Ghodssi R., Lin P., MEMS Materials and Processes Handbook

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404 S. Tadigadapa and F. Lärmer

7.1 Dry Etching

Etching is the process of controlled removal of a desired material from a substrate

via physicochemical methods. Etching can be performed either using chemicals in

liquid state – deﬁning a process commonly referred to as wet etching – or can be

performed using chemicals in gaseous or plasma phase – deﬁning the process of

dry etching. Dry etching processes include plasma etching, reactive ion etching,

chemically assisted ion beam etching, ion milling, and gas phase chemical etching.

The chemical species used for the controlled removal of the material of interest is

called an etchant. When the process of etching uses chemical species that can spon-

taneously react with the material to be etched and form volatile or gaseous products,

the etching method is a purely chemical process governed essentially by the ener-

getics of the reaction. However, most etching processes require additional energy

to speed up the kinetics of the reaction and to engineer reactions onto pathways

resulting in the formation of volatile products. This is often achieved by imparting

considerable physical energy to the etchant molecules, radicals, and ions thereby

resulting in a physicochemical reaction.

In the limiting case where a particular material cannot be removed easily by

chemical reactions, a purely physical etching process known as ion beam etching

or ion milling is used. Most dry etching processes lie in between the two extreme

regions of pure chemical and pure physical etching processes. Excellent references

on plasma etching for micromachining applications are available and the interested

reader is referred to these for more detailed information [1–5]. A general overview

on reactive ion etching can be found in [6], a review on silicon RIE is given in [7].

The capability of reactive ion etching (RIE), namely crystal orientation indepen-

dent etching to realize arbitrarily shaped high-aspect-ratio microstructures, makes

it an invaluable micromachining technology suitable for the micro- and nanoscale

systems. Most of the present-day dry etching techniques have their origins in the

classical RIE approaches developed for semiconductor manufacturing in the 1970s

and 1980s [8–11]. This chapter focuses on presenting dry etching techniques from

a process development perspective for a broad range of commonly encountered

materials in microsystems applications.

7.1.1 Etch Metrics

Etching can be characterized by several important metrics. These metrics include

etch rate, anisotropy, selectivity, etch stop, loading effects, uniformity, and so on.

Figure 7.1 schematically depicts some of the important etch metrics. Brieﬂy, the

deﬁnitions can be given as follows.

Etch Rate: Deﬁnes the rate at which the exposed material is removed under given

etch conditions. Etch rate is typically expressed in terms of thickness of the material

removed per unit time or in the units of nm/min or μm/min. Fast etch rates are often

desirable.

7 Dry Etching for Micromachining Applications 405

Etch Rate (R

) (nm/min)

Anisotropy (A)

Selectivity (a:1)

Selectivity is given as a ratio: a:1,where a is the ratio of the etch rate

of substrate to the etch rate of the mask.Typical selectivities in the

range of 10: 1 to 100:1 are desirable.

Etch Rate Uniformity (%)

The uniformity of etch rate measured across a wafer or from wafer to

wafer is given by:

Notching

Microtrenching

Perfectly Anisotropic Etch

A=1

Perfectly Isotropic Etch

A=0

Partially Anisotropic Etch

A −= 1

%100(%) ×

−

MinMax

RateEtchRateEtch

UniformityRateEtch

Notching arises due to transient charging of

exposed insulator underneath theetched layer

which in turn severely affects the trajectories of

energetic ions impinging on the substrate.

Microtrenching describes the appearance of

narrow grooves at the feet of the sidewalls in the

direction of ion-bombardement. The etch profile

irregularity is attributed to the forward scattering

of the ions at sidewalls.

+ ++ ++ +

Insulator

Silicon

Photoresist

Time (min)

Etch Depth

etch

(nm)

etch

Etch Rate =

Photoresist

Silicon

Fig. 7.1 A brief summary of various etching metrics and parameters

Anisotropy: The degree of anisotropy (A) depends upon the etch rates in the ver-

tical (perpendicular to the substrate surface) and the lateral (in-plane) directions. It

can be mathematically expressed as

A = 1 −

, (7.1)

where R

is the vertical etch rate and R

is the lateral etch rate. An ideal anisotropic

etch has a value of 1 where the lateral etch rate is insigniﬁcantly small in relation

to the vertical etch rate and a completely isotropic etch has an anisotropy value of 0

when the two etch rates are equal.

