Chapter-5 Lithography

Fabrication Processes for VLSI Devices Chip Fabrication Processes Photo- lithography Silicon Wafer Manufacture Epitaxial Growth Diffusion (Ion Implantation) Metalization oxidation Etching Packaging

Lithography A light sensitive photoresist is spun onto the wafer forming a thin layer on the surface. The resist is then selectively exposed by shining light through a mask which contains the pattern information for the particular being fabricated. The resist is then developed which completes the pattern transfer from the mask to the wafer. Lithography comes from two Greek words, “lithos” which means stone and graphein which means write. “ writing a pattern on stone”

Lithography Lithography is the most complicated, expensive, and critical process of modern IC manufacturing. Lithography transforms complex circuit diagrams into pattern which are define on the wafer in a succession of exposure and processing steps to form a number of superimposed layers of insulator, conductor, and semiconductors materials. Typically 8-25 lithography steps and several hundred processing steps between exposure are required to fabricate a packed IC. The minimum feature size i. e., the minimum line width or line to line separation that can be printed on the surface, control the number of circuits that can be placed on the chip and has a direct impact on circuit speed. The evolution of IC is therefore closely linked to the evolution of lithographic tools.

review

Basic lithography process flow-chapter-5

Photolithography: Positive and Negative photo resist

Photoresist Coating

Photoresist Coating

Spin, Rinse and Dry

Schematic Diagram of Exposure mercury lamp mirror lens mirror mask wafer

Introduction to the Lithography Process 1. Surface Preparation 2. Photoresist Application 3. Soft Bake 4. Align & Expose* 5. Develop 6. Hard Bake 7. Inspection 8. Etch 9. Resist Strip 10. Final Inspection Ten Basic Steps of Photolithography This following slides addresses Engineering Technology Content Standard 3 and 4 on Manufacturing and Materials. * Some processes may include a Post-exposure Bake 13 13 14 14 14 13

Lithography Overview 0.7X in linear dimension every 3 years. While the lithography concept is very simple, the actual implementation is very complex, because of the following demands placed on this process: Resolution - demand for smaller device structures Exposure field - chip size and need to expose at least one full chip (8” wafer) Placement accuracy - alignment with respect to the existing pattern Throughput - manufacturing cost Reduction of defects density - yield loss 0.7X in linear dimension every 3 years. • Placement accuracy  1/3 of feature size. •  35% of total wafer manufacturing costs for lithography. • Note the ???. This represents the single biggest uncertainty about the future of the roadmap.

Light Sources Decreasing feature sizes require the use of shorter . Traditionally Hg vapor lamps have been used which generate many spectral lines from a high intensity plasma inside a glass lamp. (Electrons are excited to higher energy levels by collisions in the plasma. Photons are emitted when the energy is released.) g line -  = 436 nm i line - = 365 nm (used for 0.5 m, 0.35 m)

Historical Development and Basic Concepts Patterning process consists of mask design, mask fabrication and wafer printing. It is convenient to divide the wafer printing process into three parts A. Light source - Shorter wavelength photons B. Wafer exposure system. C. Resist.

Light Sources: Laser Sources The most powerful and commonly used laser sources for deep UV photolithography are the excimer lasers. Kr +NF3  KrF photon emission Energy KrF -  = 248 nm (used for 0.25 mm) ArF -  = 193 nm Issues include finding suitable resists and transparent optical components at these wavelengths. The combination of the high power and deep UV lines makes ArF and KrF attractive sources for advance optical lithography

Wafer Exposure Systems Contact printing is capable of high resolution but has unacceptable defect densities. Inexpensive, diffraction effects are minimize. • Proximity printing cannot easily print features below a few m (except for x-ray systems). Poor resolution due to diffraction effects, required 1 X mask. • Projection printing provides high resolution and low defect densities and \ dominates today. • Typical projection systems use reduction optics (2X - 5X), step and repeat or step and scan mechanical systems, print  50 wafers/hour and cost $5 - 10M.

