Lithography Lecture 1 PDF

Chapter 5 - Lithography Lecture 1 Dr. Panagiotis Dimitrakellis & Prof. Dr. Evangelos Gogolides 1 Outline – Lecture 1 ❑ Introduction to lithography ❑ Historical Development and Basic Concepts ✓ Optical lithograp...

Chapter 5 - Lithography Lecture 1 Dr. Panagiotis Dimitrakellis & Prof. Dr. Evangelos Gogolides 1 Outline – Lecture 1 ❑ Introduction to lithography ❑ Historical Development and Basic Concepts ✓ Optical lithography process ✓ Light sources ✓ Wafer exposure systems ✓ Optics basics (ray tracing and diffraction) ✓ Projection systems (Fraunhofer diffraction) ✓ Contact and proximity systems (Fresnel diffraction) 2 Introduction 3  Introduction ❑ Lithography → most important process step in modern IC manufacturing ❑ Ability to print patterns with features 10 — 20 nm → place on a substrate with a few nm precision ❑ Almost all ICs are manufactured today with DUV (deep-UV) optical lithography operating with 193 nm photons ❑ EUV (extreme ultraviolet, X = 13.5 nm) lithography is beginning to be used for some critical mask levels in the latest technology generations because the DUV systems have reached their practical limits in resolution 4 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 5  Introduction ❑ Actual implementation is very expensive and very complex → demands placed on this process for resolution, exposure field, placement accuracy, throughput and defect density ❑ Resolution requirements result from the ever-increasing demand for smaller device structures ❑ Exposure field requirements result from the need to fabricate large chips ❑ Placement accuracy is an issue because generally each mask layer needs to be carefully aligned with respect to the existing patterns already on the wafer ❑ Throughput and defect density are issues because of the competitive the semiconductor industry ✓ Throughput translates directly into manufacturing cost ✓ Defects translate directly into yield loss → less profitability in the finished chips (defects during the lithographic process are a significant contributor to final chip yields) 6  Introduction ❑ "International Technology Roadmap for Semiconductors" or ITRS used to guide progression of new silicon technology generations on a 2 - 3 year cadence ❑ The feature size reduction in the ITRS corresponded to a factor of 0.7X in linear dimension or a decrease in the area required per transistor by 50% approximately every two to three years 7  Introduction ❑ Minimum features not only decreased in average size with each technology generation, but the variation of these feature sizes also decreased with time in the ITRS ❑ Feature size control required to be about ±5% of the smallest feature size → this requirement usually expressed in terms of the 3σ control. ❑ E.g., if minimum feature size is 60 nm, standard deviation of the distribution would be required to be σ = 1 nm so that 3σ = 3 nm (5%) ❑ If the distribution is normal, this would imply that the minimum features printed on the wafer would be within 60 ± 3 nm 8  Introduction ❑ ITRS discontinued in 2015 (companies following their roadmaps) ❑ Last versions of the ITRS (2013 edition) the resolution limit of the most advanced tools at that time (193 nm immersion projection tools) given as roughly 40 nm lines and spaces ❑ There would be no further significant advances in DUV lithography tools ❑ How smaller and smaller devices continue to be fabricated? → "tricks" increasingly used to advance to today's 10 nm and smaller technology nodes ❑ Today a radically new lithography approach - EUV lithography, with a much smaller wavelength - is entering manufacturing 9  Introduction ❑ State-of-the-art 193 nm lithography tools today cost > $50M because of the precision required ❑ Lithography 1/3 of total wafer manufacturing costs → wafer manufacturing cost a few $K for 12 inch silicon wafer → machines must print patterns on several wafers per hour to justify their cost ❑ 193 nm DUV systems typically process as many as 250 wafers per hour ❑ Radically different EUV lithography tools cost > $100M for each machine 10 Historical Development and Basic Concepts 11  Overall lithography process ❑ Patterning process consists of mask design, mask fabrication and wafer printing 12  Mask design ❑ Patterns of various layers in ICs are designed using Computer Aided design (CAD) ❑ Such systems contain many advanced capabilities that greatly improve the efficiency of designing chips with millions of components: ✓ Libraries of previous designs that are known to work are usually available → basic functions or circuits can be cut and pasted into new designs ✓ Software tools → help route or wire the connections between functional blocks ✓ Additional tools → check the design to make sure that there are no violations of design rules ✓ Simulation tools are available to predict the performance of the new design 13  Mask fabrication ❑ Information for each mask level transferred to a mask making machine → electron beam or laser pattern generator ❑ Pattern for each mask written on a mask blank using the scanning electron or laser beam ❑ The mask is usually a fused silica plate (silicon oxide) covered with a thin layer (≈ 80 nm) of chromium and a layer of photoresist ❑ The beam exposes the resist → developed and used as an etch mask to transfer the mask pattern to the chromium ❑ The chromium is dry etched (or wet etched for large dimensions) and the photoresist is removed ** The clear areas of the mask where the chromium has been etched away have high transparency at the wavelength used in the wafer exposure system → at 193 nm of DUV the SiO2-based masks are reasonably transparent 14  Mask fabrication ❑ Mask fabricated with pattern dimensions 4X to 5X larger than the features desired on the wafer → the wafer exposure system reduces the image by the same factor ❑ Demagnification makes the mask easier to fabricate and easier to check for imperfections ❑ Defects on the mask directly imaged on every wafer exposed with the mask and contribute to yield loss → repaired either by removing the unwanted chrome areas with laser ablation or ion beams or by depositing additional chrome to fill the pinholes 15  Mask fabrication ❑ Mask writing with a laser beam or a beam of electrons in an X-Y pattern across the mask with the beam blanked on and off as necessary ❑ The beam size 0.1 - 0.5 µm for e-beam systems and slightly larger for laser- based systems → adequate for writing 4X or 5X masks for today’s steppers ❑ E-beam system could be used directly to write images by putting the electron-sensitive resist on the wafer (instead of the mask) → wafer throughput would be too slow! ❑ Typical e-beam pattern generators require tens of minutes to expose a full wafer → much slower than optical steppers which typically have throughputs of >100 wafers per hour 16  Wafer printing ❑ Pattern information transferred to the wafer by printing the mask pattern in a layer of photoresist → projection exposure system ❑ Light collimated and passed through the mask → each clear area on the mask transmits the light which is then collected and focused by a second lens system (reduce the image size typically by 4X) ❑ The field of view of such systems is typically only a few cm on a side → only a few chips are printed during each exposure ❑ Wafer physically moved (“stepped”) to next exposure field → process repeated (systems called steppers) 17 Lithography process ❑ Convenient to separate lithography process into: ✓ Light source → used to generate the photons which ultimately expose the resist ✓ Exposure system → images the mask on the wafer surface ✓ Resist → contains all the issues associated with the resist itself, exposure, developing, baking etc. 18 Wafer exposure ❑ Produces the aerial image at the top surface of the resist → pattern of optical radiation on resist surface ❑ For a positive resist (most manufacturing applications today / exposed resist dissolves when processed) → incoming photons strike the resist in the light areas, changing its properties in those regions → produces a 3D latent image of the mask pattern → after developing, resist used as an etch mask 19 Wafer exposure ❑ Aerial image is the dividing line between the major parts of the lithography system → exposure tool and resist ❑ Exposure tool → produce the best aerial image possible: resolution, exposure field, depth of focus, uniformity and lack of aberrations, photon intensity, etc. ❑ Photoresist → translate this aerial image into the best thin film 3D replica of the aerial image possible: geometric accuracy, exposure speed, and resist resistance to subsequent processing 20  A. Light sources ❑ Higher resolution in lithography systems when shorter- wavelength light used → minimum feature size can be printed with a lithography tool directly proportional to the wavelength of the light used to print the feature ❑ Lithography systems used for IC manufacturing are monochromatic (single-wavelength, easier to design) ❑ From mid-1990s feature sizes smaller than λ (subwavelength lithography) - today >10X smaller than λ History of silicon IC manufacturing ❑ EUV systems at 13.5 nm developed - 193 nm immersion optical systems dominant tools used today 21  A. Light sources Hg lamps ❑ Before 1995 → arc lamps → Hg vapor inside a sealed glass envelope ❑ Arc between the electrodes by applying high enough voltage to ionize the gas (typically several kV) → the gas is ionized and behaves like a plasma ❑ Light emission comes mostly from the Hg atoms: ✓ collisions in the plasma between high-energy electrons and Hg atoms lead to electronic excitation to higher energy levels (excitation processes) ✓ when electrons return to their lower energy states, radiate photons at specific energies (frequencies) characteristic of the allowed energy levels ❑ Strong emission at several UV wavelengths for Hg → most steppers use a single wavelength by filtering out the unwanted