c05 - 1 virginia tech effect of resist thickness resists usually do not have uniform thickness on...

C05 - 1Virginia Tech

Effect of Resist Thickness

Resists usually do not have uniform thickness on the wafer– Edge bead: The build-up of resist along the

circumference of the wafer- There are edge bead removal systems

– Step coverage

Centrifugal ForceCentrifugal Force


Effect of Resist Thickness

The resist can be underexposed where it is thicker and overexposed where it is thinner– This can lead to linewidth variations

Light intensity varies with depth below the surface due to absorption

where is the optical absorption coefficient Thus, the resist near the surface is exposed first

– We have good fortune. There is a process called bleaching in which the exposed material becomes almost transparent

i.e., decreases after exposure to light- Therefore, more light goes to deeper layers

)exp()( 0 xIxI


C. A. Mack, “Absorption and exposure in positive photoresist”, Appl. Opt. C. A. Mack, “Absorption and exposure in positive photoresist”, Appl. Opt. 2727(23), Dec. 1, 1988, (23), Dec. 1, 1988, pp. 4913-4919.pp. 4913-4919.

exposedexposed = B and = B and unexposedunexposed = A+B = A+B


Photoresist Absorption

If the photoresist becomes transparent, and if the underlying surface is reflective, reflected light from the wafer will expose the photoresist in areas we do not want it to.

However, this leads to the possibility of standing waves (due to interference), with resultant waviness of the developed resist

We can solve this by putting an antireflective coating on the surface before spinning the photoresist increases process complexity


Standing Waves due to Reflections


Standing Waves Due to Reflections

http://www.lithoguru.com/scientist/lithobasics.htmlhttp://www.lithoguru.com/scientist/lithobasics.html


(a) (b) (c)

Diffusion during a post-exposure bake (PEB) is often used to reduce standing waves.

Photoresist profile simulations as a function of the PEB diffusion length: (a) 20nm, (b) 40nm, and (c) 60nm.

http://www.lithoguru.com/scientist/lithobasics.htmlhttp://www.lithoguru.com/scientist/lithobasics.html

Removal of Standing Wave Pattern


Mask Engineering

There are two ways to improve the quality of the image transferred to the photoresist– Optical Proximity Correction (OPC)– Phase Shift Masks (PSM)

We note that the lenses in projections systems are both finite and circular

Most features on the mask are square We lose the high frequency components of the

pattern We thus lose information about the

“squareness” of the corners


Mask Engineering

The effects are quite predictable We can correct them by adjusting feature

dimensions and shapes in the masks


Mask Engineering


Phase Shift Masks

In a projection system, the amplitudes of the diffracted light at the wafer add– Closely spaced lines interact; the intensity at the

wafer is smeared If we put a material of proper index of refraction on part

of the mask, we can retard some of the light and change its phase by 180 degrees– Properly done, the amplitudes interfere

The thickness of the PS layer is

n is the index of refraction of the phase shift material

12

nd


Phase Shift Masks (PSM)

Intensity Intensity pattern is pattern is barely barely sufficient sufficient to resolve to resolve the two the two patterns.patterns.


Scanning Projection Aligners

The wafer is simultaneously scanned, thus printing the mask on the wafer

Systems of the type shown on the next page are cost effective, but they must use 1:1 masks– The concept is that it is easier to correct for

aberrations in small regions than a large area– Integration of a focusing laser and vertical positioner

allowed adjustment of imaging plane to maximize resolution

– Technology became obsolete as wafer size increased and linewidths became smaller


Scanning Projection Printer


Step and Repeat Projection Systems

Steppers expose a limited portion of the wafer at a time– Features on the masks (reticules) are 4-5X

the size of the features exposed on the wafer Steppers also allow better alignment because

they align on the exposure field rather than for the entire wafer– Integration of a focusing laser and vertical

positionerLaser is also used to read information that is scribed on wafer prior to processing


Projections Systems

Key features of steppers include– Kohler illumination– Off-axis illumination

Kohler illumination focuses the light at the entrance pupil of a projection lens, rather than on the photoresist

This setup allows the projection lens to capture the diffracted light from any features on the mask


Kohler Illumination


Off-Axis Illumination

By changing the angle of incidence of the light on the mask, we also change the angle of the diffracted light– Although some of the diffracted light is lost

in this scheme, much of the higher order diffraction is captured


Off-Axis Illumination


Step and Scan Aligners

https://www.chiphistory.org/product_content/lm_asml_pas5500-400_step&scan_system_1990_intro.htm


Step and Scan


DNQ/Novolac Resist Process

Hexamethyldisilane (HMDS) is often used as an adhesion promoter– As a liquid, drops are deposited on the wafer and then

spread by spinning at 3000 – 6000 rpm for 30 s– Sometime HMDS is applied from the vapor

The surface chemistry is that the silane end of the molecule bonds with the Si while the other end bonds with the resist

Resist is then spun on immediately following HMDS

http://bmrc.berkeley.edu/courseware/ICMfg92/images/gif/spin-on.gif



Prebake is usually done on a hot plate at 90—100oC– Infrared or microwave heating can also be used

This step:– Evaporates the last of the solvent

Solvent content in the photoresist film decreases from 25% to 5%

– Adhesion is improved because heat strengthens the bonds between the resist and HMDS

– Stresses in the resist caused by spinning are thermally relieved

The resist flows slightly



Exposure times and source intensity are reciprocal—one can reduce exposure times with more intense sources

Exposure time is increase by increasing the bake temperature (due to decomposition of the PAC and thus decreased sensitivity)

The postexposure bake is often done before development because the PAC can diffuse and this will eliminate the standing wave pattern

The developer is a basic solution such as TMAH, NaOH, or KOH and is applied by immersion, or spraying



Rinsing in H2O stops the development process The rate for developing depends strongly on

temperature, developer concentration, and the exposure and bake procedures– The chemistry is the dissolution of the carbolic acid

The final step is postbake (typically 10—30 min at 100—140 C)

This hardens the resist and improves etch resistance The resist flows a bit during the process, and all

remaining solvents are driven off

Many of the steps described above are done in a single system called a wafer track system



A SITE system ESVG 86 track and coat system


Measurement Methods

Measurements are made to determine– Mask Features and defects– Resist Patterns– Etched Features– Alignment Accuracy (x-y-theta)


Measurement of Mask Features and Defects

Because almost all production now uses reducing steppers, a defect in a mask will produce a defect in every die– Therefore the mask must be “perfect”

Because of the complexity of the masks, the inspection must be fully automated– manual observation under a microscope is

not possible The process is illustrated in the next slide


Mask Inspection System


Measurement of Mask Features and Defects

Light is passed through the mask and collected by an image recognition system– Solid state detectors are used to collect the light– The information is compared against the database of

the mask design or with an identical mask The inspection process is more difficult if the mask

contains OPC (optical proximity correction) or is a PSM (phase shift mask)

Often, defects found in this process can be corrected– Lasers can burn off excess Cr– Adding Cr to clear areas is harder

Done using chemical vapor deposition


SEM Measurement


State-of-the-Art

Capable of exposing down to ~ 10nm – E-beam lithography– X-ray lithography– Extreme UV lithography

E-beam and EUV are performed under vacuum– Throughput is very slow

New resist families are required– Most are very difficult to remove after use

Research needed on mask material for x-ray and EUV– Glass absorbs– Thickness of metal needed to block x-rays is very

thick (20-50m)

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