제6장 사진식각공정 (Lithography)

Issues

1. 미세패턴을 실리콘 웨이퍼 상에 구현하는 방법

2. 현재 기술의 한계와 이의 극복방안

Key Words

1. Positive and Negative Photo-Resist

2. cross-linking

3. additive and inhibitor

Theory

Technology

1. 현재의 기술로 0.1 micron 패턴이 가능하며 256 mega DRAM  이상에

서 활용되고 있다.

2. 기술상으로는 이보다 더한 미세 패턴이 가능하나 효율과 관련된 산업성

의 문제를 해결하지 못하고 있는 실정.

6-1 개요

The origins of todays high resolution lithography date back to the 1700's and early work done in the printing trade. An excellent brief historical perspective is given in reference 1 above.

6-2 Negative working resist

consist of : a) High molecular weight polymer ( MW ~ 65,000 )

b) Light sensitive additive which promotes crosslinking

c) Volatile solvents to aid spin application (i.e., to control                 viscosity - Xylene)

The light sensitive additive [eqn: P] is typically a di functional azide [eqn: N_3 - X - N_3] in which X is chosed to make the chemistry light sensitive in the UV ( [eqn: P] is usually ~ 1 % by weight of the resist). Upon exposure to UV light, intermediate compound called nitrenes [eqn: :~NRN~:] are formed which are very reactive and short lived because of the unsatisfied N bonds on the ends. Therefore reaction occurs with polymer chains -> Cross-Liking

Light absorption of the additive P is maximum at ~ 3,650 A which corresponds to a photon energy of 3.45 eV ( [eqn: lambda = {c`h} over E] )

[eqn: Phi ==]quantum efficiency of reaction

= [eqn: {n umber ~of~ N_2~ bonds~ broqken }over {n u mber ~ of ~ photons~absorbed }]

Typically, [eqn: 0.2 <= phi <= 1]

The dissociation of the active molecule occurs via an irreversible reaction. 따라서 It is independent of reaction products present.

[eqn: therefore {d``[P]}over dt = k `E ~[P ]`` sec^-1]

where [eqn: k] = reaction rate constant

[eqn: E] = photon energy density W/[eqn: cm^2]

[eqn: [P]] = concentration of active compound

[eqn: therefore] [eqn: [P] (t) = [P]_o~ left[ e^{-kEt}~right] / cm^3]

where [eqn: [P]](t) is the concentration of unreacted active compound at time t during the exposure.

Once the nitrene [eqn: :~NRN~:] is formed cross-liking occurs very rapidly ( [eqn: tau ~approx ~0.6 sec~~ @ 25^o] C )

Note that a single bond to a polymer chain is relatively worthless since MW 65,000 + 342 Two bonds => 130,342 which greatly decreases resist solubility in developer.

During exposure the absorption of light by the resist decreases exponentially with time ( or total energy) indicating that the [[eqn: P] ] drops off exponentially as indicated in eqn 2.

As the exposure proceeds, the concentration of single bonds and double bonds (cross-links) both rise exponentially as expected. The single bonds -> double bonds later in the exposure ( calculated results)

Advantages : 1. very sensitive : short exposure times ( [eqn: 10^-6] to [eqn: 10^-7]                  C/[eqn: cm^2] )

2. relatively tolerant of developing conditions

3. High etch resistance

Disadvantages : 1. adhesion to substrate depends on cross-linking                            reaction : may need to overexpose to penetrate                         through film.

2. " swelling" during development (resist absorbs developing chemicals as part of dissolving process) Because of this it is very difficult to resolve < 2[eqn: mu]m patterns with conventional negative resists.

3. Oxygen sensitivity  [eqn: :~NRN~:] can react with [eqn: O_2] : [eqn: N_2] ambient required.

The biggest problem with negative resists is their swelling during developing. Some newer types of negative resists are differently formulated to minimize this problem, making < 2 [eqn: mu]m features possible.

6-3. Positive working resists

positive PR 은 다음으로 구성되어 있다.

a) Low molecular wgt polymer ( MW + 5,000 ~ 10,000) , moderately       soluble in developerb) Photoactive compound, "inhibitor" which is normally organic soluble     and becomes developer soluble (weak alkaline solution) on exposure,    [eqn: approx]20% by wgt.

c) volatile solvents to aid spin application ( control viscosity)

UV light 에 쪼이면 “inhibitor"는 [eqn: N_2] 분자들을 잃게 되고 alkali 용액에 녹을 수 있는 carboxylic acid 로 변하게 된다.

현상은 “etching"과 같은 공정에 의하며 developer가 film 내로 녹아 들어가는 과정은 없다. 따라서 negative resist에서 볼수 있는 swelling 현상은 없다.

first order로 negative resist 때와 같이 positive resist exposure에 관한 model을 세울 수 있는 데 즉,

[eqn: {d```[I]} over dt = k` E` [I`]]

where [eqn: [I]] = inhibitor의 농도, [eqn: E] = photon energy density, 그리고 [eqn: k] = reaction rate constant

이식은 negative때와 마찬가지로 기하급수적인 시간의존성을 예측하고 있다.

