Jackson M.J. Micro and Nanomanufacturing

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Manufacturing High Aspect Ratio Microstructures 111

(3) Sidewall charging - if the inhibitor is an insulator it can gain

a charge, this causes problems because successive charges can be

repelled disrupting the etching process, and incident ions can leave

their charge on the wall. (4) Ion deflection - occurs when ions enter

the trench (due to the image force) and additional deflection is

caused by the negative potential of the trench walls (a result of

sidewall charging), which is believed to be one of the main sources

undercuts.

(5) Ion etching - ions should impact the inhibitor with

just enough energy to cause removal and expose the substrate to

radical attack. However, if the ions have too much energy they

themselves can etch the substrate. (6) Ion reflection - if ions collide

with a surface at a small angle the impact energy is not enough to

trigger an event and the ion is reflected away from the wall. This

usually leads to a widening of the trench because the ion is reflected

into the opposing wall where it removes the inhibitor allowing radi-

cals to attack the substrate. (7) Ion shadowing - the top of the trench

blocks ions that are not perpendicular with the substrate, in this case

part of the trench, is no longer subjected to ion bombardment that re-

sults in different etch rates. (8) Ion depletion - occurs in conjunction

with ion capture. As a trench grows deeper there is more sidewall

available for ion capture hence ion depletion increases with trench

depth. Ions captured by the trench can no longer participate in in-

hibitor removal.

(9) Radical capturing - radicals impacting trench walls can ei-

ther bond with it or be reflected. It is possible that before bonding

radicals can be transported on the surface, higher surface tempera-

tures gives the radicals greater surface mobility. Radical reflection

occurs when a radical reaches an uninhibited trench section and

there is no convenient substrate to etch, in this case the radical will

leave the surface in a random direction. These two methods can help

radicals to be transported towards the bottom of the trench. (10)

Radical etching - if the inhibiting layer is too thin it is possible for

radicals to reach the substrate and etching will occur in unwanted

places. (11) Radical shadowing - occurs in the same way as ion

shadowing, however radicals are isotropic. (12) Radical depletion -

similar to ion depletion, radicals can be captured and this becomes

more likely as the trench deepens. Radical deflection and high sur-

face mobility made possible by higher temperatures are the main

112 Micro- and Nanomanufacturing

mechanisms radicals can reach the bottom of high aspect ratio

trenches.

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} \

jj I'l t\

|.h:..;;;.

n 0! n

;ts'>'^y"

?^r "jy™'^g^- ^"^^

j^^'^t^-r^V:

\\\f

\Sy

Fig. 3.4. Ion and radical etching mechanisms shown when creating a microscale

trench in the substrate material [1]: (a) EEDF; (b) PEDF; (c) lEDF; (d) lADF; (e)

RADF;

(f) boundary distortion; (g) particle collision; (h) sidewall passivation; (i)

sidewall charging; (j) ion deflection; (k) ion capturing; (1) ion etching; (m) ion re-

flection; (n) ion shadowing; (o) ion depletion; (p) radical capturing; (q) radical

etching; (r) radical deflection; (s) radical shadowing; and (t) radical depletion

Manufacturing High Aspect Ratio Microstructures 113

When creating microstructures the process is not always perfect,

depending on the particular process parameters, different results are

obtained. These effects can usually be eliminated or reduced to an

acceptable level by optimizing the process for etching of a specific

geometry. The variables that are changed are: gas concentration

(etchant gas in the plasma), gas pressure, electric current, electrode

distance, and sheath thickness.

3.5-6 Effects of Etching

Ions ideally impact the substrate at a normal angle. However, as dis-

cussed there are effects that cause the trench to grow in shapes

dif-

ferent to the ones required. The following is a discussion of effects

observed.

