Functional Polymer/1st Semester, 2006 ______Part III. Functional Polymers for Semiconductor Applications

„ Outline of Part Photoresist for Semiconductor Applications

‰ Introduction of photolithography

‰ Photoresist Materials for Exposure at 193 nm Wavelength

‰ Chemically Amplified Resists for F2 Excimer laser Lithography

Prof. Jin-Heong Yim Motivations

„ Creation of integrated circuits, which are a major component in computer technology

„ An extension of photolithography processes are used to create standard semiconductor chips

„ Play a key role in the production of technically demanding components of advanced microsensors

„ Used to make adhesives in electronics

Prof. Jin-Heong Yim History

„ Historically, lithography is a type of printing technology that is based on the chemical repellence of oil and water.

„ Photo-litho-graphy: latin: light-stone-writing

„ In 1826, Joseph Nicephore Niepce, in Chalon, France, takes the first photograph using bitumen of Judea on a pewter plate, developed using oil of lavender and mineral spirits

„ In 1935 Louis Minsk of Eastman Kodak developed the first negative photoresist

„ In 1940 Otto Suess developed the first positive photoresist.

„ In 1954, Louis Plambeck, Jr., of Du Pont, develops the Dycryl polymeric letterpress plate

Prof. Jin-Heong Yim Microlithography

A process that involves transferring an integrated circuit pattern into a polymer film and subsequently replicating that pattern in an underlying thin conductor or dielectric film

Prof. Jin-Heong Yim How Small Can We Print ?

SEM picture of typical lithographic pattern Comparison of the dimensions of lithographic images and familiar objects

Thompson, L. F.; Willson, C. G.; Bowden, M. J. Introduction to Microlithography; 2nd Ed; ACS Professional Reference Book; American Chemical Society; Washington, DC, 1994

Prof. Jin-Heong Yim Roadmap of Semiconductor Technology

10 - C.Rubber Novolak KrF ArF F2, - Bisazide - DNQ CAR CAR EUV, X-Ray CAR 16 K 1977 Resolution 5.0 64 K 1980 λ 3.0 256 K R = k 1.0 1983 64 M NA 2.0 1 M 1995 1986 0.35 256 M 1.2 4 M 1998 1989 0.25 1 G 0.8 16 M 2001 1992 0.18 4G 0.5 2004 16 G 0.13 2007 0.09 0.1

F , Contact g-line i-line 2 KrF ArF EUV, Printer Stepper Stepper X-Ray 19801985 1990 1995 2000 2005 2010

Prof. Jin-Heong Yim Light intensity(output) of Hg-lamp as a function of wavelength

I-line G-line

DUV

Thompson, L. F.; Willson, C. G.; Bowden, M. J. Introduction to Microlithography; 2nd Ed; ACS Professional Reference Book; American Chemical Society; Washington, DC, 1994

Prof. Jin-Heong Yim Photolithography

„ In photolithography, the pattern is created photographically on a substrate (silicon wafer)

„ Photolithography is a binary pattern transfer: there is no gray-scale, color, nor depth to the image

„ This pattern can be used as a resist to substrate etchant, or a mold, and other forms of design processes

„ The steps involved are wafer cleaning, photoresist application, soft baking, mask alignment, and exposure and development

Prof. Jin-Heong Yim Photoresist

„ Photoresist is an organic polymer which changes its chemical structure when exposed to ultraviolet light.

„ It contains a light-sensitive substance whose properties allow image transfer onto a printed circuit board.

„ There are two types of photoresist: positive and negative

Prof. Jin-Heong Yim PR Process

Photoresist Coating Substrate hu

Mask Exposure e a iePositiveNegative

Develop

Etch

Strip

Prof. Jin-Heong Yim Chemical Amplification

Solution : 1. More sensitive photoresists (Very high efficiency)

Photoacid Generator(PAG)

hv

Strong Acid

H + H H3 C CH CH 2 CH CH2 + CO2 + + C CH2

n n H3C

CH3

O O C CH3 OH C CH3 O "Catalytic Cycle" H3C + t-BOC H + C CH2 H3C

2. Brighter light source (e.g. KrF laser for 248nm and ArF laser for 193nm)

Prof. Jin-Heong Yim How Novolac/DNQ Resists Work

O OH COOH N2 I I I I I I UV CH2 I I I I IICoat

R R Si CH3 n (Base insoluble) (Base soluble) DNQ(Diazonaphthoquinone) Carboxylic acid Novolac Resin (N) Inhibitor (I) Photoproduct (P) UV

novolac + photoproducts I I P P P I I Expose pure I I P P P II Develop. novolac Rate Si

I I I I I I Development novolac I I + DNQ Si

Reaction Coordinate

Prof. Jin-Heong Yim Two Types of Photoresist

Positive Photoresist Negative Photoresist

„ „ Exposure to UV light Exposure to UV light makes it more soluble causes the resist to in the developer polymerize, and thus be more difficult to „ Exposed resist is dissolve washed away by developer so that the „ Developer removes the unexposed substrate unexposed resist

remains „ This is like a

„ Results in an exact photographic negative copy of the original of the pattern design

Prof. Jin-Heong Yim Preparation and Priming

„ Prepare the substrate (silicon wafer): ‰ Wash with appropriate solvent to remove any dirt and other impurities „ Acetone, MeOH, TCE ‰ Dry in Oven at 150°C for 10 min. ‰ Place on hotplate and cover with petri dish, let temp. stabilize at 115°C. „ Deposit Primer (optional) ‰ Chemical that coats the substrate and allows for better adhesion of the resist

Prof. Jin-Heong Yim Spin Coating

„ Spin-coat the photoresist onto the surface of the wafer

‰ RPM: 1000-7000

‰ Time: ~30 sec

‰ Produces a thin uniform layer of photoresist on the wafer surface. „ Use red/amber safe light at this stage

Prof. Jin-Heong Yim Soft Baking

„ Put on hotplate, or in oven

‰ Temperature: 65°C-115°C, Time: 1-5 min

„ Removes volatile solvents from the coating

„ Makes photoresist imageable

„ Hardens to amorphous solid

„ Be careful not to overbake and destroy the sensitizer

Prof. Jin-Heong Yim Mask Alignment and Exposure

„ Photomask is a square glass plate with a patterned emulsion of metal film on one side „ After alignment, the photoresist is exposed to UV light „ Three primary exposure methods: contact, proximity, and projection

Prof. Jin-Heong Yim Exposure Methods

Prof. Jin-Heong Yim Resolution of Lithographic Process

* Use of shorter wavelength for exposure to get smaller feature size

“Lens Equation” Light R= k λ 1 NA Lens θ R: Resolution (feature size) Coated λ:wavelength of light source Wafer k1:Process factor NA: Numerical aperture (NA= sin θ)

Prof. Jin-Heong Yim Photoresist Developer

„ Highly pure buffered alkaline solution

„ Removes proper layer of photoresist upon contact or immersion

„ Degree of exposure affects the resolution curves of the resist

Prof. Jin-Heong Yim Hard Baking

„ Final step in the photolithographic process

‰ Not always necessary; depends on the resist „ Hardens the photoresist „ Improves adhesion of the photoresist to the wafer surface

Prof. Jin-Heong Yim