Jackson M.J. Micro and Nanomanufacturing

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Manufacturing High Aspect Ratio Microstructures 101

process can achieve. As demand grew for the production of smaller

and smaller feature sizes, the requirements for etching substrates

moved from isotropic to anisotropic; thus ion beam etching (IBE)

was developed. IBE, however, had its own problems such as poor

selectivity between mask and substrate leading to the production of

components that did not meet specifications. Hence, chemical as-

sisted ion beam etching (CAIBE) was developed to counteract this

particular problem. The latest technique in the evolution of this

process is called ion beam assisted radical etching (IBARE). This

process has both high selectivity and high anisotropy, making it an

ideal way of creating high aspect ratio trenches with parallel sides

and square faces.

3.3.1 Ion Beam Assisted Radical Etching

In this process, the plasma is ignited by applying a radio fre-

quency voltage between two electrodes in a dielectric gas. Electrons

have enough energy to jump from their ground state and escape the

atomic bond, making the atom ionized. This results in a large num-

ber of charged particles in the chamber, namely, free electrons and

ions.

The electrodes are positive (anode) and negative (cathode), so

the electrons (negatively charged) migrate rapidly towards the anode

and ions (positively charged) move towards the cathode. During

these motions collisions with the gas occur generating neutrals, radi-

cals,

electrons, ions, and photons. Ions generated from the plasma

are strongly attracted toward the cathode. They can travel with

enough energy so that on impact with the cathode electrons from the

surface of the electrode they are released; this is a secondary genera-

tion of free electrons. Subsequently, newly created free electrons are

attracted toward the anode, as they pass through the chamber they

collide with neutrals and other particles in the gas creating further

ionization; thus secondary generation of free electrons and subse-

quent collisions sustain the plasma. Electrons usually acquire ener-

gies between leV and lOeV before colliding with other particles,

which are dependent on the gas pressure of the system.

102 Micro-and Nanomanufacturing

Electrons can be involved in collisions where they carry insuffi-

cient energy to cause ionization. The energy they have acquired is

used up in collisions within the gas, specifically by making orbiting

electrons jump energy shells. Because this energy is not enough to

cause ionization the electron decays back to its original energy level

prior to impact. Decay occurs because the electron cannot exist at

its new orbit for long, and the extra energy is released in the form of

a photon. Release of these photons is the source of the glow of the

plasma; the color of the plasma depends on the type of atom and the

magnitude of the decaying process.

3.4 Characteristics Of The Plasma

3.4.1 The Sheath Region

Above the cathode, the region is dense with electrons that have

just been liberated. They have low energy and there are few gas par-

ticles available for collision. Hence, there is little opportunity for

photon release. Therefore no light is being emitted from this region

and it is known as dark space. Similarly the anode has many elec-

trons around it and there are few chances for collision to cause light

emission; and this region is also known as dark space. The width of

the dark space depends on the chamber pressure; at low pressures

the mean free path of the electrons increases and the width of the

dark space increases. By controlling gas pressure within the cham-

ber, the energy with which the ions strike the surface can be con-

trolled.

Substrates are placed on the cathode to take advantage of the in-

cident ions. These substrates can be insulating, ion bombardment

ejects electrons and a charge builds up on the insulator to the point at

which the plasma can no longer be sustained. Hence an alternating

current at a radio frequency is used to drive and sustain the process.

The reversal of current maintains a charge balance, but if one elec-

trode is to be used then it must be energetically favored in some

way. The potential difference between electrodes is important be-

Manufacturing High Aspect Ratio Microstructures 103

cause this field accelerates ions providing them with more impact

energy. If the electrodes have equal area then they have the same po-

tential, increasing the area of one electrode changes the potential of

that electrode and overall the potential difference is increased. If the

area of the anode is increased, which is usually achieved by connect-

ing the chamber walls to the anode, the potential is at a maximum

thus favoring ion bombardment of the cathode.

3.4.2 Boundary Region

When moving from the plasma glow to the sheath there is a dis-

tinct dividing line called the boundary region, or boundary layer. It

can be considered as a surface emitting particles from the plasma.