Selectivity: Selectivity refers to the relative etch rate of the material of interest

with respect to the masking material which is typically photoresist. A high selec-

tivity is required to achieve good masking and high ﬁdelity pattern transfer through

the process.

Etch Stop: Often etch processes need to be terminated after a deﬁned depth is

attained. Such a control on the etch depth can be achieved via timed etches or by

the use of a layer of material upon which the etching process terminates. Etch stop

layers allow for greater process control and compensation for nonuniformities in

406 S. Tadigadapa and F. Lärmer

etch rate across the wafer arising due to loading effects and/or equipment. A good

etch stop layer must exhibit high inertness in the etch process especially in relation

to the material to be etched. Typically etches are terminated on the etch stop layer

using spectroscopic methods, residual gas analysis of the volatile etch products,

laser interferometry, or laser reﬂectometry.

Etch Nonuniformity: Etch nonuniformity refers to the variation in the etch depth

of features across a wafer. To ensure meaningful measurement of the nonuniformity

across a wafer or from wafer to wafer, the etch depth must be measured on identical

features. It is well known that the etch rate in plasma etching varies depending upon

the feature width, an effect more commonly known as aspect-ratio-dependent etch-

ing (ARDE). In this effect, narrow features are found to be etched less than wider

features under same plasma conditions. Compensation f or the resulting nonunifor-

mity in etch depth typically requires 20–30% overetching of wider features and the

availability of an etch stop layer is vital to the success of achieving high uniformity.

Etch uniformity across a wafer or from wafer to wafer is deﬁned as:

Etch Rate Non − Uniformity(%) =



Etch Rate

Max

− Etch Rate

Min

2 ∗ Average



× 100%

(7.2)

Loading Effects: The etch rate is not only a function of the gas chemistry and

physical settings of the process, but also depends upon the total material (area) to

be etched. In general etch rates are found to decrease as the area of material to be

etched increases, especially in chemically dominant processes and is a direct con-

sequence of the increased consumption of the etchant (reactant) species or gases.

Loading effects manifest macroscopically at the wafer level when the total area of

the material to be etched is changed by either increasing the number of wafers or

via new mask designs. Loading effects also manifest at the microscopic level espe-

cially when etching high-aspect-ratio structures due to the local depletion of etchant

molecules in etched features. Reactive ion etching lag or aspect-ratio-dependent

etching describes the difference in the etch rate between large and small width fea-

tures within the same wafer. These effects can also manifest across a wafer where,

for example, an aluminum substrate holder is used to etch silicon in ﬂuorine plasma.

Aluminum does not react with ﬂuorine radicals, therefore there is reduced consump-

tion of the radicals around the periphery of the wafer from those in the middle of the

wafer. This results in the well-known bullseye effect whereby features at the edges

of the wafer are etched more rapidly than in the middle of the wafer. Such nonuni-

formity is typically compensated by overetching the substrate material onto a high

selectivity etch stop layer. In general the etch rate R

, when etching n-wafers each

with an etch area A, can be written as [12, 13]

1 + nkA

, (7.3)

where R

is the etch rate in empty chamber, and k is a constant related to the ratio

of the amount of etchant molecules consumed in etching a wafer to the amount

7 Dry Etching for Micromachining Applications 407

of etchant lost via recombination in the chamber volume/surface. As long as the

volume or surface recombination of etchant molecules dominates, the loss arising

due to consumption during etching can be neglected and thus loading effects are

minimal. This is practically realized in large etch chambers and large ﬂow rates.

Microscopic loading is typically mitigated by performing t he etches at reduced pres-

sure and when ion bombardment or physical etch mechanisms dominate the overall

character of the etch process.

Notching: The notching effect manifests as a narrow lateral opening (wedge)

at the interface of the conductive material being etched and an insulating under

layer. Typically, notching occurs during the overetch phase in the regions where the

insulating underlayer is ﬁrst exposed. In the case of silicon etching on a layer of

silicon dioxide, the origin of notching is found in the local electric-ﬁeld-induced

trajectory bending of energetic ions that directly bombard and etch the foot of the

Si sidewall. The process continues through the subsequent forward scattering or

electrostatic deﬂection of energetic ions at the newly exposed SiO

surface under

the notch [14].

