Prof. Dong-Weon Lee

Introduction to MEMS

(Lecture 5)

Prof. Dong-Weon Lee

MEMS & Nanotechnology Laboratory School of Mechanical Systems Engineering

Chonnam National University

Today

1. Wafer production 2. Crystal: Silicon

3. Wafer identification 4. Process overview 5. Photolithography

6. Alternative technologies for lithography ???

Integrated circuit fabrication

Silicon ingot

Silicon ingot

Unit cubic of silicon

Wafer identification

R vs. dopant level

Overview of micromachining

1. 유전체와 금속층을 기판 위에 증착하는 역할 2. 확산보다 낮은 온도에서 시행

3. CVD와 PVD는 낮은 진공조건에서 실행

1. 유전체와 금속층을 기판 위에 증착하는 역할

2. 확산보다 낮은 온도에서 시행

3. CVD와 PVD는 낮은 진공조건에서 실행

1. 유전체와 금속층을 기판 위에 증착하는 역할

2. 확산보다 낮은 온도에서 시행

3. CVD와 PVD는 낮은

Overview of micromachining

1. 포토리소그래피는 가장 중요한 공정 중의 하나 2. 웨이퍼 표면위에 디자인된 패턴을 마스크 또는

1. 감광막 패턴으로부터 보호받지 못한 부분의 웨이퍼를 식각함으로 영구적인 패턴을 형성 2. Plasma etcher와 화학용액 이용

1. 유전체와 금속층을 기판 위에 증착하는 역할 2. 확산보다 낮은 온도에서 시행

3. CVD와 PVD는 낮은 진공조건에서 실행

Clean room

Originally developed by NASA and the aerospace industry for satellite

manufacturing. Clean rooms now in use for all MEMS and semiconductor

manufacturing.

Clean room for microfabrication

Clean room for microfabrication

Clean room classification

DI water

• Water is the most prevalently used material in microfabrication processes, and is used mainly for rinsing and cleaning of wafers

• Approximately 6000 gallons of de-ionized(DI) water are required for each 6” CMOS wafer

• DI water must be manufactured on site to achieve the quality and purity levels required by modern microfabrication

• Each gallon of DI water may require as much as 4-6 gallons of raw city grade water to manufacture.

• DI water must be continuously recirculated in order to achieve the quality and purity levels

Clean water: de-ionized water

HEPA filter

How Big of a Particle is Tolerable?

– Example: 0.5 m CMOS technology

• Lateral Features:

– pattern size = 0.5 m

– pattern tolerance = 0.15 m

– level-level registration = 0.15 m

• Vertical Features:

– gate oxide thickness = 10 nm – film thicknesses = 250-500 nm – junction depths = 50-150 nm

Clean Room Air Filters

• High Efficiency Particulate Air (HEPA) Filters – most common type of clean room air filter

– high efficiency, low pressure drop, good loading characteristics – uses glass fibers in a paper-like medium

– are rated by their particle retention:

• A true HEPA-rated filter will retain 99.97 % of incident particles of 0.3 m.

Characteristics of cleanroom

– Air is recirculated through HEPA filters with about 20 % make up

• Vapors are entrained, so contamination potential is very high

• Extensive gas detection and alarm systems are installed

– Temperature is controlled to 68 - 72 °F (20 ~ 22 °C ) – Humidity is controlled to 40 - 46 % RH

– Room is held at positive pressure

• Typically 0.1 in H

O for Class 100, Class 1000, and Class 10,000

• Positive pressure constantly blows dust OUT

(Biohazard rooms operate at negative pressure to keep bugs IN)

• Doors open inward, so room pressure closes them shut

• 0.1 in H

O = 3.6 x 10

psi = 0.52 lb/ft

• This produces 9.1 lbs.(4.13kg) force on a 7’ x 30” door

Types of cleanroom

Laminar Flow Cleanroom

Turbulent Cleanroom

Dos and don’t in cleanroom

• Don’t:

– touch your face or skin with gloves

– touch building hardware, oily machinery, or wafer loading areas – lean on equipment

– wear cosmetics, powders, or colognes

– wear anything on fingers-- remove all rings and bracelets – use paper, pencils or markers that leave dust or lint