Wafer Exposure Systems Contact Printer Proximity Printer Projection Printer

Wafer Exposure Systems electronic interface computer Stepper E-Beam Lithography

Wafer Exposure Systems

VLSI Silicon Devices resist silicon O. Rohde, M. Reidiker, S. Schaffner, and J. Bateman, Solid State Technology vol. 29 no. 9 (Sept 1986) p. 110

Photomask A mask for optical lithography consists of a transparent plate called blank, covered with a patterned film of opaque material. The blank is made of soda lime, borosilicate glass, or fused quartz. The advantage of the quartz is that it is transparent to deep UV (365nm) and has a very low thermal expansion coefficient.

Photolithography Three ways to improve resolution Wmin (also R is used in the text) We will derive this expression and analyze all the different means of reducing Wmin(also R)

An approach to improve resolution

Pattern Generation using CAD tools In a typical CAD system pattern is designed with a light pen on a cathode ray tube. The output of the CAD system is usually in the form of a binary data. The data are first translated into machine language and then transmitted to an optical, electron beam, or laser system and finally design shapes on the mask. A laser pattern generation system

Optics - Basics and Diffraction In order to understand the capabilities of modern wafer exposure systems we will need to review some basic concepts about light and optical system. Ray tracing (assuming light travels in straight lines) works well as long as the dimensions are large compared to  i. e. particle nature of the light. At smaller dimensions, diffraction effects dominate i.e., wave nature of the light. Dimensions on the mask are comparable to the wavelength of the light. If the aperture is on the order of , the light spreads out after passing through the aperture. (The smaller the aperture, the more it spreads out.) The light that passes through the aperture (mask) carries with it the information on the size and shape of that aperture (device pattern).

The light diffracted to wider angels carries the information about the finer details of the Aperture (device pattern), which are lost first when lens of finite size is used.

size of the image (diameter).

Projection Systems ( Fraunhofer Diffraction) These are the dominant systems in use today. Performance is usually described in terms of • resolution • depth of focus • field of view • modulation transfer function • alignment accuracy • throughput

How close together can they be and still be resolved in the image plan?  The formula suggests That decreasing the and /or increasing the NA are ways to print a smaller image size. f Two small adjacent features on a mask NA is the ability of Lens to gather light. = K1 NA Where n has been included for generality and is the index of refraction of the material between the object and the lens and R is the minimum feature size.

size of the image (diameter).

Depth of the focus (DOF) Aside from the difficulty of large lenses, there is also a significant draw back to using higher numerical aperture (NA), of a lens. If  is the axis path length difference at the limit of the focus, then the path length difference for a ray from the edge of the entrance aperture is simply cos. The Raleigh criteria for depth of the focus is simply that these two lengths not differ by more than /4 i. e., (is a measure of how much of the diffracted light the lens accepts and image) k2 is usually experimentally determined.

Photolithography-DOF The defocus tolerance (DOF) Much bigger issue in miniaturization science than in ICs A small aperture was used to ensure the foreground stones were as sharp as the ones in the distance. What you need here is a use a telephoto lens at its widest aperture.

Photolithography-DOF

This depth of focus is on the same order as the resist layer thickness itself.

Modulation Transfer Function (MTF) MTF is a measure of the optical contrast in the aerial image by the exposure system. The higher the MTF the better the optical contrast. MTF of an image can be defined as MTF increases with decreasing wavelength. For large features size MTF is unity. As the features size decreases diffraction effects cause the MTF degrade to finally reached zero when the features are so closely spaced that there is no remaining contrast in the image. Partially dark

Spatial Coherence A useful definition of the spatial coherence of practical light sources for lithography is simply S= light source diameter condensed lens diameter Practical light sources are not point sources. Therefore, the light striking the mask will not be plane waves. Typically, S ~ 0.5 to 0.7 in modern systems.

