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444 S. Tadigadapa and F. Lärmer

much lower than those achieved using chlorine plasma. In general, hydrogen-based

etches result in very smooth surface morphologies and are not much affected by

water vapor contamination allowing for the use of nonload locked RIE machines.

Furthermore, hydrogen plasma has been demonstrated to result in the least damage

to the substrate [84]. The etching itself is considered to occur through the formation

of AsH

and (CH

)

Ga as volatile reaction products. Figure 7.22 shows the depen-

dence of GaAs etch rate as a function of percentage of CH

in H

plasma [84].

Fig. 7.22 Etch rate of GaAs as a function of %CH

in H

plasma. The data were obtained for a

RIE plasma at a pressure 1.84 Pa and a power density of 0.4 W/cm

. The reduction in the etch rate

with increasing methane is due to the formation of an inert polymer layer. The maximum etch rate

is found to increase to a value of ∼180 nm/min when the etch is performed at a pressure of 5.26 Pa

and 0.75 W/cm

Technology, IET)

7.7.2 Case Study: Etching of InP, InGaAs, InSb, and InAs

Reactive Ion Etching in Cl

Plasma: Indium phosphide related materials have

been successfully etched using chlorine plasmas [86–88]. However, owing to the

nonvolatile nature of InCl

, etches performed at room temperature including ion

bombardment have been found to exhibit low etch rates and rough morphologies.

Etches performed at elevated temperatures of >140

◦

C show an appreciable increase

in the etch rate of >2 μm/min due to the increased volatilization of InCl

.Asthe

temperature is increased to 200–250

◦

C, enhanced energy-driven anisotropy of the

etched features has been observed. However, chemically assisted ion beam etching

performed at high temperatures is prone to microloading-related roughness arising

due to the sputter deposition of nonvolatile residues from hard masks, etch chamber,

and substrate chuck.

Silicon dioxide and photoresist have been found to be suitable etch masks while

graphite and silicon have been found to be suitable for the substrate chuck materi-

als [89]. Ko et al. have reported the etching of smoothly etched vias in InP using

7 Dry Etching for Micromachining Applications 445

Fig. 7.23 30 μmwide,

92 μm deep via holes etched

in InP using Cl

/Ar (50%

) plasma. An ECR source

driven at 500 W along with

100 W RF substrate power at

0.26 Pa and 20

◦

C temperature

was used. Local surface

heating due to ion

bombardment of the substrate

is considered to provide the

elevated temperature [90]

(Used with permission,

Electrochemical Society)

Fig. 7.24 0.4 μm wide slabs etched in InP using Cl

/Ar in an ICP-RIE. A substrate tem-

perature of 250

◦

Springer)

a ECR-RIE with Cl

/Ar as the etch gases (see Fig. 7.23)[90]. A ﬂow ratio of

2:1 of Cl

: Ar has been found to result in very smooth etched surface morpholo-

gies. Addition of hydrogen or HCl to the plasma has been shown to improve the

etched surface morphologies. The likely effect of the addition of hydrogen in chlo-

rine plasma is the effective removal of phosphorus as PH

. Figure 7.24 shows a very

smooth and vertical etch surface obtained using Cl

/Ar in an ICP-RIE [37].

Etching InP in CH

Plasma: Alkane–hydrogen etching of InP provides an

alternative to etching using halogen gases. Although the etch rates are low in

the range of 20–60 nm/min, the etch morphologies are extremely smooth and the

damage due to etching is limited to less than 4 nm into the surface [91]. In alkane–

hydrogen plasma, phosphorus is removed via the formation of PH

whereas indium

is removed via the formation of (CH

)

In. Due to the high volatility of PH

,the

446 S. Tadigadapa and F. Lärmer

etched surface is found to be In-rich. CH

ratio in the range of 0.1–0.4 is

typically used. Higher values of CH

lead to polymer deposition on the surfaces.