3.5.7 Tilting

Tiling can occur for a number of reasons (Fig. 3.5). If the cathode

is inert to the particular etching chemistry radicals tend to home to-

ward the center of the wafer. Radicals have now gained direction

and are no longer isotropic. Ions still remove the inhibiting layer

anisotropically, but radicals now enter a trench and etch in a pre-

ferred direction related to the center of the substrate. This problem is

usually amplified by using unusual mask shapes. As previously dis-

cussed, ion impact can be abnormal compared to the substrate,

which is usually due to boundary distortion. It occurs when trenches

are present or because geometrical features are incorrectly spaced

between the boundary of the plasma and the cathode. These prob-

lems usually occur when the sheath region is between features. In-

creasing the thickness of the sheath reduces the severity of tilting.

114 Micro- and Nanomanufacturing

Fig. 3.5. Effects of tilting on the resultant trench [1]

3.5.8 Bowing

Bowing describes excessive undercutting of the mask to form a

nearly oval trench where a square trench was originally intended; it

is usually found only when the substrate is conducting. If the side-

wall inhibitor is not strong enough sidewall passivation may not be

efficient and unwanted etching will occur. The primary mechanism

contributing to this unwanted etching is ion deflection, and its effect

can be reduced by increasing sidewall passivation, wall charging,

ion energy, and by reducing the image force. Cooling the sample

can also decrease bowing effects by decreasing the energy of the

Manufacturing High Aspect Ratio Microstructures 115

substrate, which can be done cryogenically. In addition, it does not

follow that as inhibitor thickness increases its protection increases.

>''^^.

'j^^A

ov;-:"^'

'^xi,

'i^''/^-

"T^-'

r/-

y'^'.'-^^^:^3fiti:-1^^^SA^^'fSS^SSS^A^'^i>^M

09KU 10.eK>< jaSn 0d79

Fig. 3.6. Effects of bowing on the resultant trench [1]

3.5.9 Bottling

Bottling is a three-phase phenomenon. First, initial ions that re-

move the inhibitor are off normal angles, usually because of unusu-

ally high operating pressures, which causes significant undercut of

the trench (Fig. 3.7). After a period of etching the trench deepens

and undercut continues until a critical point is reached when the op-

posite sidewall begins to shadow some of the off-angle ions. At this

point, the sidewall angle is such that incident ions are either captured

or reflected and undercut ceases. Reflected ions then travel to the

bottom of the trench where they can remove the inhibitor and cause

etching. As there is no sidewall etching from this point on, the

116 Micro-and Nanomanufacturing

trench cross section begins to approach the width of the mask, thus

creating a "bottle" shape. To decrease the effect of bottUng it is im-

portant to reduce the number of off-angle ions, which can be

achieved by reducing the gas pressure or the direct current self bias.

Reducing the sheath thickness may also aid this effect because there

are fewer opportunities for collisions to take place, and the beam

will retain its collimated profile longer.

(a) (b)

Fig. 3.7. The effects of bottling on the resultant trench [1]: (a) effect of broad-

shaped ion beam; (b) effect of decreased sheath thickness; and (c) effect of using a

conventional IBARE system

Manufacturing High Aspect Ratio Microstructures 117

3.5.1

OTADTOP

An interesting etch characteristic is TADTOP, or trench area de-

pendent tapering of profiles (Fig. 3.8). Different trench widths have

different cross-sectional profiles because etch rates are different for

wide and small trenches. Ion deflection is the mechanism responsi-

ble for creating TADTOP, hence in wide trenches ions become at-

tracted to a sidewall. Ions traveling toward the center of the trench

are usually unaffected, but ions near a sidewall are subjected to a

negative charge; hence the ion is attracted toward the sidewall and

can remove the passivating layer. In narrow trenches, the ions ex-

perience a negative attraction from either sidewall and these forces

tend to cancel each other out. Counteracting this effect means coun-

teracting ion deflection, which can be achieved by the same method

as counteracting bowing.