Microtrenching: Microtrenching manifests as the appearance of rapidly etched

narrow grooves at the bottom corners of etched vertical sidewalls. This enhance-

ment in the etch rate in these corner regions is typically attributed to the preferential

shallow angle forward scattering of energetic ions by the mask and etched substrate

sidewall. Microtrenching is dominant in high-energy ion-bombardment-driven etch-

ing processes including ion beam etching. Although the phenomenon is observed in

all kinds of substrates, charging effect in insulating substrates can also enhance the

microtrenching processes. A detailed exposition on this subject can be found in [15].

7.2 Plasma Etching

Plasma etching (PE) and/or reactive ion etching (RIE) are well established process

technologies in the semiconductor industry [3, 6, 7]. The classical tool is the diode

reactor, as shown in Fig. 7.2. The scheme depicts the key elements of a typical

plasma etching system: a number of reaction gases provided to the chamber by

adjustable mass ﬂow controllers, a grounded showerhead electrode as a gas inlet, a

substrate electrode carrying the wafer, and a power supply to the substrate electrode

by radio frequency (RF) via a coupling capacitor (which is mostly part of a more

complex matching-unit to adapt RF impedances). Detailed descriptions of different

plasma reactors and processes can be found in [16].

In RIE conﬁguration, ions from the plasma are accelerated to high energy lev-

els (several 100 eVs) and perpendicularly directed towards the powered substrate.

In the plasma etching (PE) conﬁguration, the showerhead is powered and the sub-

strate electrode grounded. In this case, acceleration of ions from the plasma to the

substrate is restricted to minor energy levels of only a few eVs and the directional-

ity of the i ons is lower than in RIE conﬁguration. In the triode conﬁguration, both

the substrate electrode and the showerhead electrode are powered simultaneously,

408 S. Tadigadapa and F. Lärmer

Platen

Reaction Gases

Pumping

Helium

Backside

Cooling

Electrostatic

Chuck

Gas Inlet

Wafer

Showerhead

Gas1

Gas2

Mass Flow Controllers

Fig. 7.2 Schematic

illustration of a diode plasma

or reactive ion etcher. The

reactor is pumped to

low-pressure conditions.

Reaction gases are supplied

to the chamber via t he upper

showerhead electrode. The

lower electrode is powered by

RF to drive the plasma

discharge and accelerate the

ions towards the substrate

(RIE conﬁguration)

either at the same or at different frequencies, see below. The goal is to generate

the plasma at the desired plasma density controlled by the power level supplied to

the upper electrode, and to accelerate the ions towards the substrate at the desired

energy value controlled by the power level supplied to the substrate electrode, more

or less independently of each other.

Radio frequencies generally used are in the low-frequency (380 kHz), mid-

frequency (2 MHz), or high-frequency (13.56 MHz) regime. Typically, 13.56 MHz

is used as a standard for plasma generation.

Pumping provides the low-pressure conditions in the reactor chamber needed for

the electrical discharge to form between the powered and the grounded electrode,

which builds up the plasma in an atmosphere of one or more compound gases, at a

pressure ranging from 0.1 to 100 Pa.

The coupling capacitor allows the powered electrode to ﬂoat at a DC-potential

generated from the applied RF voltage and arising due to the difference in the mobil-

ities of negative charges (electrons) and positive charges (ions) in the plasma. This

is the so-called DC-bias voltage, which is responsible for energizing the ions.

7.2.1 Types of Etching

Isotropic Plasma Etching: With regard to etching, a reactive plasma consists of

two relevant kinds of species: ions and radicals. The plasma is by far dominated

by chemically active neutral species, the so-called radicals (∼10

− 10

−3

)in

comparison to the electron and ion densities (10

− 10

−3

). Active radicals

are created by the decomposition of gas molecules into fractional parts after colli-

sions between gas molecules and energetic electrons in the plasma. In most cases, an

electron-neutral collision breaks up the neutral molecule into neutral radicals. Being

electrically uncharged, radicals reach the substrate with no or only little direction-

ality, bond to the surface, and react readily with the material to etch, to more or less

volatile compounds. If the reaction takes place instantaneously and the volatility

7 Dry Etching for Micromachining Applications 409

of the reaction products is high enough, the volatile compounds leave the etched

surface quickly yielding isotropic etching behavior. This so-called chemical etch

depends mainly on the properties of the materials involved, provides high selectiv-

ity, and makes it, in general, easier to ﬁnd appropriate etching masks (e.g., SiO

for

masking a Si-etch).