왼쪽 실험결과는 노광에 따른 positive resist을 통과한 빛의 세기를 나타내고 있는데 빛의 통과도는 inhibitor가 망가질수록 커지게 되며 항상 1 이하를 나타내는 이유는 inhibitor 말고도 다른 첨가물에 의해 빛의 흡수가 이루어지기 때문이다.

In actual fact, there will be reflections from underlying interfaces ) particularly the Si substrate)

Since monochromatic radiation is usually used for exposure ( Hg line  3600 - 4400 A), standing waves will be set up in the resist.

왼편그림은 Si 기판에 도포된 6300 A 두께의 감광제 경우  계산된 빛의 세기인데 사용된 노광 빛의 파장은 [eqn: lambda] = 4040 A 이다.

exposure destroys most inhibitor at intensity maxima. [I] shows a depth distribution which is the inverse of the light intensity.

Advantages : 1. 분해능이 좋다. - no swelling 따라서 0.25 [eqn: mu]m 이하의 선폭 형성이 가능하다.

2. adhesion이 노광에 영향을 받지 않는다.

3. high etch resistance

4. no oxygen sensitivity

Disadvantages :  negative resist에 비해 감광효과가 떨어지므로

오랜시간의 노광이 필요하다.

Note that the 2D edge effects are common to all resists and can be minimized by

1. Using multiple wavelengths for exposure

2. Multi-layer resists in which a thin resist on top of a thicker planarization layer is used. The lower layer can be absorbing to minimize reflections.

6-4 Other types of resists

1. Deep UV resists

Shorter wavelengths -> higher resolution

일반적으로 2000 에서 2500A에 해당하는 짧은 파장을 다루기 위해 quartz optics를 필요로 한다. Positive resist가 주로 쓰인다.

2. E-beam and X-ray resists

1 에서 25 A 에 해당하는 극히 짧은 파장에 관한 것으로 x-ray는 5-25 A, E-beam의 경우는 1A 이하의 파장을 지닌다. 따라서 resist polymer의 모든 bonding에서 energy를 흡수하게 되므로 감광에 대한 기본 기구는 열에 의한 thermal bond breaking에 가깝다. 따라서 resist는 positive 또는 negative 일 수 있는데 다음의 사항을 고려한다.

a) Broken bonds reform in new compounds with higher MW (negative resist) Bell labs COP is the best known example

b) Broken bonds simple break polymer chains. 따라서 Lower MW ( Positive resist) PMMA, PBS, are typical examples.

6-5 Exposure systems

노광기는 다음과 같은 option이 있다.

1. Contact printing

2. proximity printing

3. 1G:1 projection printing

4. 1/N step and repeat projection

5. E-beam direct write              6. X-ray proximity printing

In contact or proximity printing the mask is in physical contact or separated by a few [eqn: mu m] and flood exposure is used.

- line widths of < 1 mu m (contact) and 1.5 -    2[eqn: mu] (proximity) are achievable and cost is      low.

- defect density can be a problem especially in contact printing

These systems will likely see limited use in the future because of the defect density problem.

Step and repeat projection printing with 10X to 2X masks is in widespread use today.

1. resolution of 1.5 mu m

2. alignment per exposure

3. +- 0.25 mu m alignment

4. $500 k - 1,000 k+

5. very complex refracting optics

1:1 projection aligners with reflecting optics are very widely used (Perkin Elmer). With deep UV exposure, 1 mu m resolution is obtainable with +- 0.25 mu m alignment. Optics are simple compared to refracting optics.

E-beam can be used to directly expose resist. This process is widely used toady for mask making and in R&D for direct writing on wafers. Excellent resolution ( < 0.5 [eqn: mu] m ) and alignment ( < 0.2 [eqn: mu]m ) are achievable but cost is high and vacuum operation is required.

X-ray generated from an intense e^- beam ( 10 - 20 kWatts)J can be used to flood expose a wafer using a mask in 치ose proximity. two major problems here are

1. Intense X-ray source so exposure time is short

2. Masks -g thin substrates ( < 5 [eqn: mu]m) must be used to make them transparent. Often thinned Si wafers or S[eqn: i_3 N_4] films with Cr-Au patterns are used. These masks must be extremely stable thermally and mechanically strong.

Resolution and alignment are excellent - [eqn: approx] < 0.5 [eqn: mu]m and < [eqn: +- 0.2 mu]m typically.

Core Findings

1. Negative resist 보다는 positive resist 가 미세패턴을 이루는데 유리하다.

2. 미세패턴의 한계는 빛이 간섭효과에 있으며 보다 미세한 패턴 형성을 위해서는 파장이 짧은 빛을 사용하는 것이 필요하다.

3. 감광제의 개발이나 새로운 팬턴 형성 방식이 개발 및 새로운 노광 시스탬의 개발은 그 한계에 달할 것으로 보이며 따라서 nano-strucrue의 개발은 lithography에 의하지는 않을 전망이다.