emissions → most common are 436 nm (g-line) and 365 nm (i-line) 22  A. Light sources Excimer lasers ❑ Brightest sources in deep UV spectrum are excimer lasers → KrF (248 nm) and ArF (193 nm) ❑ In excimer lasers two elements are present which do not normally react in their unexcited state (often a noble gas and a halogen-containing compound) → if these elements (Kr and NF3 for example) are excited, a chemical reaction forming, for example KrF, is possible. ❑ Excited molecule returns to ground state → photon emitted in DUV and the molecule breaks up ❑ Some problems exist with these exposure sources, e.g., reliability and longevity of the laser, transparency of optical components in the lens systems at these wavelengths, finding suitable resists ❑ First excimer laser systems in the mid-1990s used KrF sources (248 nm) - in the early 2000s were gradually replaced by ArF systems (193 nm) – used ever since 23  A. Light sources Extreme UV (EUV) ❑ EUV using 13.5 nm wavelength photons → such photons have energies of about 92 eV so generating such photons and at high intensities (needed for lithography) is a major challenge ❑ 98% of the EUV energy does not reach the wafer → compared to 193 nm laser source with refracting optics, EUV source must be much brighter (10X) to achieve reasonable throughput in manufacturing ❑ The dominant method to produce 13.5 nm photons for EUV systems is a laser-pulsed Sn plasma ❑ EUV systems just entering high-volume silicon manufacturing for latest generation chips (7 nm , 5 nm) → these systems should make possible continued printing of smaller features for some years to come ❑ ❑ Enormously complex and expensive and only a few leading-edge companies are likely to use them → there is only one manufacturer of EUV exposure systems in the world today — ASML 24 B. Wafer exposure systems ❑ System used to create the aerial image at the photoresist surface ❑ Three general classes of optical wafer exposure tools: ✓ Contact ✓ Proximity ✓ Projection ❑ Only the last is in widespread high- volume manufacturing use today 25  B. Wafer exposure systems ❑ Mask placed chrome side down in direct contact with the resist layer Contact printing → resist exposure takes place by shining light through the mask ❑ Aligning to patterns already on the wafer takes place before exposure ❑ Capable of high-resolution printing because - with the mask and wafer in contact - diffraction effects are minimized ❑ Machines relatively inexpensive - cannot be used in high-volume manufacturing of complex chips → hard contact between the mask and the wafer results in damage to both the mask and the resist layer and therefore results in high defect densities ❑ The chip yields are not compatible with the economic manufacturing of today's chips → only suitable for applications where low volumes or small chip sizes make the economics of these systems more attractive 26  B. Wafer exposure systems Proximity printing ❑ Proximity printing largely solves the defect issues associated with contact printing → the mask and wafer are separated by 5 - 25 μm ❑ Not suitable for manufacturing most of today's chips → the separation of the mask and wafer degrades the resolution of the printed patterns due to diffraction effects ❑ In practice it is not possible to print features smaller than a few μm with UV exposure and gaps on the order of 20 μm ❑ Both contact and proximity systems require 1X masks, which are more difficult to produce than masks for reduction systems. 27  B. Wafer exposure systems Projection printing ❑ The dominant method of wafer exposure today ❑ ❑ Provide high resolution but without defect problems of contact printing → mask physically separated from wafer and optical system used to image the mask on the wafer (resolution still limited by diffraction effects) ❑ Reduction of mask image by 4X to 5X (only a portion of the wafer printed during each exposure) → steppers can print 0.25 μm features over an exposure field of several cm2 and have a throughput of 25 - 50 wafers per hour ❑ Systems designed for high-volume manufacturing cost around $50M and are high-precision machines 28  B. Wafer exposure systems Optics basics – Ray tracing and diffraction ❑ Light travels as an electromagnetic wave through space ❑ Optical system in which all dimensions are very large compared to the wavelength of light → light can usually be treated as particles traveling in straight lines between the optical components → this simplifies the problem to one of "ray tracing" ❑ In projection lithography systems this approach fails when the light passes through the mask because the feature dimensions on the mask are comparable to the wavelength of the light ❑ To understand the behavior of the light as it passes through the mask, the reduction lens and onto the wafer, we must include the wave nature of light ❑ The single most important effect that we must account for is diffraction 29  B. Wafer exposure systems Optics basics – Ray tracing