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

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474 D.W. Burns

Various test equipment can aid in the development and monitoring of wet etches,

as listed in Table 8.2. The trained, unaided human eye can effectively see when an

etch has been completed for many ﬁlms such as shiny metal ﬁlms, although it is

generally unsuitable for determining adequate clearing of the ﬁlm between small

patterned features. By temporarily withdrawing a wafer from an etchant, an opera-

tor can look brieﬂy at large features on the wafer topside or backside and observe

interference fringes, color, or sheen changes on the wafer to determine if the etch

is complete. Hydrophobicity changes may be observed as the wafer is temporarily

withdrawn from the etchant. Excessive withdrawal of the wafer for inspection may

slow the etch, however, and in some cases cause the etch to stop due to surface

reactions that may occur when the wafer is exposed to air.

Table 8.2 Test equipment for wet-etch development

Equipment Advantages Disadvantages

1 Unaided eye Allows rapid inspection during

etch

No intermediate rinsing and

drying steps

Allows etch continuation

Provides immediate veriﬁcation

Overetch can be timed from

etch clearing

Good for pilot wafer

veriﬁcation

Can slow etch

Not applicable for many ﬁlms

Difﬁcult to tell progress of an

incomplete etch

Requires larger features for

inspection

Requires manual handling

Difﬁcult to assess small

features

2 Microscope Readily available

Works well for metal ﬁlms and

ﬁlms with high visual

contrast

Small features can be inspected

Requires rinsing and drying

before each inspection

Transparent ﬁlms are difﬁcult

to assess

Difﬁcult to assess very ﬁne

features

3 Microscope with

depth gage

Works well for deep etches

Upgrade available for most

microscopes

Limited resolution due to depth

of ﬁeld limitations

Not valid for transparent layers

4 Ellipsometer Noncontact

Provides refractive index and

thickness

Difﬁcult on small features

Not suitable for metal ﬁlms

Limited applicability to

multilayer ﬁlms

5 Optical ﬁlm

thickness spectro-

reﬂectometer

Noncontact

Repeatable quantitative metrics

Suitable for test structures

Provides refractive index and

thickness

Requires rinsing and drying

before inspection

Not suitable for metal ﬁlms

Limited applicability to

multilayer ﬁlms

6 Optical

interferometric

proﬁlometer

Noncontact

Wide-area 3-D step heights

Suitable for test structures

Requires rinsing and drying

before inspection

Not suitable for transparent

ﬁlms

7 Mechanical

proﬁlometer

Relatively low cost

Uses patterned features

Presents contamination

concerns due to probe

contact

Requires patterned features

8 MEMS Wet-Etch Processes and Procedures 475

Table 8.2 (continued)

Equipment Advantages Disadvantages

8 Depth gauge,

mechanical

Low cost

Re-zero to wafer reference

surface

Moderate accuracy

Requires large feature sizes

9 SEM High resolution Requires wafer fragments or

cross-sectioned features

Destructive

10 FIB/SEM Accuracy

In situ cross-sectioning

Requires patterned features

Destructive

11 EDS, STEM, XPS,

scanning Auger

Provides chemical analysis Difﬁcult on small features

Destructive

12 AFM Provides surface texture and

small step heights

Incurs physical contact

Small sample area

Viewing the wafer with an optical microscope may help determine etch com-

pletion of a patterned feature for many ﬁlms, although etch rates for incompletely

etched ﬁlms are difﬁcult to determine when limited to observations through the

microscope. Deep etches can often be determined with a quality microscope by

focusing on the substrate surface and then on the bottom of the etched fea-

ture, and calculating height differences from readings off the mechanical dials on

the microscope. To improve accuracy, the z-translation stage on the microscope

can be outﬁtted with an interferometric readout for determining distance between

one in-focus surface and another, from which etch depths and etch rates can be

determined.