Ions are directed by the field lines existing between the electrodes,

this causes particles to leave the boundary layer perpendicular to the

substrate. The ions can now be considered as a collimated stream of

particles. As ions leave the boundary layer they pass through the

sheath on their way to the substrate. The sheath region contains par-

ticles so that ion collisions are possible; if they occur it will cause

ions to deflect and they will no longer travel perpendicular to the

boundary layer or substrate. In addition, the collision has made ions

lose energy and therefore reduced the effectiveness of impact when

it reaches the cathode or substrate. These are two important effects

known as the ion angular distribution function (lADF), which is a

measure of how collimated the particles are before they reach the

substrate, and the ion energy distribution function (lEDF), which is a

measure of how much energy the ions have remaining. Radical colli-

sions in this region are unimportant because their initial direction is

isotropic so that further collisions producing random motions have

no effect on the overall direction.

3.5 Etching of Microstructures

Etching can occur if a chemical reaction producing a gaseous, or

high vapor liquid, or solid, is energetically favored. In other words.

104 Micro-and Nanomanufacturing

if a reaction can lower the overall energy of the system, then it is

possible for it to be energetically favorable to cause etching. In high-

pressure plasma reactors etching of Silicon, Si, is achieved using

Chlorine, C, and Fluorine F. After the reactions have taken place the

system must be at a lower energy; this is achieved because the radi-

cals (in this case F) are freely available and the bond energy between

Si and F is less than the bonding energy between Si and Si. In other

words it is energetically more favorable for the reaction to take

place. The reaction produces a SiF molecule resulting in an overall

net loss of Si; this is etching. A typical gas used would be CF4,

which will not etch silicon directly. F radicals are produced by ioni-

zation in the plasma. Other bonding groups in the system are Si2,

SiF2,

and SiF4. The Si substrate surface is coated with F, a bulk Si

atom is bonded to an Si surface atom which is itself bonded to two F

atoms, making SiF2. However, because it is bonded to the wafer it is

not free to move. An incoming F atom can replace the Si-Si bond

with SiF2, because Si already shares another surface F. The SiF2 then

leaves the surface and a small portion of Si has been removed. Al-

ternatively the incoming F atom could attach itself to SiF2, embed-

ded on the surface making SiFa, subsequently another F will join the

system producing SiF4, which will then leave the surface.

In the case of reactive ion etching a physical process for model-

ing the etching mechanism has been proposed. Chlorine gas in the

forms,

CI or CI2, can etch undoped silicon but only very slowly. N-

type doped silicon is etched spontaneously by CI, only. In the case

of undoped silicon, chlorine atoms migrate toward the surface and

chemisorbs. However, this does not break the Si-Si bond. Once a

layer of chlorine builds on the surface further absorption is pre-

vented by stearic hindrance. After a short time this surface becomes

negatively charged, in this case ionic bonding between the Si and CI

can occur. When this happens chemisorption sites are freed, thus in-

creasing the chances that CI atoms will penetrate the bulk and pro-

duce volatile SiCl. This process is dramatically enhanced by ion

bombardment, so areas subjected to ion bombardment etch much

faster than other regions and this produces anisotropic etch profiles.

However, the more heavily doped the substrate the more pronounced

the charging effect is. If reasonable etch rates are to be achieved then

Manufacturing High Aspect Ratio Microstructures 105

high doping means that undercut of the sidewalls is increased and

etching characteristics are no longer anisotropic.

3.5.1 Etching Phenomena

For IBARE techniques regardless of plasma chemistry, smaller

width trenches are etched more slowly than wider trenches, the con-

trolling factor being the aspect ratio of the trench rather than its

depth or width. One method of reducing this effect is to change the

geometry of the shape and control the aspect ratio.

3.5.2 Inhibitor Depletion in a Trench

In small trenches the supply of inhibitors is reduced partly due to

the narrow opening. The resulting depletion of inhibitors can lead to

a higher etch rate, and inverse reactive ion etching (RIE) lag should

be the result (i.e., the smaller trench is etched more quickly due to

radical ion etching lag).