Figure 7.25 shows the etch proﬁle obtained in CH

/Ar RIE plasma for an

InP/InGaAsP mirror. An overhang and positive etch slope can be seen due to the

buildup of polymer on the sidewall of the etched feature. Cyclically alternating

between O

plasma to clear the polymer buildup and etch step using CH

solves

this problem at the cost of greater process complexity. Alternatively a small amount

of O

can be continuously added to the CH

plasma to prevent the buildup of the

polymer on the sidewalls [92]. A comparison of these methods and their advantages

has been presented by Schramm et al. [93].

Fig. 7.25 A 45 period, gas

source MBE grown

InP/InGaAsP mirror etched

using CH

/Ar plasma

under conditions: pressure =

6.57 Pa, substrate bias =

500 V, CH

/Ar : 4/20/10

sccm [93] (Used with

The American Vacuum

Society)

7.8 Case Study: Ion Beam Etching

Ion-beam etching is a purely physical, plasma-based etch process and excellent

reviews are available [94, 95]. During milling, the sputtering of material occurs by

momentum transfer between the impinging ions and the surface. To ﬁrst order, the

etch rate, ER, is proportional to the product of the ion ﬂux density F, and the sputter

yield S(θ), and can be given as [94]

ER(θ, IE) ∝ F cos(θ )S(θ , IE),

where θ is the angle of incidence of the ion beam and IE is the energy of the ion

beam. In general, sputter yield is a function of the angle of incidence, the atomic

7 Dry Etching for Micromachining Applications 447

weight and incident energy of the ions, and the target material. Most materials

exhibit an increase in sputter yield with θ, up to a maximum at about 40

◦

–60

◦

followed by a sharp drop. The overall increase in etch rate for oblique angles of

incidence is because the increased sputter yield generally exceeds the reduction in

ﬂux density. The sputter yield also varies with ion energy exhibiting an exponential

increase between 20 and 100 eV, followed by a linear increase up to about 500 eV,

and ﬁnally increasing more gradually up to a saturation point above 10 keV [94].

The process is highly anisotropic so undercutting is generally not a concern, and

very high resolutions with features of better than 100 Å have been demonstrated.

The physical nature of the process means that all materials can be etched, allowing

a single-step approach for patterning multimaterial ﬁlm stacks. However, the major

drawback of ion milling is the poor selectivity, and there is typically less than a

tenfold difference in the etch rate between materials. The etch depth is therefore

controlled by timing, or with mask layer thicknesses adjusted to compensate for

the low selectivity. Table 7.6 lists the etch rates observed in milling experiments

using an Oxford Series-300 dual-source etch system with a 15 cm Kaufman-type

ion source. All tests were performed using argon at a pressure of 0.026–0.039 Pa.

The use of oblique angle increases the etch rate as well as avoids trenching and

redeposition problems.

Table 7.6 Ion-beam etch rates for some commonly ion-milled materials

Material Current density (mA/cm

) Voltage (V) Angle (

◦

) Etch rate (nm/min)

PZT 0.42 1000 0 14

PZT 0.45 1000 45 24

Platinum 0.42 1000 0 30

Platinum 0.45 1000 45 34

0.45 1000 45 28

Zirconia 0.45 1000 45 20

Milling conditions: 0.026–0.039 Pa [26]

The addition of a partial pressure of oxygen during ion-beam etching has been

noticed to reduce the sputter yield of certain metals that oxidize easily [96].

Figure 7.26a shows the etch rate as a function of oxygen concentration for various

materials. The effect is likely the result of competition at the surface between the

chemisorption of oxygen, and the sputtering removal of both the oxygen and metal

atoms. This method can be used as a means to increase the selectivity of a mask

over nonreactive materials. This is well illustrated for patterning complex oxides

such as lead zirconate titanate (PZT), where reactive sputtering in oxygen ambient

has little effect on its milling rate, however, the ion milling etch rates of metals

such as chrome, titanium, and aluminum are signiﬁcantly reduced due to harder to

etch metal–oxide formation (see Fig. 7.26b). Figure 7.27a shows PZT etching with

nearly vertical sidewalls achieved using a titanium mask layer. Using this technique

it is possible to pattern thin PZT features down to ∼1 μm as shown in Fig. 7.27b.

