Lithography Lecture 2 PDF

Chapter 5 - Lithography Lecture 2 Dr. Panagiotis Dimitrakellis & Prof. Dr. Evangelos Gogolides 1 Outline – Lecture 2 ❑ Historical Development and Basic Concepts ✓ Photoresists ✓ g-line and i-line resist...

Chapter 5 - Lithography Lecture 2 Dr. Panagiotis Dimitrakellis & Prof. Dr. Evangelos Gogolides 1 Outline – Lecture 2 ❑ Historical Development and Basic Concepts ✓ Photoresists ✓ g-line and i-line resists ✓ DUV resists ✓ Basic properties and characterization of resists ✓ Mask engineering – optical proximity correction and phase shifting ❑ Manufacturing methods and equipment ✓ Wafer exposure systems ✓ Photoresists ❑ Models and simulation ✓ Wafer exposure systems 2 Photolithography Basic lithography process ✓ light-sensitive photoresist spun onto the wafer forming a thin layer ✓ resist selectively exposed by photons passing through a mask that contains the pattern information for the particular layer being printed ✓ resist developed which completes the pattern transfer from the mask to the wafer ✓ resist may be used as a mask to etch underlying films or it may be used as a mask for an ion implantation doping step 3 Photoresists 4  Photoresists ❑ Photoresist materials → designed to respond to incident photons by changing their properties when they are exposed to light ❑ Many materials absorb light → typically absorption results in electronic processes rather than chemical changes ❑ Semiconductors absorb photons and the energy is given to electrons and holes → absorbed energy dissipated (recombination or phonon interactions) or collected (solar cells) ❑ Not useful in lithography because in photoresists we require a material that maintains a latent image of the impinging photons at least until the resist is developed ❑ A long lived response to light generally requires a chemical change in the material 5  Photoresists ❑ Almost all resists today fabricated from hydrocarbon based materials ❑ When the materials absorb light → energy from the photons generally breaks chemical bonds → the resist material chemically restructures into a new stable form ❑ Positive resists respond to light by becoming more soluble in the developer solution ❑ Negative resists respond to light by becoming less soluble in the developer solution ❑ Current practice in the semiconductor industry → relies primarily on positive resists because they generally have better resolution than do negative resists 6  Photoresists ❑ Photoresists in use today are liquids at room temperature ❑ Applied to the wafers by placing the liquid on the wafer and spinning at several thousand RPM (spin-coating technique) ❑ Spin speed and viscosity of the resist → determine final resist thickness (usually 0.6 - 1 μm) ❑ Viscosity controlled by a solvent (constituent of the resist) Spin coating technique ❑ Resist is spun onto the wafer → a baking step (prebake) generally used to drive off the remaining solvent 7  Photoresists ❑ Next steps → resist exposure → developing ❑ Developing done using liquid developers ✓ immersion of the wafer in the liquid ✓ spraying the developer on the wafer ✓ placing a "puddle" of the developer on the wafer (most common) ❑ Upon developing resist is baked again (postbaked) → harden and improve its ability to act as an etch mask or ion implantation mask ❑ Upon etching or implantation process the resist is removed → oxygen plasma (or chemical stripping) Many important parameters determine the usefulness of a particular resist 8  Photoresists A. Sensitivity ❑ A measure of how much light is required to expose the resist (measured in mJ cm-2) ❑ For g-line and i-line resists typically 100 mJ cm-2 ❑ Deep UV (DUV) resists often achieve sensitivities of 20-40 mJ cm-2 (due to chemical amplification) ❑ Generally high sensitivity desired → decrease of exposure time of the resist → improve throughput in the lithography process. 9  Photoresists A. Sensitivity ❑ Extremely high sensitivity usually not desired: ✓ make the resist material unstable ✓ sensitive to temperature ✓ problems with statistical variations due to shot noise during exposure ❑ Higher contrast and more process latitude are achieved at lower sensitivities ❑ DUV chemically amplified resists achieve both higher sensitivity and higher contrast than the older g-line and i-line resists ❑ Light used to expose the resist is at a specific wavelength → also important that the sensitivity of the resist be optimized for the exposure wavelength 10  Photoresists B. Resolution ❑ Quality of resist patterns limited by the exposure system (aerial image) and not by the resist ❑ However the resist materials and the process steps (exposure dose, baking and developing