and diffraction ❑ Light does not actually travel in straight lines → diffraction ❑ Simple case: light passing through a small aperture → light pattern on the screen (image plane) covers much larger area ❑ The smaller the aperture the more spread out the screen image becomes diffraction effects ❑ Similar situation found in modern lithography where light passes through a mask with apertures (clear regions) with dimensions on the order of the light wavelength 30  B. Wafer exposure systems Optics basics – Ray tracing and diffraction diffraction effects ❑ Huygens-Fresnel Principle used to construct the wavefront versus position as it propagates: every unobstructed point of wavefront acts as a source of a spherical secondary wavelet of the same frequency as the primary wave ❑ Amplitude of optical field by superimposing wavelets: (a) superposition results in a propagating plane wave - (b) only the source points in the aperture serve as sources of Huygens wavelets → resulting propagating pattern beyond (a) (b) the aperture involves diffraction A plane wave is shown propagating through space, unobstructed in (a) and ❑ Smaller aperture → more light spreads out (fewer wavelets passing through an aperture in (b) pass through aperture) → more spherical propagation beyond the aperture 31  B. Wafer exposure systems Optics basics – Ray tracing and diffraction ❑ Diffraction: "bending" of light when passing through aperture ❑ Passing light carries information about the size and shape of that aperture → if the aperture is part of a mask this information needs to be carried to the photoresist on the wafer ❑ Information spreads out due to diffraction → must be collected to convey perfect information about the aperture to the resist ❑ Finite size of the lens → collects only part of the total diffraction pattern - light diffracted to wider angles carries the information about the finer details of the aperture → lost first when a lens of finite size is used to collect and focus the light 32  B. Wafer exposure systems Optics basics – Ray tracing and diffraction ❑ Actual image produced for a small circular aperture known as Airy's disk after Airy who first derived the expression describing the central intensity maximum (approximates the image of the circular aperture ❑ Because of diffraction the image is composed of a bright center disk surrounded by a series of faint rings - intensity described mathematically and approximate size given by where d is the focusing lens diameter, f is the focal length and λ is the wavelength of the light - a point source only produces a point image if d→∞ or f (and λ)→0 33  B. Wafer exposure systems Optics basics – Ray tracing and diffraction ❑ Two types of diffraction are usually distinguished: Fresnel diffraction: ✓ Fresnel or near field diffraction the image plane is close to the ✓ Fraunhofer or far field diffraction aperture → the light travels ❑ Both are fundamentally due to the same effect - directly from the aperture to the wave nature of light the plane where the image is ❑ In modern lithography systems, Fresnel formed (no intervening lens) diffraction applies to contact and proximity exposure systems and Fraunhofer diffraction applies to projection systems Fraunhofer diffraction: ❑ Mathematical descriptions and simulation tools the image is far from the aperture and a have been developed for both → allow the lens is normally placed between the calculation of the aerial image formed by the aperture and the image plane to capture wafer exposure system and focus the image 34  B. Wafer exposure systems Projection Systems – Fraunhofer diffraction ❑ Performance of projection printers → specified in terms of: ✓ resolution ✓ depth of focus directly related to the basic ✓ field of view properties of optical systems ✓ modulation transfer function (MTF) ✓ alignment accuracy more associated with the ✓ throughput mechanical design of the system 35  B. Wafer exposure systems Projection Systems – Fraunhofer diffraction ❑ Two point sources close together (e.g., two adjacent features on a mask we are trying to print on resist) ❑ How close they can be and still be resolved in the image plane? ❑ Images produced by the two point sources → Airy disks ❑ Rayleigh suggested a reasonable criterion for resolution was that the central maximums of each point image lie at the first minima of the adjacent point image the distance is 36  B. Wafer exposure systems Projection Systems – Fraunhofer diffraction ❑ Resolution R of the lens is index of refraction of the material between the object and the lens (usually air with the maximum half angle of the n =1 in lithography systems) diffracted light that can enter the lens → may be limited by the physical size ❑ The angle α is a measure of the ability of the lens to of the lens or by an entrance collect diffracted light → this property was named aperture in front of the lens the numerical aperture (NA) by Ernst Abbe. 37  B. Wafer exposure