Fluorine radicals delivered in a plasma discharge from ﬂuorine-rich gaseous

compounds such as sulfur hexaﬂuoride SF

, carbon tetraﬂuoride CF

, or nitrogen

triﬂuoride NF

, exhibit isotropic etching behavior towards silicon. Once adsorbed to

the silicon surface, ﬂuorine radicals react spontaneously with silicon atoms to form

silicon ﬂuorides SiF

(x = 1,2,3,4). The reaction main products, SiF

and SiF

,are

volatile enough to leave the surface without a need for physical assistance, and in

the process remove silicon atoms from the etched surface. Because there is no direc-

tionality contained in the mechanism, silicon etching proceeds with no preferential

orientation, and the proﬁles are isotropic. This is an ideal behavior for underetch-

ing micromechanical structures in a so-called sacriﬁcial etching or release-etching

step, for the fabrication of free-standing and movable elements on top of a wafer.

In contrast to that, etching masks such as SiO

or photoresist are not or only slowly

attacked by ﬂuorine radicals, without physical assistance by the energetic ions. A

number of advanced micromachining technologies make use of combinations of

anisotropic and subsequent isotropic silicon etching: ﬁrst etch deep in the vertical

direction, then protect the structures and underetch in the lateral direction [17, 18].

A well-known process example is single-crystal reactive etching and metallization

(SCREAM) [19].

Plasmaless Isotropic Etching: For the purpose of silicon sacriﬁcial etching, it is

not even necessary to use a plasma source of ﬂuorine radicals. A number of gaseous

compounds are well known for the ability to etch silicon spontaneously, without a

need for plasma excitation. XeF

[20, 21], BrF

[22], and ClF

[23] deliver ﬂuorine

radicals which once adsorbed onto the surface, lead to the spontaneous etching of

the silicon. For example, XeF

is a white solid at room temperature and atmospheric

pressure. However at ∼395 Pa and room temperature the solid sublimates and in this

form is able to etch silicon. Gas phase xenon diﬂuoride isotropically etches silicon

according to the chemical reaction:

2XeF

+ Si → 2Xe + SiF

The reaction is exothermic and may result in an increase in the local tempera-

ture, especially in thermally isolated structures. This effect is mitigated by the

use of pulsed rather than continuous etch systems [24]. The absence of any liq-

uid phase eliminates stiction-related problems. Typical etch rates greater than

1 μm/min have been reported which can be reduced by diluting XeF

with nitrogen.

Furthermore, the etch process is a purely chemical isotropic process and requires

no other form of physical energy input for it to occur. XeF

has very high etch-

selectivity between silicon and all other known materials used in the semiconductor

processes [25]. Selectivities may well exceed values of 1000 for most metals or

dielectrics.

410 S. Tadigadapa and F. Lärmer

A typical XeF

etching process in a pulsed mode is as follows: The cycle begins

by evacuating the expansion and etching chambers to a pressure of ∼13.2 Pa. This

allows the XeF

crystals to sublimate and ﬁll the expansion chamber to a selected

pressure. The etching and expansion chambers are then connected and the pressure

is allowed to equilibrate to 130–400 Pa. Typically etching begins immediately and

the pressure begins to rise due to an increase in the total number of moles of the

gases. After a selected etch-time has passed, both chambers are again evacuated

to a 13.2 Pa, held for 10 s, and the process is repeated. The advantage of pulsed

systems over continuous systems is that the quantity of etchant delivered each cycle

is known and can be controlled. However, it is important to note that the etch rates

are dependent on the quantity of silicon present in the chamber. The presence of

water vapor can also have a deleterious effect on the etch performance inasmuch

as water vapor can react with XeF

to form HF molecules which are ineffective in

etching silicon. Furthermore, the etch rate in the ﬁrst few cycles can be lower than

later cycles, due to the presence of native oxide on the silicon. Silicon dioxide is

found to offer a good selectivity whereas nonstoichiometric silicon nitride (Si

)

is found to be etched in XeF

. Table 7.1 shows the etching rates obtained for single-

crystal silicon and polysilicon for the conditions used [26].