cycles) must be carefully controlled to achieve diffraction limited resolution in the resist images 11  Photoresists C. The ‘resist’ function of photoresists ❑ "resist" describes the need for the photoresist to withstand etching or ion implantation after the mask pattern is transferred to the resist ❑ Practical resists need to have reasonable robustness → must be able to resolve small features even when the resist is of reasonable thickness 12  Photoresists ❑ g-line and i-line photoresists generally consist of three components: ✓ An inactive resin → usually a hydrocarbon which forms the base of the material ✓ A photoactive compound (PAC) → also a hydrocarbon ✓ A solvent → used to adjust the viscosity of the resist ❑ DUV resists replace the PAC component with a photo-acid generator (PAG) which acts as a chemical amplifier or catalyst (often add other components as well for stability) ❑ Most of the solvent evaporates during spin-coating and prebake processes before the resist is exposed → leaving a material that is about 1:1 base and active components 13  g-line and i-line resists ❑ Most common g-line and i-line resists today → diazonaphthoquinone or DNQ materials ❑ Base resin is generally novolac → polymer material consisting of basic hydrocarbon rings with 2 methyl groups and 1 OH group attached. Novolac will readily dissolve in the developer solution at a typical dissolution rate of about 15 nm sec-1 Structure of novolac 14  g-line and i-line resists ❑ PACs in these resists are often diazoquinones ❑ Photoactive part is the portion above the SO2 – remaining molecule often abbreviated ❑ Role of the PAC → inhibit the dissolution of the resist material in the developer ❑ Diazoquinones insoluble in typical developers → reduce the overall dissolution rate of the resist to Basic structure of approximately 1-2 nm sec-1 diazoquinones ❑ DNQ material is essentially insoluble in the resist developer before it is exposed to light 15  g-line and i-line resists ❑ When the resist is exposed to light the diazoquinone molecules chemically change ❑ N2 molecule is weakly bonded in the PAC and the first part of the photochemical reaction involves the light breaking this bond → leaves behind a highly reactive carbon site ❑ PAC structure can stabilize itself by moving a carbon atom outside the ring with the oxygen atom covalently bonded to it → Wolff rearrangement Decomposition of DNQ upon exposure to light 16  g-line and i-line resists ❑ Resulting ketene molecule finally transforms into carboxylic acid in the presence of water ❑ Carboxylic acid → readily soluble in a basic developer (typically TMAH - tetramethyl ammonium hydroxide, KOH or NaOH dissolved in H20) ❑ Novolac matrix material also readily soluble in this solution ❑ Exposed resist material now dissolves at a rate of 100 -200 nm sec-1 Unexposed regions of the resist essentially unaffected by the developer → if the exposed pattern accurately reproduces the mask pattern, the photoresist can produce a high resolution image of the mask 17  Deep Ultraviolet (DUV) resists ❑ DUV resists in use today: NOT modified DNQ resists → based on a completely new chemistry and make use of chemical amplification (CA resists) ❑ Standard DNQ resists achieve quantum efficiencies (QE) of about 0.3 → 30% of the incoming photons interact with PAC molecules and are effective in exposing the resist (higher possible sensitivity improvement a factor of about 3) 18  Deep Ultraviolet (DUV) resists ❑ CA resists use a different exposure process: ❑ Incoming photons react with photo-acid generator (PAG) molecule → create an acid molecule ❑ Acid molecules act as catalysts during a subsequent resist bake (PEB) to change the resist properties in the exposed regions ❑ Both positive and negative resist versions possible: Principle of CA resists Positive resist case → PAG initiates a chemical reaction that makes the resist soluble in the developer (opposite for the negative resist) 19  Deep Ultraviolet (DUV) resists ❑ Key point: the reactions are catalytic ❑ Acid molecule regenerated after each chemical reaction → may participate in tens or hundreds of further reactions! ❑ Overall quantum efficiency in a CA resist = initial efficiency of the light/PAG reaction x number of subsequent reactions that are catalyzed Principle of CA resists ❑ Product can be >1 → responsible for the improvement in sensitivity of DUV resists compared to DNQ resists (20-40 mJ cm-2 compared to 100 mJ cm-2) 20  Deep Ultraviolet (DUV) resists ❑ Chemical amplification → very powerful approach to create resists ❑ Number of possible acid catalyzed reactions is much larger than the number of photochemical reactions → explosion of new resist possibilities occurred Principle of CA resists 21  Deep Ultraviolet (DUV) resists ❑ Positive resists consist of a PAG and a blocked or protected polymer which is insoluble in the developer because of attached molecules (labeled INSOL) ❑ Example would be a polyhydroxystyrene polymer with attached acid labile groups (unstable, easy to cleave) ❑ Incident DUV photons react with the PAG molecules to create an acid molecule Principle of CA resists 22  Deep Ultraviolet (DUV) resists ❑ Spatial pattern of acid molecules in the resist after the exposure → a "stored" or latent acid 3D image of the mask pattern! ❑ After exposure wafer baked at a T ≈ 120°C for a few min (post exposure bake or PEB) → heat provides the energy needed for the reaction between the acid molecules and the insoluble fragments on the polymer chains Principle of CA resists ❑ Heat also provides mobility (through diffusion) for acid molecules to reach the insoluble fragments and react with many of such fragments 23  Deep Ultraviolet (DUV) resists ❑ During PEB the insoluble blocked polymer converted into an unblocked polymer soluble in aqueous alkaline developer ❑ PAG catalyzes a reaction which crosslinks the polymer chains making the resist insoluble in the developer The key mechanism in either process is the catalytic behavior of the acid molecules that are regenerated after each reaction ❑ DUV resists require careful control of PEB conditions → bake is used to drive the chemical reaction Chemical reactions and diffusion associated with acid molecules during PEB generally depend exponentially on T - requiring control on the order of a fraction of a °C during PEB 24 Basic properties and characterization of resists ❑ Two basic parameters often used to describe the properties of photoresists: ✓ Contrast ✓ Critical modulation transfer function (CMTF) 25  Basic properties and characterization of resists Contrast ❑ A measure of the resist's ability to distinguish light from dark areas in the aerial image the exposure system produces ❑ Diffraction effects and other possible imperfections in the exposure system result in an aerial image that does not have abrupt transitions from dark to light ❑ Important question: how the resist responds to the "gray" region at the edges of features in the aerial image? 26  Basic properties and characterization of resists Contrast ❑ Contrast experimentally determined for each resist and its value extracted from plots → data obtained by exposing resist layers to a variety of exposure doses ❑ Each sample developed for a fixed period of time → measure thickness of resist remaining after developing ❑ Positive resists → samples receiving small exposure doses will not be attacked by the developer to any appreciable extent - those receiving large doses will completely dissolve in the developer (intermediate doses result in partial dissolution of the resist) ❑ Negative resists → the opposite behavior occurs 27  Basic properties and characterization of resists Contrast ❑ Contrast simply the slope of the steep part of the curve, defined as where Qo is the dose at which the exposure first begins to have an effect and Qf is the dose at which exposure is complete ❑ Typical g-line and i-line resists achieve contrasts of 2-3 and Qf values of about 100 mJ cm-2 ❑ DUV resists better contrast and sensitivity → because the chemical amplification steepens the transition from the unexposed to the exposed condition - typically achieve y values of 5 - 10 and Qf values of about 20 - 40 mJ cm-2 28  Basic properties and characterization of resists ❑ γ not a constant for a particular resist composition - Contrast experimentally extracted value that depends on process parameters (development chemistry, bake times and temperatures before and after exposure, wavelength of the exposing light, underlying structure of the wafer) ❑ Desirable to have a resist with a high contrast because this produces better (steeper) edge profiles in the developed resist patterns High contrast implies that the resist sharply distinguishes between dark and light areas in the aerial image → resists with high contrast can actually "sharpen up" a poor aerial image 29  Basic properties and characterization of resists Critical modulation transfer function (CMTF) ❑ Modulation transfer function (MTF) of the aerial image → measure of the "dark" versus "light" intensities in the aerial image produced by the exposure system ❑ Useful to define a similar quantity for the resist: critical MTF or CMTF ❑ Roughly the minimum optical transfer function necessary to resolve a pattern in the resist ❑ Typical CMTF values for g-line and i-line