systems Projection Systems – Fraunhofer diffraction ❑ Equation derived from the Fraunhofer diffraction pattern for an Airy Disk and as such strictly applies only to point sources ❑ 0.61 factor is often replaced by k ❑ Real lithography systems → mask contains a variety of shapes ❑ k depends in practice on the ability of the resist chemistry to distinguish closely spaced features, on the wafer structure below the resist (topography, reflectivity etc.) and on defocusing at the image plane ❑ Actual k values achieved in practical optical lithography systems are 0.6 to 0.8 38  B. Wafer exposure systems Projection Systems – Fraunhofer diffraction ❑ Shorter exposure wavelengths → better image resolution ❑ Lenses with higher numerical apertures also achieve better resolution → lens able to capture more of the diffracted light and construct a better image ❑ There is also a significant drawback to using higher NA lenses → the depth of focus (DOF) ❑ Factor ½ often replaced by k2 (experimental value) 39  B. Wafer exposure systems Projection Systems – Fraunhofer diffraction ❑ Another useful concept regarding optical exposure systems → modulation transfer function (MTF) ❑ Generic projection lithography system with a reducing lens used to image a mask pattern in resist MTF concept 40  B. Wafer exposure systems Projection Systems – Fraunhofer diffraction ❑ Diffraction effects only important after the light passes through the mask → optical intensity pattern as the light exits the mask almost ideal representation of the mask ❑ Because of diffraction effects and other non-idealities in the optical system → aerial image produced at the resist plane not perfectly black and white ❑ Features widely separated → aerial image may approach the ideal shown on the left MTF concept ❑ Features move closer together → aerial image looks more like that the one on the right 41  B. Wafer exposure systems Projection Systems – Fraunhofer diffraction ❑ Two Airy disks partially overlapping → useful measure of the quality of the aerial image is the MTF where I is the intensity of the light ❑ The MTF is really a measure of the contrast in the aerial image produced by the exposure system ❑ An exposure system needs to achieve MTF > 0.5 for the resist to properly resolve the features ❑ Recently developed deep UV (DIN) resists can work with smaller MTF values 42  B. Wafer exposure systems Projection Systems – Fraunhofer diffraction ❑ MTF depends on the feature size in the image ❑ For large features → resulting aerial image produced by the exposure system has excellent contrast → MTF = 1 ❑ Feature size decrease → diffraction effects cause the MTF to degrade and finally reach zero when the features are so closely spaced (no remaining contrast in the aerial image) 43  B. Wafer exposure systems Projection Systems – Fraunhofer diffraction ❑ MTF also affected the spatial coherence of the light source ❑ An ideal point source produces light in which the waves are in phase at all points along the emitted wavefronts ❑ A condenser lens can then convert these waves to plane waves → all strike the mask at exactly the same angle ❑ Such a source is an ideal coherent source Concept of spatial coherence 44  B. Wafer exposure systems Projection Systems – Fraunhofer diffraction ❑ As the physical size of the source increases → light emitted from a volume rather than a point → waves not perfectly in phase everywhere ❑ If the same condenser lens is used to convert the light to plane waves, the result will be light arriving at the mask from a variety of angles ❑ Such a source is a partially coherent source ❑ In the limit, if the size of the source became infinite (and the condenser lens also infinite to collect all the light) → Concept of spatial coherence the source would become completely incoherent 45  B. Wafer exposure systems Projection Systems – Fraunhofer diffraction ❑ A useful definition of the spatial coherence of practical light sources for lithography is simply ❑ Alternatively, S is defined in terms of the NA of the condenser and projection optics in the exposure tool, Concept of spatial coherence 46  B. Wafer exposure systems Projection Systems – Fraunhofer diffraction ❑ It might appear at first glance that we would choose to have an ideal coherent source (s = 0) for optical lithography → not practically true ❑ First, as s→0, optical intensity also goes to zero, resulting in infinite exposure times to print the mask pattern ❑ Second, MTF also affected by the value of s → having s = 0 not the optimum choice Concept of spatial coherence 47  B. Wafer exposure systems Projection Systems – Fraunhofer diffraction ❑ If the light passing through the mask is partially coherent (at need to consider diffraction different angles) → diffraction patterns resulting from mask effects once again! features will be smeared out (blurred / less sharp) ❑ The diffraction pattern for a given feature will be spread over an angle larger than α ❑ Initially seeming