm -2 ) time (s) Q = It Q 0 Q 0 Q f Q f 61/64

62 Developed Thickness Resist Contrast Quantitatively, the contrast is defined by γ: γ = log 10 1 Q f Q 0 Where: Q 0 is the onset of exposure effect. Q 0 Q 0 Q f is the dose at which the exposure is complete γ is typically Log 10 (Q) Q f 62/64

63 Lift Off Not used in VLSI, but can be used in research. Avoid etching of difficult materials Can produce a wider range of structures. 63/64

64 Next Time We will be talking about advanced techniques. Techniques for going down to sub-100nm resolution. 64/64