resists are around 0.4 - higher γ values in DUV resists achieve significantly smaller CMTF values (0.1 - 0.2) 30 Basic properties and characterization of resists ❑ Assumption of uniform resist thickness and exposure process as occurring simultaneously throughout the volume of the resist ❑ Resist thickness nonuniform across the wafer → spun liquid fills in the "hills and valleys" of the underlying topography ❑ Exposing regions with different resist thickness (thinner on top of high structures and thicker over low lying structures) → problem at the edges of underlying thin films where the resist thickness can change abruptly ❑ Resist can be effectively underexposed where it is thicker and overexposed where it is thinner 31 Basic properties and characterization of resists ❑ Light absorption varies with depth below the surface of the resist and also changes with time during the exposure process ❑ PAC concentration assumed uniform throughout resist → PAC molecules near the resist surface absorb light photons → those photons not available for exposing the deeper layers of the resist → light concentration drops with depth ❑ Light intensity drops exponentially with distance into the resist where z is the depth below the surface, I is the intensity at the surface and α is the optical absorption coefficient in the resist. ❑ Resist first exposed near the top surface 32  Basic properties and characterization of resists ❑ "bleaching" occurs in g-line and i-line DNQ resists: upon exposure PAC altered, absorbing less and less light, and becomes more and more transparent → as the top layers are exposed, they transmit more of the light to the deeper layers which are subsequently exposed → more uniform exposure ❑ Bleaching typically does not occur in DUV resists: light can reflect off surfaces below the resist during exposure → mitigated by using antireflection coatings below the DUV resist to minimize reflections 33 Basic properties and characterization of resists ❑ Highly reflective layers below the photoresist → light passing without being absorbed will be reflected by the underlying layers and pass through the resist again ❑ Speed up the exposure process BUT also has the potential for setting up standing wave light patterns in the resist (constructive and destructive interference between the incoming and outgoing waves) ❑ If the light is scattered sideways, image resolution can be degraded ❑ Antireflective coating (ARC) deposited on the wafer prior to spinning of resist → minimizing standing wave effects (but increases process complexity). 34 Mask engineering – optical proximity correction and phase shifting 35  Mask engineering – optical proximity correction and phase shifting ❑ A mask has clear areas and dark areas → the patterns represent exactly the pattern that we wish to print in the resist on the wafer ❑ Possible to do better designs if the objective is to produce the highest quality aerial image ❑ Two approaches: optical proximity correction (OPC) and phase shift masks (PSM) ("mask engineering" or“ wavefront engineering“) 36 Mask engineering – optical proximity correction and phase shifting Optical proximity correction ❑ Finite aperture of projection systems → loss of some of the light diffracted from the mask features ❑ Apertures and lenses in projection systems circular (not square or rectangular like most of the features on masks) ❑ What are lost are the fine details (e.g., sharp edges and small features) ❑ Lost information results in an aerial image which has rounded rather than square corners, changes in linewidth between isolated and grouped lines and shortening of the ends of narrow linear features 37  Mask engineering – optical proximity correction and phase shifting No OPC OPC Optical proximity correction ❑ Predictable effects → can be Mask compensated by adjusting the feature dimensions and shapes on the mask (purely through software once the mask design is complete – difficult due to the complexity of modern masks) Aerial image 38 Mask engineering – optical proximity correction and phase shifting Phase shifting ❑ Changing the transmission characteristics of the mask in selected areas → improve resolution of printed aerial image ❑ A periodic mask with equal lines and spaces (diffraction grating) is used as the mask ❑ Electric field, ε , associated with the light just after it passes through the mask and also at the wafer (far field diffraction pattern) without (left) and with phase shifting (right) ❑ Photoresist responds to the intensity of the light or (ε field)2 → intensity pattern on bottom left barely sufficient to resolve the two lines 39 Mask engineering – optical proximity