448 S. Tadigadapa and F. Lärmer

Oxygen Partial Pressure (x10

–3

Pa)

Etching Rate (nm/min)

1 2 3 4 5 7 10 20 30 50 100 200 500 1000 2000 5000

Vanadium

Chromium

Bulk Aluminum

Aluminum (Film)

Manganese

Silicon

Silicon Dioxide

Etch Rate (nm/min)

PZT Chromium Titanium Aluminum

0.53 Pa, Ar Only

0.53 Pa, 2:1::O

:Ar

Fig. 7.26 (a) Etch rate of various materials with the addition of oxygen at 1 keV and 0.6 mA/cm

sputtered metals and sol-gel deposited PZT with and with without oxygen using 0.45 mA/cm

current density, 1 kV acceleration potential, angle of 45

◦

and a pressure of 0.53 Pa

7 Dry Etching for Micromachining Applications 449

Fig. 7.27 Ion-beam etching PZT with hard masks in O

, showing excellent resolution: (a)PZT

patterned using titanium hard mask, and (b) using chromium hard mask [26]

7.9 Summary

To summarize, dry etching processes are at the heart of MEMS fabrication and mate-

rials technology. The use of plasma creates a reactive gaseous environment in which

various substrate materials can be removed as volatile reaction products. Most of the

etching processes are based on various halogen chemistries. The overall etch results

depend upon the speciﬁc material, etch chemistry, etch equipment, and various other

parameters including masking materials, history of the chamber, and so on, thus it

is nearly impossible to give speciﬁc process recipes that can be reliably reproduced

across various laboratories or facilities. What is important is to realize the deter-

mining factors of the overall etch characteristics in a given process. In Table 7.7,

process recipes for etching various metals, semiconductors, and dielectrics are

listed. These are not intended as a list of the current state-of-the-art numbers,

but to provide the reader with nominal etch-related parameters to be expected

on three different but typical pieces of etch equipment. Recent developments in

high vacuum technology with high-throughput turbomolecular pumps, high-density

and stable inductively coupled plasma sources, fast switching, precision mass ﬂow

controllers, and process control instrumentation have enabled the design and man-

ufacturing of next-generation etching equipment. However, challenges remain in

the development of high-throughput, high-deﬁnition etching processes, especially

for high-aspect-ratio etching of ceramic materials such as complex oxides encoun-

tered in piezoelectric and magnetic materials, and wideband gap semiconductors.

In addition, challenges lie ahead as top-down fabrication techniques are rapidly

approaching the 10 nm scale with high-aspect-ratio etch requirements.

450 S. Tadigadapa and F. Lärmer

Table 7.7 Typical plasma etch recipes including gas chemistry used, etch rates obtained, and equipment type

Material

Pressure

(Pa)

Source power

(W)

Substrate

power (W) Gases

Etch rate

(nm/min) Comments

Metals

Aluminum 1.32 800 175 Cl

: 75 100 Tool: Applied Materials DPS System, Mask:

PR 3012

Platinum 1.32 800 175 Cl

: 75 sccm 100 Tool: Applied Materials DPS System, Mask:

PR 3012

Chromium 10.5 75 − Cl

: 60 sccm,

: 20 sccm

10 Tool: Plasma Therm PT 720 Mask: ZEP-530

(Selectivity: PR:Cr::3:2) Note: 60% over

etch recommended for small features (sub

750 nm)

Semiconductors

Silicon 1.32 1000 150 Cl

: 20 sccm 490−520 Tool: Applied Materials DPS System, Mask:

