By Madhushankar 10 Step Patterning

Previous Classes Raw wafer preparation Wafer Fabrication Oxidation

Reference Chapter 8: Microchip Fabrication Peter Van Zant 5 th edition

Definition - Patterning Patterning is the series of processes that establishes the shapes, dimensions, and placement of the required physical “parts” (components) of the IC in and on the wafer surface layers. Patterning is also called photolithography, photomasking, masking, oxide removal (OR)), metal removal (MR), and microlithography. At the end, surface layer is left with either a hole or an island

Patterning It is the process that sets the surface (horizontal) dimensions on the various parts of the devices and circuits. First is to create, in and on the wafer surface, pattern with the dimensions established in the design phase of the IC or device. referred to as the resolution Second goal is the correct placement of the circuit pattern on the wafer. called alignment or registration

Five mask set silicon gate transistor.

IC Processing Flow Materials IC Design Masks IC Fab Test Packaging Final Test Thermal Processes Photo- lithography Etch PR strip Implant PR strip Metallization CMP Dielectric deposition Wafers

IC Fabrication e-Beam or Photo EDA PRChip Photolithography Ion Implant Mask or Reticle Etch EDA: Electronic Design Automation PR: Photoresist

Photolithography Temporarily coat photoresist on wafer Transfers designed pattern to photoresist Most important process in IC fabrication 40 to 50% total wafer process time Determines the minimum feature size

Photolithography Process

Applications of Photolithography Main application: IC patterning process Other applications: Printed electronic board, nameplate, printer plate.

Overview of the Photo masking Process 1 st step  Pattern on the mask is transferred into a layer of photoresist polymerization.  Removing the soluble portion with chemical solvents (developers) leaves a hole in the resist layer corresponds to the opaque pattern

Overview of the Photomasking Process 2 nd step  Transfer takes place from the photo resist layer into the wafer surface layer  transfer occurs when etchants remove the portion of the wafer’s top layer that is not covered by the photo resist.

Concept of Holes & Islands Previous example - > created holes The hole came about because the pattern in the mask was opaque to the exposing light A mask whose pattern exists in the opaque regions is called a clear-field mask clear-field

Concept of Holes & Islands The pattern could also be coded in the mask in the reverse, in a dark-field mask. If the same steps were followed, the result of the process would be an island of material left on the wafer surface

Types of photo resists 1. -ve: previous example 2. +ve: Within the resists, the light changes the chemical structure from relatively non soluble to much more soluble -> “photosolubilization” An island is produced when a light-field mask is used with a positive photoresist Mask and photoresist polarity results.

10 step process (clear field mask & -ve photo resist)

10 step process

2 nd Step 10 step process

Assignment: List the steps and draw the cross sections for Dark field mask & +ve photo resist

Photo resist components and their roles 1. Polymers Commonly used resists are designed to react to ultraviolet or laser sources -- optical resists In a negative resist, the polymers change from un- polymerized to polymerized after exposure to a light or energy source (polyisopreme) - polymerization The basic positive photoresist polymer is the phenol- formaldehyde polymer photo -solubilization

Photo resist components and their roles 2. Solvents Largest ingredient by volume in a photoresist Makes the resist a liquid and allows the resist to be applied to the wafer surface as a thin layer For negative photoresist, the solvent is an aromatic type, xylene. In positive resist, the solvent is either ethoxyethyl acetate or 2-methoxyethyl.

Photo resist components and their roles 3. Sensitizers. Chemical sensitizers are added to the resists to cause or control certain reactions of the polymer Sensitizers are added to either broaden the response range or narrow it to a specific wavelength.

Photo resist components and their roles 4. Additives Various additives are mixed with resists to achieve particular results. Some negative resists have dyes intended to absorb and control light rays in the resist film. Positive resists may have chemical dissolution inhibitor systems. inhibit the dissolution of non exposed portions of the resist during the development step.

Photoresist Performance Factors Dimensions required on the wafer surface. Function as an etch barrier during the etching step, a function that requires a certain thickness for mechanical strength. must be free of pinholes In addition, it must adhere to the top wafer surface.

1.Resolution capability The smallest opening or space that can be produced in a particular photoresist is generally referred to as its resolution capability Smaller the opening or space produced, the better the resolution capability. Thinner – more pinholes Thicker – less pinholes and acts etch barrier – Trade off The capability of a particular resist relative to resolution and thickness is measured by its aspect ratio

1.Resolution capability Positive resists have a higher aspect ratio as compared to negative resists for a given image-size opening, the resist layer can be thicker. The ability of positive resist to resolve a smaller opening is a result of the smaller size of the polymer.

2.Adhesion capability A photoresist layer must adhere well to the surface layer to faithfully transfer the resist opening into the layer. Lack of adhesion results in distorted images. Resists differ in their ability to adhere to the various surfaces used in chip fabrication. Negative resists generally have a higher adhesion capability than positive resists

3.Photoresist exposure speed, sensitivity, and exposure source The primary action of a photoresist is a change in structure in response to an exposing light or radiation. The faster the speed, the faster the wafers can be processed through the masking area. Negative resists typically require 5 to 15 sec of exposure time, whereas positive resists take three to four times longer.

Photoresist exposure speed, sensitivity, and exposure source The sensitivity of a resist relates to the amount of energy required to cause the polymerization or photo-solubilization to occur. Common positive and negative photo-resists respond to energies in the ultraviolet and deep ultraviolet (DUV) X-rays or electron beams. Resist sensitivity, as a parameter, is measured as the amount of energy required to initiate the basic reaction.