correction and phase shifting Phase shifting ❑ Material with thickness and index of refraction chosen to phase shift the light by exactly 180° is added to the mask. The thickness of this layer is given by where n is the index of refraction of the phase shift material ❑ Light intensity at the aerial image is the square of the ε field intensity → quality of resulting aerial image significantly improved 40 Manufacturing methods and equipment 41  Manufacturing methods and equipment ❑ Lithography process dominates the cost and the throughput of modern IC manufacturing ❑ Contact and proximity printers played a major role in the early days of the silicon industry, they no longer do so because of unacceptable defect levels and/or limited resolution → only projection aligners ❑ DNQ positive resists and DUV resists 42  Wafer exposure systems Projection aligner ❑ Projection aligners are the dominant exposure tools in the silicon industry - Perkin-Elmer Corp. pioneered the earliest systems of this type under the name Micralign (scanning projection aligners that used 1:1 masks) ❑ Basic idea: easier to correct optics for aberrations in a small area rather than a large area ❑ Illumination source produces a slit of light mechanically scanned across the mask - wafer simultaneously scanned so that the mask pattern is printed across the wafer 43  Wafer exposure systems ❑ Scanning systems: cost effective and high throughput BUT require 1X masks that contain the pattern information for all the chips to be printed on each wafer ❑ Chips become more complex and wafers become larger → difficult to produce perfect full wafer masks and to make optical systems that can scan entire wafers with the required resolution (also need for placement accuracy and local alignment) ❑ Scanning systems replaced by steppers that exposed a limited portion of the wafer (typically a few cm2) at a time → this eliminates wafer size as a major issue ❑ Mask issue also addressed by making the steppers reduce the image by 4X or 5X → masks use much larger dimensions which makes them more easily repairable ❑ Further key additional ideas used in practice to improve the performance of these systems → use of Kohler illumination and off-axis illumination 44  Wafer exposure systems Hybrid system ❑ Increasingly difficult to build steppers that have the resolution and field of view required → hybrid type of system which uses both stepping and scanning come into use ❑ Stepping to move the wafer between major exposure fields - within each exposure field, the mask pattern is scanned across the wafer ❑ Advantages of scanners → optical system "perfect" in a smaller area ❑ Advantages of steppers → 4X or 5X reduction to simplify mask fabrication and total field printed at each exposure much smaller than wafer (eliminating wafer size as major issue and simplifying lens design) ❑ Complex and very expensive → disadvantages of each type of system (the synchronized mechanical motion of scanners, and the costly refractive lens design of steppers) 45 Photoresists ❑ Actual details associated with photoresist exposure and processing → very complicated ❑ Typical process flow associated with photolithography ❑ Specific numbers refer to typical conditions for DNQ g-line or i-line resists ❑ DUV resists generally utilize a similar process flow with some changes in the process details ❑ In modem lithography many of the steps accomplished in single integrated machine known as wafer track system - $1M (usually integrated with the exposure tool so that easy wafer transfer can occur between the two systems) 46 Photoresists Step 1 ❑ Ensure that the resist will adhere well to the wafer ❑ Depending on the stage in the process flow, this may involve one or more operations ❑ Normally wafer is clean before resist application → resist commonly deposited on wafers just after thin film deposition or just after some other high temperature process step ❑ Otherwise needs to be chemically cleaned or heated to several hundred °C to remove any water vapor 47  Photoresists Step 2 ❑ Even with clean and dry surface the adhesion to silicon may not be as good → common practice to use an adhesion promoter (typically hexamethyldisilane – HMDS) ❑ Applied in liquid form at room temperature by spinning (3000 - 6000 rpm for 30 sec) - in most cases HMDS introduced as a vapor into a chamber containing the wafers (more easily produces the desired single monolayer) ❑ One end of the HMDS molecule bonds readily with SiO2 surfaces and the other bonds with the resist (for other surfaces other adhesion treatments may be used) 48  Photoresists Step 3 ❑ Spinning the resist - normally just after HMDS application ❑ Resist dispensed on wafer → spun (3000 - 6000 rpm for = 30 sec) to produce a thin (0.6 - 1 μm) uniform layer ❑ The acceleration to its final spin speed is important - typically done rapidly (fraction of a second) → rapid acceleration leads to more uniform films ❑ The solvent begins to evaporate rapidly after the resist is dispensed and accelerated ❑ Viscosity of liquid resist (solvent content) and the spin speed are the primary factors affecting final resist thickness 49  Photoresists Step 4 ❑ Pre-baking normally on a hot plate at 90 - 100°C (IR or MW heating also used for faster processes) ❑ Pre-bake step accomplishing several things: ✓ remaining solvent evaporated, reduced from = 25% to = 5% of the resist content ✓ adhesion improved as heating strengthens bonds between resist - HMDS - substrate ✓ stresses in the resist due to spinning are relieved through thermal relaxation ❑ Chemical changes take place in resist during high temperature bake → required exposure times are increased as the bake temperature increases (possible mechanism is decomposition of the PAC at high temperature → resist sensitivity is degraded) 50  Photoresists Step 5 ❑ Exposure process creates a latent image in the resist that can be later developed ❑ Most resists exhibit reciprocity → light intensity and exposure time can be directly traded off with each other (increase in exposure system aerial intensity directly reduce exposure times) ❑ Required exposure time also affected by pre-bake thermal cycle and thickness of the resist → all must be controlled ❑ Exposure doses designed to >Qf → latent images with sharpest edges (limited by the quality of the aerial image) ❑ Typical DNQ resists: dose > 100 mJ cm-2 - DUV resists (chemical amplification): typically 20 - 40 mJ cm-2 51  Photoresists Step 6 ❑ Post exposure bake (PEB) ❑ g-line and i-line resists: before development of resist latent image → minimize standing wave effects in the resist ❑ Mechanism: at high T the PAC can diffuse → if T and time well controlled in this bake, the PAC molecules can diffuse far enough to alleviate the standing wave effects along the edge of the resist features (but not far enough to significantly distort the image features) ❑ Typical bake cycle might be around 10 min at 100°C ❑ If antireflective coatings are used under resist, PEB may not be necessary (standing wave problem less severe) 52  Photoresists Step 6 ❑ Post exposure bake (PEB) ❑ DUV resists → PEB necessary and critical in the process ❑ PAG reacts with the polymer chain to complete the exposure process ❑ The time and especially the temperature must be very tightly controlled because chemical reaction rates and diffusion typically depend exponentially on temperature 53  Photoresists Step 7 ❑ DNQ resists developed in basic solutions (normally TMAH - tetramethyl ammonium hydroxide solutions diluted with H2O - NaOH or KOH solutions also used) ❑ Developing process → wafers immersed in developer, developer sprayed on wafers, a puddle of developer placed on wafer (rinsing the wafers with H2O stops the developing) ❑ Developing rate highly dependent on: ✓ temperature ✓ developer concentration ✓ exposure and bake procedures used before developing 54  Photoresists Step 7 ❑ DNQ resists: developing proceeds by dissolving the carboxylic acid that results from the exposed PAC in the alkaline solution ❑ Developing rate: dependent on local carboxylic acid concentration → proportional to local exposure intensity in the resist ❑ Positive working DUV resists → developing takes place in a similar fashion since the unblocked polymer chains are soluble in the basic developer 55  Photoresists Step 8 ❑ Higher T than the earlier bakes (typically 10 - 30 min at 100 - 140°C ) ❑ Designed to harden resist and improve its etch resistance ❑ Causes the resist to flow slightly and hence may also modify edge profiles, removes any remaining solvents, and adhesion to the underlying substrate also improved ❑ When resist must withstand particularly harsh etching process or high current ion implantation (may raise its temperature considerably) → further hardening and crosslinking to enhance the robustness of resist layer 56 Models and simulation 57  Models and simulation ❑ Lithography simulation relies on: ✓ Optics → mathematical description of the behavior of light in the exposure systems ✓ Chemistry → tools to treat exposure, baking and developing of the resists used to convert the aerial image to a 3D replica of the mask patterns ❑ Simulation tools commercially available (PROLITH, DEPICT, ATHENA) 58  Wafer Exposure System Models ❑ Consider only projection exposure systems → model far field or Fraunhofer