• Do:

– change gloves whenever they get dirty or torn

– use a fresh pair of gloves whenever handling wafers

– wipe down wafer handling areas with isopropanol

– use clean room paper and dust-free ball point pens

Clean benches

Vertical and Horizontal Laminar Benches

기본적인 Cleaning 개념

유기물 제거 금속불순물 제거 산화막 제거

이온성 불순물 제거 입자성 불순물 제거

산화막 제거 DI water Dry

RCA 1 (Organic dirt)

DI water(5):NH

OH(1), in a Pyrex beaker, and then heat to 70±5C on hot plate.

Remove from hot plate and add a part of H

O

. Solution will bubble vigorously after 1–2 minutes, indicating that it is ready for use.

RCA 2 (metal ions: Au, Cu, Cr, Na etc.)

DI water(6):HCl(1) in a Pyrex beaker, and then heat to 70±5C on hot plate. Remove from hot plate and add 1 part of H

O

. Solution will bubble vigorously after 1–2 minutes, indicating that it is ready for use.

Cleaning process : wafer

RCA 1

RCA 2

Steps in photolithography

1. Wafer cleaning

2. Apply adhesion promoter (HMDS)

3. Spin coat photoresist at 1000 – 10,000 rpm

4. “Soft bake” (90 – 120°C for 60 –120 sec) to remove solvent 5. Alignment

6. Exposure

7. Development

8. “Post bake” (100 – 180°C) to increase adhesion 9. Etch exposed regions

10. Strip resist

http://www.youtube.com/watch?v=9x3 Lh1ZfggM&feature=related

Key steps in photolithography

Photolithography: transfer patterns from a mask into photoresist

Optical lithography: Types

Photolithography: Mask aligner

Mercury bulbs (Hg Arc Lamp)

High pressure Hg Arc lamp spectrum

Photolithography

Positive: gets more soluble after exposure

Negative: gets less soluble after exposure.

Photoresist

- Photoresist is an organic polymer which becomes soluble

when exposed or non-exposed to ultraviolet light (UV)

Photoresist: Positive & Negative

Positive PR

DQ:

PAC는 developer에서 용해 비율을 10배 혹은 그 이상의 차이를 만듦  N

의 약 결합이 원인

(UV light를 가지고 exposure  Energy를 받아 N

의 연결 고리를 끊음)

Phenolic resins which contain ICA

instead of DQ are readily dissolved by aqueous alkaline developers.

Photonic Active Compound (PAC)

Wolff arrangement

탄소고리로부터 질소는 자유로워지고 이 반응이

안정화 되기 위해 고리 바깥쪽으로 하나의 탄소원자가 이동

Positive PR

Negative PR

N₃의 활성화  N₂ gas 배출

다른 고분자 연결체와 결합으로 분자량 증가

 녹지 않게 됨

현상 도중에 현상액을 흡수하여 패턴의 팽창이 발생: swelling effect

Comparison of PRs (Important)

Several PRs

SU-8 photoresist

General PR과의 차이점

SU-8 photoresist

Spin coating process

- Thickness of the photoresist depends on

Concentration, Viscosity, Spin speed, Spin time

Spin speed

PR thickness

Spin time

PR thickness

- Photolithography steps

Photoresist spinning, 1 to 50 mm spin coat Optical exposure through a photomask

Developing to dissolve exposed or unexposed resist

Bake to drive off solvents

Spin coating process

Photolithography 1: make a check list

Piranha, RCA 1 and RCA 2 cleaning

Ex. AZ 1512

1.2 m at 4000 rpm

Check run-out

8.5s at 12mW

Photolithography 2

Soft bake

목적: 용제(solvent)를 증발시켜 감광막을 건조시키고, 웨이퍼와의 접착력을 향상시키며, 열에 의한 annealing effect로 응력을 완화

장비Convection oven: 10 ~ 30분, 모든 웨이퍼에 동일 온도 유지 가능

Infra-red oven: 3 ~ 4분, 웨이퍼에 종류에 따른 반사 및 흡수의 차이로 동일 온도 유지가 어려움 Hot-plate: 0.5 ~ 1분, 가장 단시간에 가능, 진공흡착 등의 장비가 필요