SiO

Selectivity: Si:SiO

::8:1

Silicon 1.32 175 − Cl

: 20 sccm 60 Tool: Plasma Therm PT 720, Mask: Cr,

Selectivity: Si:Cr :: 10:1. Process relies on

oxidation of chromium for good selectivity

Silicon 26.3 200 − SF

: 50 sccm 300−400 Tool: Plasma Therm PT 720, Mask: SiO

Selectivity: Si:SiO

:: 60:1

Gallium arsenide 0.132 200 − Cl

: 20 sccm 500 Tool: Plasma Therm PT 720, Mask: PR,

Selectivity: GaAs:PR :: 20:1, Mask: Cr,

Selectivity: GaAs:Cr :: 50:1

AlGaAs 1.32 250 − Cl

: 29 sccm 400 Tool: Plasma Therm PT 720, Mask: PR,

Selectivity: AlGaAs:PR :: > 10:1

7 Dry Etching for Micromachining Applications 451

Table 7.7 (continued)

Material

Pressure

(Pa)

Source power

(W)

Substrate

power (W) Gases

Etch rate

(nm/min) Comments

Dielectrics

Silicon dioxide 1.97 − 100 CHF

: 75 sccm

: 30 sccm

37−40 Tool: Applied Materials MERIE System,

Magnetic Field: 60 gauss

Silicon dioxide 13.2 200 − CF

: 45 sccm

: 5 sccm

110 Tool: Plasma Therm PT 720 Mask: PR 1813

Silicon nitride 1.97 − 50 CHF

: 25 sccm 10 Tool: Applied Materials MERIE System,

Magnetic Field: 60 gauss

Silicon Nitride 26.3 300 − CF

: 40 sccm

: 8 sccm

Ar: 7 sccm

167 Tool: Plasma Therm PT 720 Mask: PR 1813,

Selectivity: < 1

Quartz 6.57 − 200 CHF

: 45 sccm

Ar: 140 sccm

46−49 Tool: Applied Materials MERIE System,

Magnetic Field: 22.5 gauss, Mask:

Chromium, Selectivity: Quartz:Cr :: 12:1

SilSpin



2.63 − 25 O

: 30 sccm

CHF

: 12 sccm

130 Tool: Applied Materials MERIE System,

Magnetic Field: 45 gauss, Cathode

temperature: 3

◦

Polyimide 2.37 − 150 O

: 90 sccm 682 Tool: Applied Materials MERIE System,

Magnetic ﬁeld: 45 gauss, Cathode

temperature: 3

◦

C,Mask:CrorNiHard

Mask

Polypyrrole 5.26 350 − O

: 30 sccm 300 Tool: Plasma Therm PT 720 Mask: PR 1813,

Selectivity: Polypyrrole:PR ::1:1

Benzocyclobutene (BCB) 26.3 200 − O

: 45 sccm

Ar: 5 sccm

: 10 sccm

1000 Tool: Plasma Therm PT 720, Process used for

BCB etch back

Source: The Pennsylvania State University node of the National Nanofabrication Infrastructure Network

Note: The PlasmaTherm PT-720 is a parallel plate etcher (i.e., diode type etcher), AMAT-MERIE is a magnetically enhanced reactive ion etcher and uses

a magnetic ﬁeld for increasing the plasma density, and AMAT-DPS is a high-density inductively coupled plasma etcher in which the plasma power and

substrate power are independently controlled

452 S. Tadigadapa and F. Lärmer

Acknowledgment The author (ST) would like to thank the graduate students Steven Gross, Han

Guen Yu, Abhijat Goyal, Saliya Subasinghe, and Ping Kao in the group and Guy Lavallee and

Changjun Huang at the Pennsylvania State University (PSU) Nanofabrication facility. ST acknowl-

edges the use of facilities at the PSU Site of the NSF NNIN under Agreement 0335765. ST also

acknowledges ﬁnancial support from the US Army Research Ofﬁce (Grant # W911NF-07-1-0327)

and NSF (Grant # 0925438).

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