diffraction ❑ Light travels as an electromagnetic wave → traveling electric field at a point P: or in complex exponential function: Generic projection lithography system 59  Wafer Exposure System Models ❑ Digital transmission function of the mask ❑ After the light is diffracted, the electric field intensity pattern is described by the Fraunhofer diffraction integral where fx and fy are the spatial frequencies Generic projection lithography system of the diffraction pattern, defined as 60  Wafer Exposure System Models ❑ Electric field pattern is the Fourier transform of the mask pattern ❑ The light intensity I is simply the square of the magnitude of the electric field Generic projection lithography system 61  Wafer Exposure System Models ❑ After passing through the mask, light is collected by the objective lens → only a portion of the light is collected → characterized by a pupil function ❑ Objective lens → target to reconstruct the diffraction pattern and focus on the wafer → now performs the inverse Fourier transform Mathematical models resulting in a light intensity at the resist surface (aerial image) given by 62  Chapter 5 - Lithography Lecture 2 Thank you for your attention Dr. Panagiotis Dimitrakellis [email protected] Prof. Dr. Evangelos Gogolides [email protected] 63 Additional slides 64  Wafer exposure systems Kohler illumination source ❑ Light passing through the mask focused at the entrance pupil of the projection lens (rather than having collimated light pass through the mask) ❑ Projection lens can capture the diffracted light from any of the features on the mask equally well ❑ Triangles represent the angular spread of the diffracted light through the mask apertures ❑ If collimated light were used though the mask → much of the diffracted light would be lost from mask features near the outside edge of the mask 65  Wafer exposure systems Off-axis illumination ❑ Light from a coherent source is incident at an angle to the mask rather than normal to the mask (changes the angle of light passing through the mask) → changes the angle of the diffracted light ❑ Some diffracted light will be lost (outside the projection lens aperture) BUT some higher order diffracted light will be captured in the off-axis system → net result: resolution can be improved ❑ Basic idea - partially coherent illumination better than coherent illumination → attention must be paid to the illumination part of the optical system as performance in projection aligners is pushed towards physical limits 66 Measurement methods 67  Measurement methods ❑ Measurement issues associated with lithography: ✓ Masks: we need to know the dimensions of the features on the mask and to verify that they correspond to the intended design – also important to ensure no defects on the mask ✓ Exposure process (optical system that produces the aerial image and resist processing that transfers the aerial image into a resist profile) → aerial image itself is not normally directly measurable, so that measurement issues associated with lithography normally focus on the resist pattern after it is developed ✓ Alignment of patterns with respect to underlying features on the wafer (earlier mask layers) 68  Measurement of Mask Features and Defects ❑ All exposure systems used today in high volume manufacturing are reducing steppers or reducing step and scan projection systems ❑ Mask contains the features for only a few dies and pattern is exposed multiple times on each wafer → defect on the mask will affect a large fraction of all chips on every wafer the mask is used to print (intolerable yield losses) ❑ The mask must be "perfect" → there are no printable defects on the mask ❑ Since the mask has 4X to 5X larger feature sizes than the wafer patterns, mask defects below some critical size will not print on the wafer ❑ Two issues with respect to the mask: Does the mask pattern correspond to the design for that mask level? Are there any defects large enough to print on the wafer? 