3.5.3 Radical Depletion in a Trench

Radicals can etch a surface only if they are close by when it is ex-

posed to ions. Radicals can arrive at this site either by gas transport

or by surface flow. Radicals will attack any available site, in this

case the whole surface. Therefore, surface transport is not possible

because the path of each radical is blocked by its surrounding

neighbors. If this is the case the only way for radicals to enter the

trench is by transport in the bulk gas. This is called microloading;

the etch rate decreases and is inversely proportional to the amount of

substrate exposed to the plasma. Generally this effect is present

when radical density is exposed to an unusually high substrate area,

and gives rise to inverse RIE lag.

Now consider a masked wafer with a hole in the center. Surface

transport is now possible because there is no surface silicon to con-

sume the radicals. This is the so-called micro-loading effect, which

106 Micro-and Nanomanufacturing

occurs when the reactant density is depleted as a result of an exces-

sive substrate load. Etch rate will decrease inversely to the silicon

area which is exposed to the plasma glow, giving inverse RIE lag.

However, etch rate seems to be influenced by the geometry of the

mask shape. For a long, small silicon structure etching is faster than

the square of the same area. Hence, it is important to know the

transport mechanism so that RIE lag can be predicted. It is therefore

important to know how the radicals reach the trench because this de-

termines whether there will be any RIE lag.

3.5.4 Volume Transport

The gas flow within the system is one of three types: viscous, mo-

lecular, or a combination of both called transitional. The Knudsen

number is defined as the ratio of the mean free path of a molecule, X.,

to a characteristic dimension, d, of the channel through which the

gas is flowing, (a) Viscous flow - is characterized by a small Knud-

sen number (high pressure) because the high-pressure particle den-

sity is very high and particles are most likely to collide with each

other rather than the wall. Therefore, most collisions in this flow

type are intermolecular and characterize the gas as X/d < 0.01. (b)

Molecular flow - is characterized by large Knudsen numbers (low

pressure) because particles are more likely to collide with the walls

rather than each other because low pressure reduces the density.

Upon collision with a wall the particle or molecule is often ab-

sorbed. After a short time the molecule desorbs from the wall in a

random direction, these collisions characterize the nature of the gas.

Molecular flow restrictions result from the geometry being yd> 1.

(mid-pressures). Collisions with the wall and molecules are equally

as likely in this range. 0.01 < A./d < 1.

In a realistic situation where a substrate is covered by a mask it

is possible for radicals to move on the mask; therefore surface ge-

ometry determines radical supply to the bottom of the trench. Ions in

a trench can become depleted for a number of different reasons, the

most important is ion deflection. There are different forces that con-

tribute to the final force that is deflecting the ions. It is possible for

Manufacturing High Aspect Ratio Microstructures 107

the mask to become charged and the charge estabhshed on this sur-

face can deflect ions. Charged particles are attracted towards solid

bodies with a force inversely proportional to the square of their dis-

tance. In the case where a particle enters a trench the particle is sub-

jected to two forces, one from each wall. Any ions that are captured

by the walls due to image forces are unable to assist in etching at the

base of the trench. This effect is usually observed in small trenches

and can be reduced by increasing ion velocity; this is because the

ions have less time to become captured although that in turn can lead

to increased sidewall etching (Fig. 3.1).

The forces between charged particles (known as Coulomb and

Lorentz forces) are not considered to be significant because the

chances of two ions existing in a trench together are small, however

the significance increases as the density increases. Initially when the

material is placed on the cathode the distance between the boundary

layer and substrate is constant and the associated electric field is

constant; therefore etching ions have equal properties along the sub-

strate length. However, when trenches form the distance from the

boundary layer to substrate changes locally, and in turn the electric

field also changes. It becomes distorted by an amount related to the

trench size. This is yet another cause of RJE lag, although insignifi-

cant at the microscale. In this case RIE lag is initiated by the distor-

tion that intensifies the electric field around it. Therefore, a concen-

trated number of ions build up to attack the inhibitor. After a short

time the charge builds up and incoming ions are repelled to the point

where they spread out and widen the trench producing inverse RIE

lag.