Contact and Proximity Systems ( Fresnel Diffraction) Contact printing systems operate in the near field or Fresnel diffraction regime. There is always some gap between the mask and resist. The aerial image can be constructed by imagining point sources within the aperture, each radiating spherical waves (Huygens wavelets). Interference effects and diffraction result in “ringing” and spreading outside the aperture. Fresnel diffraction applies when Within this range, the minimum resolvable feature size is Wmin kg Typical value of k is close to 1 and deepens on resistor process. Example: if g = 20 µm and an g-line light (436nm) source is used, Wmin 3 µm. This is much larger then the dimension used in modern VLSI chips. However, for application in which features size are compatible with them, proximity printers are an economical solution.

Summary of wafer printing systems In the contact printing system , a very high resolution image is produced i. e., minimum diffraction effect. In a proximity printing system, the resolution degrade because of near field Fresnel diffraction effects. In the projection printing system , diffraction effects are minimized by placing a lens between mask and the wafer. And focus the aperture on the wafer. It is clear from the figure that the resolution of the proximity system is inferior to both of the other systems. This is why projection systems are used in manufacturing today.

Discuss implications of following calculation for the technologist that must manufacture transistors with 0.5 m features. R R Example R

The liftoff process for pattern transfer.

Photoresist Composition The base resin is novolac a long chain polymer consisting of hydrocarbon rings with 2 methyl groups and 1 OH group attached. Photoresist Composition The most commonly used positive resist consist of diazonaphtoquinone (DQ), which is the photoactive compound (PAC), and novolac (N), a matrix material called resin. Upon absorption of UV light, the PAC undergoes a structural transformation which is followed by reaction with water to form a base soluble carboxylic acid, which is readily soluble in basic developer (KOH, NAOH, TMAH etc.)

Basic Properties of Resists: Contrast Curves Two basic parameters are used to describe resist properties, contrast and the critical modulation transfer function or CMTF. Contrast is a measure of the ability of a resist to distinguish between light and dark portion of the mask defined as , The higher the contrast, the sharper the line edge. Resist with high contrast Can actually “sharpen up” a poor aerial image.

Contrast curves are Depend on the Development process Soft back Post exposure back Wavelength of system Surface reflectivity of the wafer At ET the resist Become complete Soluble. Exposure response curve and cross section of the resist image after development. (a) Positive photoresist. (b) Negative photoresist.

Example Find the parameter  for the i Positive photoresist Df=90mJ/cm2 and Do=45 mJ/cm2 ii Negative photoresist Df=7mJ/cm2 Do=12mJ/cm2

Typical g-line and i-line resists achieve  values of 2 - 3 and D100 values of about 100 mJ cm-2. DUV resists have much higher  values (5 - 10) and D100 values of about 20 - 40 mJ cm-2. Gradual Edges on the Resist profile Shows poor Image. Qf Q0 Steep resist edge Shows sharp image. It is important to note that  is not constant for a particular resist composition It depends on process parameters like the development chemistry, bake time Temperature before and after exposure, wavelength of the light, and substrate.

Critical modulation transfer function (CMTF) By analogy to the MTF (dark versus light intensities in the arial image produced by the exposure system) defined earlier for optical systems, the CMTF for resists is defined as In terms of the contrast, can be written as, In general CMTF < MTF is required for the resist to resolve the aerial image. •There are often a number of additional issues that arise in exposing resist.

A 0.6 mm thick layer of a particular photoresist has D0=40mJ/cm2 and D100 =85mj/cm2. (a) Calculate the resist contrast (b) Calculate CMTF © If resist thickness is cut in half, D100 reduced to 70mJ/cm2 while D0 is Unchanged . What is the highest contrast possible in this resist without changing the Resist processing?

Exposures issues: subsurface reflectivity

Exposures issues: subsurface reflectivity

Resolution enhancement techniques: Mask engineering or wavefront engineering Sharp features are lost because higher frequencies are lost due to diffraction. These effects are calculable and can be compensated for. The resolution of an optical system can be improved by increasing the numerical aperture and reducing the wavelength. Increasing the numerical aperture and reducing the wavelength, however, decrease the depth of the focus. Further reduction in the wavelength requires the development of new optical systems and resist compositions.