69 Measurement of Mask Features and Defects ❑ Mask inspection with a microscope unworkable today due to complexity of new chips → highly automated systems developed ❑ Light passed through the mask and collected by imaging system → a solid state image sensor detects pattern of transmitted light ❑ Information can be compared either with the design database used to generate the mask, or with a second identical mask pattern, if the mask contains the patterns for more than one chip ❑ Scanning over the mask provides full information on defects - if mask incorporates optical proximity correction (OPC) or phase shifting, the inspection process more difficult (structures on the mask smaller than the minimum feature size and therefore might be interpreted as "defects" by the inspection system) 70  Measurement of Mask Features and Defects ❑ Detected defects can be corrected ❑ Opaque defects (chrome on the mask in areas where it should not be) corrected with lasers or ion beams focused on the defect area and evaporate excess chrome ❑ Clear defects (chrome not present where it should be) more difficult to correct → require deposition of opaque material to cover the clear area - laser assisted deposition from a chromium bearing gas introduced above the mask or ion beam deposition 71  Measurement of Mask Features and Defects ❑ Crucial issue is the actual size of the features on the mask ❑ Several issues with e-beam mask making that lead to disparities in feature size between the design database and the mask ❑ Spot size of e-beam is finite (0.125 - 0.5 μm) → important when writing very small features ❑ For small features "proximity effects" can be an issue when closely spaced → from electron backscattering in the resist on the mask and can result in pattern distortion ❑ Many masks made with laser based systems instead of e-beam systems (less expensive) – BUT have similar resolution limits 72  Measurement of Resist Patterns ❑ Resist developing → three dimensional structure ❑ Resist edges may be sloped and they may contain standing wave patterns ❑ Definition and measurement of the "linewidth" (smallest width of a feature that can be accurately patterned onto the wafer) in the resist not straightforward ❑ Linewidths in chips > 1 μm → optical methods using a microscope commonly used to measure resist feature sizes ❑ Small features on today's chips → SEM measurements have largely replaced optical methods for accurate linewidth measurements (potential for very high resolution, operation in-line) 73  Measurement of Resist Patterns SEM imaging ❑ SEM images of resist pattern after development ❑ Difficulties in "measuring" linewidths → possibility of sloped edges, standing wave patterns and variations in the linewidth along photoresist lines → difficulties in defining exactly what is meant by the linewidth ❑ Linewidth defined as the width of the photoresist material at a specific height above the resist - substrate interface ❑ Often sophisticated algorithms are used to translate an SEM line scan into a "number" representing the linewidth 74  Measurement of Etched Features ❑ Photoresist patterns transferred to underlying thin films by etching → quality of pattern transfer process equally important to photoresist pattern (same linewidth measuring methods based on SEM used to measure the etched patterns in oxide, polysilicon, Al, or other materials) ❑ Etched thin film patterns (unlike the photoresist patterns) remain as part of the final chip → possible to make measurements on thin film layers after wafer processing is complete → electrically during wafer testing ❑ Electrical test structures measure parameters like linewidth and alignment accuracy → complete test chips have been developed which provide detailed information about the process (very useful during development stages of new processes) ❑ Specific test structures also sometimes incorporated onto product chips once a process is in manufacturing → provide ongoing information about manufacturing tolerances (usually placed in the scribe lines between chips so they do not consume chip area) 75  Measurement of Etched Features ❑ Overall structure assumed a conducting material Electrical test structures (polysilicon, silicide, aluminum etc.) ❑ Right hand part (pads 3-6) is a van der Pauw structure designed to extract the sheet resistance of the material making up the test structure ❑ Geometry chosen to define one square of the material (labeled with the ρs symbol) ❑ If a current I5-6 is forced between terminals 5 and 6, and the voltage V3-4 is measured between terminals 3 and 4, then the sheet resistance is given simply by 76  Measurement of Etched Features ❑ Once the sheet resistance is measured, then the linewidth W can be extracted from the rest of the structure through an additional electrical measurement. In this case, a current l1-5, is forced between terminals 1 and 5, and a voltage V2-3 is measured between terminals 2 and 3 ❑ The linewidth W is then given by ❑ Linewidth extracted from test structures may be different than the linewidth measured optically or with an SEM ❑ Modifications can allow similar structures to extract information about alignment between levels on a chip → parts of the structure are fabricated in different levels of the circuit and measurements of resistances structure provide the alignment information 77