적정온도 유지 필요: 과소  Orange peel (Nega), mask 접착 (Posi, Nega) 과대  열 다중화에 의한 Scum 현상

Pattern size가 줄어들수록 soft bake의 온도와 시간은 매우 중요한 변수가 됨

Photolithography 3

AZ 400K:DI(1:4) or AZ 300 MIF

 Inspection

AZ remover Inspection

Photolithography 4

Hard bake

목적: 잔여용제(solvent)를 제거하여 감광막을 건조시키며 용액에서의 공정에서 기판에 대한 감광막의 접착도 증가

장비Convection oven: 30분, 모든 웨이퍼에 동일 온도 유지 가능

Infra-red oven: 3 ~ 4분, 웨이퍼에 종류에 따른 반사 및 흡수의 차이로 동일 온도 유지가 어려움 Hot-plate: 1 ~ 3분, 가장 단시간에 가능, 진공흡착 등의 장비가 필요

적정온도 유지 필요: 과대  Puddling: 감광막이 오므라드는 현상(Nega) 다음 공정 후 감광막의 제거가 힘이 듬 (Posi)

Photolithography 5

Importance of a bake process

How to remove PRs

Photoresist Removal (Stripping)

• Want to remove the photoresist and any of its residues.

• Simple solvents are generally sufficient for non-postbaked photoresists:

– Positive photoresists:

• acetone

• trichloroethylene (TCE)

• phenol-based strippers (Indus-Ri-Chem J-100) – Negative photoresists:

• methyl ethyl ketone (MEK), CH3COC2H5

• methyl isobutyl ketone (MIBK), CH3COC4H9

• Plasma etching with O2 (ashing) is also effective for removing organic polymer debris.

– Also: Shipley 1165 stripper (contains n-methyl-2-pyrrolidone), which is

effective on hard, postbaked resist.

Mask design

Photomask

- Photomasks

Layout generated from CAD file (GDSⅡ file)

Mask reticle: chrome or emulsion on a glass or a quartz 1-3 $k depends on mask materials

• Types:

– film mask (200$/mask, cheapest)

– photographic emulsion on soda lime glass – Fe

O

on soda lime glass

– Cr on soda lime glass

– Cr on quartz glass (2000$/mask, most expensive, needed for deep UV litho)

• Dimensions:

– 4” x 4” x 0.060” for 3-inch wafers – 5” x 5” x 0.060” for 4-inch wafers

• Polarity:

– “light-field” = mostly clear, drawn feature = opaque

– “dark-field” = mostly opaque, drawn feature = clear

Degradation of mask

Mask Wafer

Degradation of mask

Cr mask fabrication

Gap between mask and wafer

Development process

Limitation of UV-based litho.

“Features” of lithography - 2-dimensional exposure - requires flat surfaces

- resolution limited by wavelength of light

- projection errors

How to make nano-sized patterns

Alignment process (P: Positive, N: Negative, A/B/C mask grade)

Mask to wafer align

Mask to wafer align

• Usually takes 1-5 seconds to align and expose on a modern stepper.

• Human operators usually take 30-45 seconds with well-designed alignment marks.

Mask to wafer align

Mask to wafer align

Multiple patterned layers of different materials

Report: Design of alignment marks (in case of 4 inch wafer)

Summary of steps in photolithography

SPM-based lithography

Diameter Range 10 - 30 nm Nanotube Length Range 500 - 3000 nm

Tip Angle +/- 5 deg Resonant Frequency 300 KHz Spring Constant 40 N/m Cantilever Length 125 um

Reflective coating 30 nm Al on cantilever

SPM-based lithography

http://www.nanoink.net

Lift-off process

Lift-off process

Lift-off process

Lift-off process

Summary: MEMS design & fabrication

Device/system design:

Mechanics, electronics, electrostatics, fluidics

System analysis:

Modeling, simulation

Process design:

Chemistry and physics of fabrication

Fabrication:

Clean room

Device/system testing:

Microscope, probe station, SEM, laser Problem;

Compare with IC industry:

Mask fabrication

(1000 $/5 inch)