제8장 DRY ETCHING TECHNIQUES

Issues

1. Advantages of dry etching over wet etching

2. Fundamental mechanism for etching process

3. current technologies and future trends

Key Words

1. anisotropic etching

2. plasma

3. selectivity

4. Reactive Ion Etching (RIE)

Theory

Technology

1. 현재 critical dimension 의 구현을 위해 가장 많이 쓰이고 있는 건식식각 방법은 선택성이 좋고 비등방성이 우수한 RIE 이다.

2. 비등방성 식각은 미세소자 형성에 매우 중요한 역할을 하지만 ion milling 등은 장비 가격이 비싸 실제 공정에의 적용은 활발히 이루어지고 있지 않다.

3. 최근에는 Wet etching은 꼭 필요한 경우를 제외하고는 폐수의 문제 및 공정흐름의 청결을 위해 거의 사용되고 있지 않다.

4. 건식 식각은 선택성과 비등방성의 중요함에 따라 공정 별로  다양한 방법이 사용된다.

8-1 개요

Plasma : Ionized gas consisting of nearly equal numbers of + ions and - electrons. Neutral species dominate. Energy to create ions and electron is usually provided by RF source ( often 13.56 MHz - not critical)

Typically,

Neutrals [eqn: approx] [eqn: 10^15] / [eqn: cm^3]

ions + [eqn: e^-] [eqn: approx] [eqn: 10^10] /[eqn: cm^3]

[eqn: CF_4] plasma generates both neutrals (expecially F) and ions. In plasma etching, the neutral species (F) are the most important.

Any material which forms a volatile fluoride or chloride can be plasma etched.. The process is essentially a chemical reaction between the substrate and neutral F or Cl atoms.

example : [eqn: CF_4] (gas) -> [eqn: CF_3] + F

Si + 4F -> [eqn: SiF_4] (gas)

The plasma is simple a means of obtaining the atomic F. The lifetime and diffusion coefficient of F are approx qq0.3 sec and approx 500 [eqn: cm^2] /sec @[eqn: 25^o C] and 1 Torr. Therefore the F can travel [eqn: approx] 2 [eqn: sqrt{Dt}] [eqn: approx] 25 cm before recombination takes place.

[eqn: therefore] geometry of system must place plasma close to or in contact with wafers.

The other extreme from a purely chemical process as described above, is a purely physical process - SPUTTERING

Ar ions are accelerated to the substrate and physically sputter atoms off. This process involves no chemistry, just physical collisions.

In fact, most dry etching techniques involve some physical and some chemical components to the etching.

8-2 Barrel Reactor

1. Ions are kept away from the wafers by shield

2. Neutral species ( F, Cl) diffuse to wafers [eqn: therefore] distance < 25 cm (F) is required.

3. Etching is purely chemical

4. Isotropic etching results

5. Low cost, high throughput, P = 1 Torr

In the barrel reactor, only neutral species [eqn: R_1] are involved. These have a relatively long lifetime. Probability of reaction [eqn: <= 10^-2] each time [eqn: R_1] strikes surface [eqn: therefore] many rebounds and isotropic. ( Long lived species must be involved because of geometry of barrel  : plasma is not in contact with wafers.)

Practical reactions:

[eqn: CF_4 + O_2 -> CO_2 + COF_2 + F]

a) 4F + Si -> [eqn: SiF_4] (polysilicon)

b) 12F + [eqn: Si_3 N_4] -> 3 [eqn: SiF_4] + 2 [eqn: N_2] ( Nitride)

[eqn: O_2 -> 2 O]

c) O + [eqn: C_x H_y O_3] -> [eqn: CO_2] + [eqn: H_2 O] ( Photoresist)

[eqn: O_2] is added to the [eqn: CF_4] gas to increase the amount of atomic F Produced and hence increase the etch rates. ([eqn: SiO_2 ^*] = deposited oxide)

Practical etch rates when power and pressure are optimized are

[eqn: Si_3 `N_4] : 200 A /min

poly Si : 1200 A /min

Photoresist : 1000 A /min

[eqn: SiO_2] : 80 A /min

Note that these systems cannot be used to etch [eqn: SiO_2] on Si because of selectivity  i.e., selectivity [eqn: approx] 80/1200 [eqn: approx] 1/15

Etch rates of the various materials vary with T in an exponential fashion

[eqn: E_A] [eqn: approx] 0.1 - 0.2 eV

[eqn: therefore] selectivity can be controlled to some extent by optimizing T

Even with the [eqn: O_2] added to the [eqn: CF_4] mixture, the [eqn: Si_3 N_4] etch rate [eqn: E_A] is [eqn: approx] same, indicating that the chemistry has not changed just more F is produced in the plasma.