Resolution enhancement techniques: Mask engineering (1) Optical proximity corrections (OPC) It is known that in the sub 0.5m range, a perfect image on the mask can, from diffraction effect, result in a distorted pattern in the resist. OPC mask attempt to reverse the situation by having a distorted image on the mask that is design to, produce a perfect image on the resist. A computer is used to analyze exposure process conditions. However, the use of OPC are so difficult that they are unlikely to be implemented on a large scale in the near future.

Resolution enhancement techniques: OPC

Photolithography- OPC Optical Proximity Correction (OPC) can be used to compensate somewhat for diffraction effects. Sharp features are lost because higher spatial frequencies are lost due to diffraction. These effects can be calculated and can be compensated for. This improves the resolution by decreasing k1.

Resolution enhancement techniques: (2) Optical Phase Shifting Diffraction problem could be more pronounced as two mask patterns get closer together OPS uses phase shifting to “sharpen” printed images. These techniques can allow existing exposure tools to be used in manufacturing at least one more technology generation. Unresolved pattern

(2)

(3) Off-axis Illumination technique When the angle of illumination and the angle of diffraction are well matched, the amount of light diffracted can be enhanced and the contrast of the image improved.

Photolithography-l

Photolithography-NA At the same time that exposure wavelengths have been reduced, improvements in lens design has led to improvements in the NA of exposure systems lens, see figure . In the mid eighties an NA value of approximately 0.4 was typical, today 248nm exposure systems are available with an NA greater than 0.8. The physical limit to NA for exposure systems using air as a medium between the lens and the wafer is 1, the practical limit is somewhere around 0.9, with recent reports suggesting that an NA as high as 0.93 may be possible for ArF systems in the future .

Photolithography- k1 The third element in the Rayleigh equation is k1. k1 is a complex factor of several variables in the photolithography process such as the quality of the photoresist and the use of resolution enhancement techniques such as phase shift masks, off-axis illumination (OAI) and optical proximity correction (OPC). While exposure wavelengths have been falling and NA rising, k1 has been falling as well, see figure . The practical lower limit for k1 is thought to be about 0.25.

Photolithography-Immersion Litho From the discussion to this point, the resolution limit for 193nm exposure systems may be calculated using the Rayleigh equation with, l = 193nm, NA = 0.93 and k1 = 0.25 or From the above a highly optimized ArF exposure system has an absolute maximum resolution of 52nm, sufficient for 65nm linewidths forecast in 2005, but not capable of meeting the 45nm linewidths forecast in 2007.

Photolithography-Immersion litho NA is determined by the acceptance angle of the lens and the index of refraction of the medium surrounding the lens. The physical limit for an air based system is clear, but what if a medium with a higher index of refraction is substituted for air? Microscopy has for years used oil between the lens and the sample being viewed for resolution enhancement and it is somewhat surprising that the semiconductor industry has taken this long to seriously consider the merits of replacing air with an alternative.

Photolithography-Immersion Litho The medium between the lens and the wafer being exposed needs to have an index of refraction >1, have low optical absorption at 193nm, be compatible with photoresist and the lens material, be uniform and non-contaminating. Surprisingly, ultrapure water may meet all of these requirements. Water has an index of refraction n = 1.47, absorption of <5% at working distances of up to 6mm, is compatible with photoresist and lens and in it’s ultrapure form is non-contaminating.

The primary factor limiting resolution in optical lithography is diffraction. However, because of advancement in excimer lasers, photoresist chemistry and resolution enhancement techniques , optical lithography will remain the main stream technology , at least to the 100nm generation.

Next Generation lithographic methods Why is optical lithography so widely used and what makes it such a promising method? It has high throughput, good resolution, low cost and ease in operation. However, due to deep submicron IC process requirements, optical lithography has limitation that have not yet been solved. Therefore, it is required to find alternatives to optical lithography. The possible promising techniques are: Electron beam lithography Extreme Ultraviolet Lithography X-ray lithography Ion beam lithography

Next Generation Lithography (NGL) 3 3 3 3 3 3 4 4 4 3 3

Schematic of an electron beam lithography machine. Advantages: Generation of submicron Resist geometries Greater depth of focus Direct patterning on a Semiconductor without Using a mask. Currently EBL is the Technology of choice for Mask generation due to Its ability to accurately define small features. Disadvantage: Low throughput