Table 7.1 Measured etch rates in XeF

gas phase etching system [26]

Material

XeF

pressure (Pa)

pressure

(Pa)

Etch time

(s/cycle)

Cycle time

(s)

Etch rate

(nm/cycle)

Si (SC) 395 0 45 90 100

Polysilicon 395 0 45 90 2500

SiO

395 395–790 45 90 0.1

395 395–790 45 90 10

Figure 7.3 shows the typical silicon surface obtained after etching in XeF

.The

uniform and randomly nucleated spontaneous etching process causes the surface to

roughen the longer the etching is performed. Typical RMS surface roughness val-

ues range from 100 nm to 1 μm. The process can also be controlled and slowed

down by the addition of nitrogen to provide controlled thinning of silicon structures

at the nanometer scale. In a recently published paper, high selectivity was demon-

strated for the ﬁrst time for selectively etching a SiGe sacriﬁcial layer with respect

to Si functional structures in chlorine triﬂuoride gas [27]. This makes any need for

passivation and protection of the functional silicon during the release step obsolete.

XeF

,BrF

, and ClF

react with silicon via a physisorbed surface layer.

However, in comparison to XeF

and BrF

,ClF

shows slower reaction kinetics

due to the formation of a ﬂuorosilyl skin [28]. The hampering effect of the ﬂuo-

rosilyl layer is explained via a diffusion like transport of ClF

molecules through

a ﬂuorosilyl layer [29]. This is the basis for anisotropic etching of Si at low tem-

peratures and the reason for the extremely high selectivity of Si over SiGe, where

Ge atoms act as accelerators of the ClF

decomposition reaction. ClF

shows crystal

7 Dry Etching for Micromachining Applications 411

Fig. 7.3 Typical texture of

the single-crystal silicon

surface resulting after etching

in XeF

gas phase etching

orientation-dependent “anisotropic” etching of silicon below 0

◦

C. The <110> direc-

tion shows a maximum whereas <100> direction shows a minimum of the angular

etch-rate distribution, with a ratio of <110>/<100> ≈ 1.5.

Anisotropic Etching: The second major species of importance in plasma etch-

ing of substrates is the positively charged ions also known as cations. Although less

numerous than radicals, ions are, however, entirely responsible for any directionality

and anisotropic nature of the etch. They are created during collisions of neutral gas

molecules with energetic electrons in the plasma in which an electron is knocked off

from the molecule and leaves behind a positive charge. Being electrically charged,

ions can be accelerated towards the substrate by negatively biasing the latter with

respect to the plasma potential using DC or RF voltage. This results in the i ons bom-

barding the substrate surface in a more or less perpendicular orientation. The ions

represent the directional contribution provided by the plasma, and all approaches to

create anisotropic plasma etching rely on a perpendicular ion ﬂux to the substrate.

Owing to their directionality, ions do preferentially hit the bottom of a structure

and only to a lesser extent their sidewalls. The incoming ions transfer their kinetic

energy to the local area of impact. Although the number of ions is small compared

to the number of chemically active neutral species, their energetic impact can sig-

niﬁcantly affect or drive the progress of the etching reaction locally. Ion-assisted

directional etching can be caused by one or several of the following mechanisms.

(i) Induction of chemical reactions between adsorbed radicals from the plasma

and the material to etch, by providing the activation energy through ion impact,

especially if the reactivity of etching species with the material is low or

hindered by an energy barrier.

(ii) Desorption of reaction products from the surface by the sputter action of

incoming ions, especially if the volatility of the reaction products is low and the

412 S. Tadigadapa and F. Lärmer

latter stick to the material surface, forming an inhibiting ﬁlm there. In this case,

ion impact removes the inhibiting ﬁlm of reaction products from the surface.

(iii) Desorption of other inhibiting ﬁlms by the sputter action of incoming ions,

for example, polymer ﬁlms formed from polymerizing additives to the plasma,

or ﬁlms formed from nonvolatile reaction products between the material to be

etched and other additives to the plasma.