like a bad thing (means that some information will be lost because the finite aperture of the objective lens will not collect it ❑ However, this smearing also means that information from closely spaced features that would have been completely lost outside the aperture of the focusing lens, is now partially collected because it is smeared inside the lens aperture 48  B. Wafer exposure systems Projection Systems – Fraunhofer diffraction ❑ The result of these effects is that the MTF behavior with feature size is modified ❑ s increases → source becomes more incoherent → MTF is degraded for large features ❑ Improved for the smallest features! ❑ Usually a good tradeoff in projection imaging systems that are being pushed towards their resolution limits ❑ In practice, spatial coherence of 0.5 – 0.7 often used in chip manufacturing 49  B. Wafer exposure systems Contact and Proximity Systems - Fresnel Diffraction ❑ Contact and proximity exposure systems operate in the near field or Fresnel diffraction regime ❑ There is no lens between the mask and the resist on the wafer → the diffraction pattern resulting from the light passing through the mask directly impinges on the resist surface ❑ The aerial image depends on the near field diffraction pattern. 50  B. Wafer exposure systems Contact and Proximity Systems - Fresnel Diffraction ❑ Mask and wafer separated by a small gap g ❑ Plane wave incident on an aperture in a mask The diffraction pattern on the opposite side of the mask can be constructed by imagining Huygens wavelets as emanating from each point in the aperture. 51  B. Wafer exposure systems Contact and Proximity Systems - Fresnel Diffraction ❑ Several features in the light distribution of interest: ✓ intensity rises gradually near the edges of the mask aperture → because of diffraction the light "bends" away from the aperture edges producing some resist exposure outside the aperture edges ✓ "ringing" in the intensity distribution within the aperture dimension → because of constructive and destructive interference between the Huygens wavelets emanating from the aperture ❑ Contact and proximity printers often use multiple wavelengths for exposure + not use light sources with perfect spatial coherence → minimize the ringing effects (of course do not eliminate diffraction effects) 52  B. Wafer exposure systems Contact and Proximity Systems - Fresnel Diffraction ❑ Increase of g → quality of the aerial image at the resist surface degrades (diffraction effects more important) ❑ Contact printers minimize these effects by attempting to reduce g to zero (in most practical systems not strictly zero due to topography) ❑ In the limit when the mask and wafer are pressed into hard contact, the resolution of such systems can be very good (well below 0.1 μm) ❑ Resist finite thickness → resolution still limited by a) light scattering in the resist and b) light reflection from surface features on the underlying wafer (scatter the light laterally into regions adjacent to the mask apertures) 53  B. Wafer exposure systems Contact and Proximity Systems - Fresnel Diffraction ❑ Aerial image calculated using Fresnel diffraction theory whenever the gap g falls within the limits where W is the size of the mask aperture (feature size) ❑ Lower limit on g certainly satisfied by proximity printing and often satisfied by contact printing systems (unless hard contact is used) ❑ If gW2/λ → Fresnel diffraction theory replaced by Fraunhofer diffraction theory to calculate aerial image 54  B. Wafer exposure systems Contact and Proximity Systems - Fresnel Diffraction ❑ Within the Fresnel diffraction range, the minimum resolvable feature size is on the order of ❑ Proximity exposure system operating with a g = 10 μm and an i-line light source (λ = 365 nm) can resolve features slightly smaller than 2 μm ❑ Much larger than the dimensions used in modem chips → such systems not useful for manufacturing ❑ Proximity printers much less expensive than the projection systems → for applications in which features sizes are compatible, proximity printers offer an economical solution 55  B. Wafer exposure systems Summary of exposure systems ❑ Plane wave passing through a mask aperture → imaged on the resist through one of the three types of exposure systems ❑ Contact printing → a very high resolution image is produced because the mask and resist are assumed to be in hard contact ❑ Proximity printing → resolution degrades because of near field Fresnel diffraction effects ❑ Projection printing → if we place a lens between the mask and wafer and focus the aperture on the wafer, an image characterized by Fraunhofer diffraction is produced resolution of the proximity system image is inferior to both of the other systems → projection systems are used in manufacturing today 56  Chapter 5 - Lithography Lecture 1 Thank you for your attention Dr. Panagiotis Dimitrakellis [email protected] Prof. Dr. Evangelos Gogolides [email protected] 57