Note that Cl based processes are not often used in barrel reactors because many of the reaction products are too short lived.

Applications

1. Stripping photoresist - however [eqn: Na^+] may be problem

2. Etching [eqn: Si_3 N_4] in LOCOS process with potoresist as a mask

3. Etching plasma [eqn: Si_3 N_4] over Al - excellent selectivity

4. Isotropic etching of poly on [eqn: SiO_2]

8-3 Planar Type Reactor

Advantages : almost anisotropic etching possible

acceptable throughput

Disadvantages : process are critical

photoresist erosion limits

dimensional control ( 10 - 100 [eqn: ANGSTROM] / min

etch rate for photoresist)

In plasma etching in a planar reactor, the wafers sit on a grounded electrode in the plasma, The RF electrode will tend to take on a DC potential with respect to plasma because mobile electrons reach the electrode more easily than larger ions.

[eqn: Therefore] 1. Ions can be accelerated towards both electrodes depending on biases ( applied and induced) and some physical etching can take place. This is not too significant in planar plasma etching because pressure is relatively high ( 0.1 - 0.8 Torr)

2. Short lived neutral species can play a major role in the process because of the close proximity of the plasma to the wafers.

Short lived neutral species [eqn: R_S] give less undercutting because fewer "bounces" [eqn: Therefore]Much more anisotropic etching is possible.

Any physical etching contribution due to I is also anisotropic.

Note the almost vertical walls that are possible in a planar reactor. Etching is not always anisotropic, however, it depends on whether long lived or short lived species are involved in the etching.

True anisotropic etching is only possible with ions i.e., physical etching and this plays a minor role in a parallel plate plasma system.

8-4 Reactive Ion Etching

In these systems, ions play an important role in the etching process.

a) wafers are on RF electrode which takes on a DC potential [eqn: Therefore] ions attracted

b) P [eqn: approx] 0.01 - 0.1 Torr

Larger mean free path for ions to reach wafers.

[eqn: Therefore] 1. etching is anisotropic ( ion dominated) provided wafers are securely connected to susceptor to keep T < [eqn: 100^o] C ( at higher T, chemical processes become important because of [eqn: e^{-E_A /kT}] dependence)

2. In general , selectivity is worse than with chemical processes.

3. Under most conditions ions and neutrals both play a role in the etching.

Advantages 1. Excellent dimensional control

2. Easy processes, simple gases

Disadvantages : 1. radiation damage ( > [eqn: 600^o] C to anneal)

2. Severe photoresist erosion

3. contamination from sputtering off reactor walls

4. often poor selectivity

8-5 Sputtering and Ion Milling

systems are conceptually similar to reactive ion etching.

[eqn: Ar^+] ions are used so the process is purely physical ( no chemistry)

The principal advantage of the physical etching processes is that all materials can be "etched". Principal difficulty is poor selectivity because no chemistry involved.

Core Findngs

1. 일반적으로 비등방성이 높으면 선택성이 낮으며 비등방성을 높이기 위해서는 고가의 세련된 장비가 필요하다.

2. etching gas의 Plasma에 의해 건식식각이 가능하며 이때 이온들은 방향성이 있으므로 wet etching 과는 달리 비등방성 식각이 이루어진다.

3. under cutting의 해소는 resolution에 직접적 관계가 있으며 비등방성 식각은 실제 공정상에 유리한 점과 불리한 점이 있다.

4 현재 etching gas는 두로 Fluorine 과 Chlorine 계통의 chemical 이며 앞으로 다양한 etching gas의 개발이 필요하다. (예를 들어 Cu의 경우는 이들 chemical에 의해 etching 되지 않음.)