Etching based on the halogens of lower reactivity such as chlorine and bromine is

strongly ion-driven, which gives anisotropy to the process, however, limits achiev-

able etch rates (typically to below 1 μm/min), etch depths (a few μms) and mask

selectivities (e.g., 20–30:1 for SiO

-hardmask). Because physical impact from the

ion ﬂow to the substrate plays a key role in the etch, selectivity as a result of individ-

ual chemical properties of the involved materials becomes less signiﬁcant. Typically,

there is a fundamental tradeoff between increasing the anisotropy by stronger ion

bombardment on the one side, and maintaining mask selectivity sufﬁciently high on

the other side.

7.2.2 Plasma Sources

Reactive ion etching, except in the case of gas phase chemical etching involves the

use of various plasma sources into which the substrates can be introduced for etch-

ing. Depending upon the exact conﬁguration of the electrodes and how the RF power

is coupled into the chamber, plasma reactors can be classiﬁed as: (i) capacitive,

(ii) inductive, and (iii) wave etchers. Furthermore, depending upon the location of

the plasma with respect to the substrate, etchers can be classiﬁed as (i) plasma con-

tact or (ii) downstream etchers. Figure 7.4 lists the major RIE systems that are brieﬂy

described next. Detailed information on plasma sources and etcher conﬁgurations

can be found in [30].

Barrel Etchers: These systems are mainly used for surface treatment and were

the plasma system conﬁguration in early designs. Plasma i s typically generated

with RF-powered electrodes (capacitive coupling) or with a coil wrapped around

the dielectric tube (inductive coupling). These systems can be used to remove

photoresist residues from wafers using oxygen glow discharge. Initial experiments

demonstrating CF

-based silicon etching were performed in these systems, which

resulted predominantly in isotropic etching due to the lack of directionality of the

ions (See Fig. 7.4).

Capacitively Coupled RF Diode System was the work horse of the 1980s for

reactive ion etching applications. The wafer is placed on the powered electrode and

the best results are usually obtained at low pressures (<13.2 Pa). This approach

does not allow independent control of the ion energy and the ion current at a ﬁxed

pressure and RF frequency. Typically ion energies of hundreds of eV are required to

achieve the desired etch rate. A particularly effective implementation of the diode

approach was the “hexode” machine developed at Bell Labs and developed to allow

7 Dry Etching for Micromachining Applications 413

Gas Inlet

Plasma

Pump

Downstream Etcher Triode Etchers Barrel Etcher

Etch Gas

Fig. 7.4 Schematic

illustration of major plasma

sources and etcher

conﬁgurations

etching of several wafers simultaneously. This plasma etcher conﬁguration was a

dominant batch-processing machine in the 1980s [31].

Capacitively Coupled Planar Diode and Triode Systems: The planar diode

plasma conﬁguration has been already discussed in the previous section. The tri-

ode geometry shown in Fig. 7.4 was well known in sputter deposition technology

and introduced in plasma etching technology to allow independent control of ion

energy and ion ﬂux. This approach allows the ion energy to be reduced while keep-

ing the etch rate constant by increasing the ion ﬂux density. The wafer is placed

on the lower electrode; the lower electrode power (bias power) is used to control

the ion energy and the upper electrode power (source power) is used to control the

ion ﬂux. It is well known that the RF voltage needed to deliver a ﬁxed power to

the plasma decreases strongly as the frequency is increased. Thus large power can

be delivered to the plasma through the high frequency (typically 13.56 MHz) with-

out high-energy bombardment of the electrode. The ion bombardment energy at the

wafer surface (lower electrode) can be controlled by a lower frequency RF power

applied at the lower electrode. The term “triode” is used for this two-electrode

system to recognize the fact that the grounded wall serves as a third (reference)

electrode.

Chemical Downstream Etching: Figure 7.4 shows the schematic illustration of

a chemical downstream etching system. In these systems, the wafer chamber is

separated from the plasma source by a tube with no line-of-sight connection to

the plasma source. Few ions and electrons reach the surface of the substrate and

etching is carried out by the energetic neutral radicals. The process is speciﬁcally

designed to minimize damage onto the surface but the etching process is isotropic