Next Generation Lithography: E-Beam Diffraction is not a limitation on resolution (l < 1 Å for 10-50 keV electrons) Resolution depends on electron scattering and beam optics the size of the beam, can reach ~ 5 nm Two modes of operation: Direct writing with narrow beam Electron projection lithography using a mask :EPL Issues: Throughput of direct writing is very low : research tool or low pattern density manufacturing Projection stepper (EPL) is in development stage only (primarily by Nikon). Mask making is the biggest challenge for the projection method Back-scattering and second electron result in proximity effect –reduce resolution with dense patterns there is also the proximity effect Operates in high vacuum (10-6 –10-10 torr) –slow and expensive 11 11 11 11 11 12 12 12 11

Next Generation Lithography: E-Beam The biggest disadvantage of electron lithography is its low throughput (approximately 5 wafers / hour at less than 0.1 µ resolution). Therefore, electron lithography is primarily used in the production of photomasks and in situations that require small number of custom circuits. The advantages of electron lithography are: (1) Generation of micron and submicron resist geometries (2) Highly automated and precisely controlled operation (3) Greater depth of focus (4) Direct patterning without a mask

Wafer Exposure Systems electronic interface computer Stepper E-Beam Lithography

In raster scan system The beam scan sequentially over every possible location (pixel) on the mask and is Turned off where no exposure Is required. In a vector scan system The beam is directed only to the Requested pattern features and jumps from features to Features.

(a) Raster scan writing scheme. (b) Vector scan writing schemes (a) Raster scan writing scheme. (b) Vector scan writing schemes. (c) Shapes of electron beam: round, variable, and cell projection.

Issue associated with EBL: Proximity effect In EBL the resolution is not limited by diffraction In EBL backscattering causes the electron beam to broaden and expose a large volume of resist then expected. The proximity effect places a limit on the minimum spacing between pattern feature.

SCALPEL® (SCattering with Angular Limitation Projection Electron-beam Lithography) EPL is e-baem with a mask for high-throughput The aspect of SCALPEL which differentiates it from previous attempts at projection electron-beam lithography is the mask. This consists of a low atomic number membrane covered with a layer of a high atomic number material: the pattern is delineated in the latter. While the mask is almost completely electron-transparent at the energies used (100 keV), contrast is generated by utilizing the difference in electron scattering characteristics between the membrane and patterned materials. The membrane scatters electrons weakly and to small angles, while the pattern layer scatters them strongly and to high angles. An aperture in the back-focal (pupil) plane of the projection optics blocks the strongly scattered electrons, forming a high contrast aerial image at the wafer plane

Possible solution of low throughput: Scattering with angular limitation projection electron beam lithography (SCALPEL) This approach has not yet been in a full scale manufacturing environment, but It appears to have significant promise for future lithography needs.

Next Generation Lithography : EUV Uses very short 13.4 nm light All reflective optics (at this wavelength all materials absorb!) Uses reduction optics (4 X) Step and scan printing Optical tricks seen before all apply: off axis illumination (OAI), phase shift masks and OPC Vacuum operation Laser plasma source Very expensive system

An extreme ultraviolet (EUV) lithography system. Challenges: EUV is strongly absorbed In all materials. Lithography process must be performed in vacuum Mask blank must also be multilayer coated to minimize Its reflection.

Schematic representation of a proximity x-ray lithography system. 1nm Advantages: Low diffraction Shorter exposure time Scattering is minimum X –rays pass through spots Problems: Masks are the most Difficult and critical Element of an XRL system lacking of photoresist 1:1 printing High energy x-ray destroy conventional optics Schematic representation of a proximity x-ray lithography system.

Although all non optical lithography techniques have 100 nm or better resolution , each process has its own limitations: Proximity effect in electron beam lithography Mask blank production difficulties in EUV lithography Mask fabrication complexity in X-ray lithography Random space charge effect in ion beam lithography