4.Process latitude Goal of the overall process is a faithful reproduction of the required image size in the wafer layer(s). Every step has an influence on the final image size, and each of the steps has inherent process variations. Some resists are more tolerant of these variations; that is, they have a wider process latitude. The wider the process latitude, the higher the probability that the images on the wafer will meet the required dimensional specifications.

5.Pinholes Pinholes are microscopically small voids in the resist layer. allow etchants to seep through the resist layer and etch small holes in the surface layer. Due to particulate contamination in the environment, the spin process, and from structural voids in the resist layer. The thinner the resist layer, the more pinholes. Thicker films have fewer pinholes, but they make the less resolution Classic trade-offs in determining a process resist thickness. Positive resists have got higher aspect ratio Allows a thicker resist film and a lower pinhole count for a given image size.

6.Particle and contamination levels Resists, like other process chemicals, must meet stringent standards for particle content, sodium and trace metal contaminants, and water content.

7. Step coverage By the time the wafer is ready for the second masking process, the surface has a number of steps. As the wafer proceeds through the fabrication process, the surface gains more layers. For the resist to perform its etch barrier role, it must maintain an adequate thickness over these earlier layer steps. The ability of a resist to cover surface steps with adequate resist is an important parameter.

8. Thermal flow 2 heating steps. soft bake, evaporates solvents from the resist. hard bake, takes place after the image has been developed in the resist layer. The purpose of the hard bake is to increase the adhesion of the resist to the wafer surface. However, the resist, being a plastic-like material, will soften and flow during the hard-bake step.

8. Thermal flow The resist has to maintain its shape and structure during the bake, or the process design must account for dimensional changes due to thermal flow. The goal is to achieve high bake temperature as possible to maximize adhesion. This temperature is limited by the flow characteristics of the resist. In general, the more stable the thermal flow of the resist, the better it is in the process.

Comparison of negative and positive results

Surface Preparation Photo masking » Painting 3 stages: Particle removal, Dehydration, and Priming.

Particle Removal - Wafer Clean Remove contaminants Remove particulate Reduce pinholes and other defects Improve photoresist adhesion Basic steps Chemical clean Rinse Dry

Dehydration Baking Due to exposure to moisture, either from the air or from post-cleaning rinses -> hydrophilic To maintain hydrophobic surface Keep the room humidity below 50 percent Coat the wafers with photo resist as quickly as possible Additional steps Dehydration bake and Priming with a chemical

Pre-bake and Primer Vapor P-Well Polysilicon Primer

Prebake Dehydration bake Remove moisture from wafer surface Promote adhesion between PR and surface Usually around 100 °C Integration with primer coating

Primer Promotes adhesion of PR to wafer surface Wildly used: Hexamethyldisilazane (HMDS) HMDS vapor coating prior to PR spin coating Usually performed in-situ with pre-bake

Photoresist Coating P-Well Polysilicon Photoresist Primer

Spin Coating Goal - establishment of a thin, uniform, defect-free film of photo resist on the wafer surface. Spin Process - designed to prevent or minimize the build up of a bead of resist around the outer edge of the wafer. Edge bead

Static dispense Wafer sit on a vacuum chuck Slow spin ~ 500 rpm Liquid photo resist applied at center of wafer Ramp up to ~ rpm Photo resist spread by centrifugal force Evenly coat on wafer surface

Dynamic dispense Wafer is rotated at a low speed of ~ 500 rpm. Resist is dispensed onto the surface. The action of the rotation assists in the initial spreading of the resist. Less resist is used, and a more uniform layer is achieved. After spreading, the spinner is accelerated to a high speed spread and thin the resist into a uniform film.

Moving-arm dispensing Improvement on the dynamic dispense technique Moving-arm resist dispenser The arm moves in a slow motion from the center of the wafer toward its edge. This action creates more uniform initial and final layers. Saves resist material, especially for larger-diameter wafers

Ready For Soft Bake Spindle To vacuum pump Chuck Wafer

Soft Bake P-Well Polysilicon Photoresist

Purpose of Soft Bake Evaporating most of solvents in PR Solvents help to make a thin PR but absorb radiation and affect adhesion Soft baking time and temperature are important parameters Over bake: polymerized, less photo-sensitivity Under bake: affect adhesion and exposure

Methods of Soft Bake Conduction- Hot plates Convection forced-air furnaces, hair dryers, air- and nitrogen fed ovens, oxidation furnaces Radiation

Baking Systems Heater Vacuum Wafer Heater Heated N 2 Wafers MW Source Vacuum Wafer Photoresist Chuck Hot plateConvection ovenMicrowave oven

Alignment and Exposure One of the basic ten patterning steps, with two separate actions positioning or alignment of the required image on the correct location on the wafer surface. encoding of the image in the photo resist layer from an exposing light or other radiation source.

Alignment P-Well Polysilicon Photoresist Gate Mask

Exposure Gate Mask P-Well Polysilicon Photoresist

Alignment and Exposure Most critical process for IC fabrication Most expensive tool (stepper) in an IC fab. Most challenging technology Determines the minimum feature size

Exposure sources Chosen to create the required image size in conjunction with a specific photo resist Dependable exposure source has been the high-pressure mercury lamp light in the UV range Resists are tailored to respond to only narrow bands (lines) in the mercury lamp spectrum shorter wavelengths of the spectrum - deep ultraviolet or DUV lasers, X-rays, and electron beams