Fig. 3.8. Effects of TADTOP on the resultant trench [1]: (a) decreasing the ARDE

effect due to stronger passivation; and (b) TADTOP creates wider openings as the

trench gets deeper

3.5.11 RIE Lag Due to Ions

Smaller trenches can etch either faster (negative lag), or slower

(positive lag) than wide trenches; this is called RIE lag (Fig. 3.9).

Positive lag is accounted for by ion deflection, ions are attracted to

the negatively charged walls of a trench, if the ion supply is low and

118 Micro- and Nanomanufacturing

the trench is deep then all the ions can be captured by the wall and

etching ceases as there are no more ions to remove the inhibitor at

the base of the trench. However, a certain point is reached where the

wall contains many ions and it becomes positively charged; then in-

coming ions experience an opposite force and are repelled. The cross

section should remain unaltered because if ion deflection begins it is

soon negated by ion reflection.

Fig. 3.9. The effect of RIE lag due to ions on the formation of a trench [1]

3.5.12 RIE Lag Due to Radical Depletion or Reflection

RIE lag due to radical depletion or reflection can cause the trench

to bow slightly, thus making the side walls unparallel. Figure 3.10

shows the effects of RIE lag on trenches with different aspect ratios.

iiiiiili

Fig. 3.10. Effects of RIE lag due to radical depletion and reflection on the forma-

tion of a trench [1]: (a) at center of silicon wafer under high pressure; (b) at the

edge of the wafer under high gas pressure, and (c) at the center of the wafer under

low gas pressure

Manufacturing High Aspect Ratio Microstructures 119

3.5.13 Micrograss

Micrograss is the formation of many small spikes at the bottom of

a trench (Fig. 3.11). The main formation mechanism for micrograss

is unwanted particles on the surface of the substrate. These particles

act as a mask and prevent ions from removing the inhibitor. There

are two primary sources for these particles: (1) contamination from

foreign bodies, which is why cleaning of the substrate and operating

in a clean environment are important; and (2) etched particles can

fall on the surface and will have reacted with the surface so that they

are no longer reactive in the environment. Of course, if there is ion

reflection from the sidewalls then the incoming ions no longer act

normal to the substrate surface, thereby undercutting micrograss that

essentially eliminates it. Hence, if incoming ions are perfectly col-

limated then this can increase the likelihood of micrograss forma-

tion.

(a)

iliiiiiiMiiiT

Fig. 3.11. Effect of micrograss formation after (a) 10 minutes, (b) 20 minutes, and

120 Micro- and Nanomanufacturing

3.6 Micromachining High Aspect Ratio

Microstructures

A method of machining high aspect ratio microstructures is via me-

chanical methods of micromachining using diamond coated cutting

tools with bespoke profiles so that trenches can be milled in materi-

als such as silicon, silicon-based materials, and engineering materi-

als such as copper, aluminum, and iron-based alloys. This type of

process is currently used to machine die steels to produce mold in-

serts and injection molds for the mass production of polymeric-

based micro-and nanostructures. Microfluidic devices are machined

using mechanical-based machining processes that use milling cutters

and diamond profiled cutting wheels. Mold inserts are also ma-

chined using mechanical micromachining techniques. When com-

pared with lithographic or etching processes, mold inserts are easily

manufactured with three-dimensional features and curved surfaces

with mechanical micromachining processes.

3.7 IVIicromolding

Micromolding of thermoplastic polymers is one of the most promis-

ing fabrication techniques for non-electronic micro-devices. There-

fore,

parts fabricated by micromolding, even from high-end materi-

als,

are suitable for applications requiring low-cost and disposable

components. Moreover, thermoplastic materials are a very large ma-

terial class, which allows one to find a suitable polymer for nearly

every application (see Table 3.1). Molded microstructures can be ei-

ther soft and elastic such as polyoxymethylene (POM) or hard and

brittle such as polysulfone (PSU). They are available from optically

transparent materials such as cyclo-olefin copolymer (COC) and

opaque ones such as polyamide (PA) filled with graphite.