Not only is it possible for ions to be depleted by forces attracting

them to sidewalls, it is possible accidental collisions with other ions

in the plasma glow and plasma sheath region (ion dispersion) can

send ions to the sidewalls causing ion depletion. Of course these col-

lisions alter the ion energy as well. Finite element models have been

constructed to simulate the etching process, thus reducing time spent

on optimizing the process. The substrate is divided into elements and

these elements are subjected to incident energies simulating ion im-

pact. When the energy exceeds a certain level the element disap-

pears,

it is assumed that immediately after an ion has removed the

inhibitor a radical etches the substrate. It is possible to alter the ion

108 Micro-and Nanomanufacturing

input angles and energies to simulate different etching conditions.

These simulations quite accurately predict the evolution of a trench

and RIE lag. The flowchart showing a typical etching simulation is

shown in Fig. 3.2.

ieft

image

charge

1 T

^ , ;

„„,

Ml '*^ \

^ • »"-fn •""——;^

^ w

4, %

^ ^

w >Q

nght

image

charge

Fig. 3.1. Ion transport during the etching of a high aspect ratio trench [1]

Figure 3.3 shows the simulation of the formation of trenches by

changing the ion energy flux distribution. It is possible for the im-

age forces at the wall to give extra energy to the incoming ions, in

this case they have enough energy to remove the sidewall inhibitor

and etching can occur. When deciding what factors cause RIE lag

there are three possible causes, inhibitor depletion, radical depletion

and ion depletion. Inhibitor depletion can be eliminated from ex-

perimental evidence. Radical depletion can be tested in such a way

that the effects of ions, and inhibitors are eliminated. In these ex-

periments RIE lag is not observed, therefore this mechanism is also

eliminated. Therefore, ion depletion must be the mechanism respon-

sible,

since ion depletion is reliant on contributions from the ion dis-

tribution and image force. A simulation has been developed that ex-

amines ion depletion due to sidewall capturing. It was found that the

average ion energy decreases with increasing incident angle. The

Manufacturing High Aspect Ratio Microstructures 109

particular ion angular distribution (IAD) associated with this effect

usually results in bottling, and also predicts RIE lag. Simulated re-

sults are backed up by experiment. Etch rates can be predicted if the

aspect ratio rather than the feature size is scaled. The aspect ratio is

time dependent because the trench becomes deeper as time in-

creases. As the trench deepens radical supply become restricted

therefore it can be observed that etch rates reduce as the aspect ratio

increases. The image force mechanism predicts RIE lag will increase

as feature size decreases; it is thought RIE lag is significant only in

high aspect ratio sub-quarter microscale trenches. Figure 3.4 illus-

trates the etching mechanisms associated with ion and radical etch-

ing of sihcon.

/ input:

mask layout,

vafues of angle B,

stepsize of angle

os.,

MAXtTER /

ion beam

^^^^^^masR

19-

hit materia! elements

in diresctian defined by

and

Fig. 3.2. Flowchart showing incremental steps in determining the etching charac-

teristics of

trench [1]

110 Micro-and Nanomanufacturing

FWHM=5 =10

=20

Fig. 3.3. Simulation of the formation of trenches is controlled by changing pa-

rameters such as the ion energy flux distribution that is varied by changing a and

6 [1], (a) the effect of IAD on RIE lag, and (b) simulated output for different ver-

tical etch times and the FWHM of triangular shaped IAD

3.5.5 Etching Disruption IVIechanisms

The following is a summary of effects that can reduce the effective-

ness of the ions and radicals entering the trench. (1) Boundary dis-

tortion - occurs when the distance between the boundary layer and

substrate changes along the length of the substrate that can be caused

by trenches. The effect causes distortion of the electric field, which

changes the concentration and direction of incoming ions, they no

longer impact the substrate at 90 degrees. (2) Sidewall passivation -

results from the protective inhibiting layer formed over the substrate.

However, in the case of a high aspect ratio trenches inhibitor build

can vary with depth, specifically it is thicker at the trench top than at

the bottom allowing the potential for undercutting.