An ellipsometer may be used effectively to determine ﬁlm thickness and removal

rates for single-layer and some multilayer stacks. A vertical noncontact spectrome-

ter with a small spot size (e.g., NanoSpec [34]) is a valuable tool for determining the

etch rate for most transparent and semitransparent ﬁlms. A mechanical proﬁlome-

ter (e.g., Alpha-Step [35] or Dektak [36]) is useful for observing etch progress for

metals and other ﬁlms, although it requires contact on the s urface with a needle

point that may present contamination problems. Mechanical drop or depth gauges

(e.g., Mitutoyo [37]) can be helpful for large, deeply etched structures such as

anisotropically etched diaphragms.

The application of adhesive tape, a stripe from a resist pen, or a drop of photore-

sist on a ﬁlm to be etched can sometimes provide suitable features for determining

ﬁlm thickness and hence the etch rate after the ﬁlm has been etched, although the

feature edges are often too poorly deﬁned to provide a sufﬁciently sharp edge.

Improvements to the edge deﬁnition can be made with use of a test or device mask

on a test or run wafer. Dedicated structures for depth measurements and etch veriﬁ-

cation can be placed on the test and device wafers, and are highly recommended for

process development, qualiﬁcation and monitoring.

476 D.W. Burns

Analytical equipment such as a scanning electron microscope (SEM) is used

occasionally for determining etch rates, however, it requires cross-sectioned fea-

tures and is generally destructive. An etch proﬁle may be determined by cleaving a

wafer across an etch hole and then viewing and measuring etch depths in the SEM.

Newer focused ion beam and scanning electron microscopy (FIB/SEM) equipment

can perform the cross-sectioning in the same chamber as the inspection. XPS and

other chemical-analysis techniques may also be used to determine the etch rate of

some material conﬁgurations, although a large, unpatterned spot size is generally

needed.

8.2.8 Additional Considerations and Alternatives

Wet-etch processes, particularly sequences that require a new setup for each use,

are prone to certain difﬁculties such as incorrectly mixed etchant, unavailable wafer

holders, temporarily unavailable services, and broken labware. Although procedures

can be set up to avoid such issues, it is generally incumbent on the user to ensure

that the etch process is successfully carried out. Two principles help avoid inadver-

tent happenings: run a pilot wafer or part of a wafer prior to the run wafer(s), and

ensure test structures are located somewhere on the wafer or device so that the etch

sequence can be veriﬁed immediately.

For those with limited access to unit processes such as wet etches, alternatives

are provided in Table 8.3. A commercial foundry may support an entire process

ﬂow that allows the user to submit a design, and from that design the foundry pro-

cesses the wafers and returns ﬁnished wafers or packaged parts to the user. Custom

process ﬂows or process variants can sometimes be made available from commer-

cial foundries, although additional negotiating and payment may be needed for

some nonrecurring engineering costs. Some foundries will perform standard unit

process steps such as thin-ﬁlm depositions, patterning, and wet or dry etches at a

ﬁxed cost, whereas custom unit processes may incur development costs. Industrial

laboratories may offer captive processes for development and limited initial pro-

duction. University facilities, such as members of the National Nanofabrication

Infrastructure Network (NNIN) in the United States, may be used by qualiﬁed stu-

dents and industrial users at each site, or may alternatively be accessible to r emote

users via a staff member or a local consultant to perform desired services [38].

Some of the university facilities accommodate and encourage industrial employees

to work on-site on a fee basis. Within the university facilities, students and faculty

members can design devices, develop processes, and perform research with appro-

priate oversight. Government laboratories and government-sponsored programs also

may be available to meet user requirements. Trade groups, networks, and exchanges

[39] have emerged as centers for connecting user needs for design services, unit

processes, or full fabrication sequences with foundries and laboratories that cater to

those needs.

8 MEMS Wet-Etch Processes and Procedures 477

Table 8.3 General comparison of wet-etch site alternatives

Facility Mode Advantages Disadvantages

Commercial

foundry

Standard process ﬂow Fixed processes

Easy design variations

Well-deﬁned start times

Reliable completion times

Fixed pricing

Ramp to manufacturing

Pay for what is used

Few or no process variants

Expensive for only a few wafers or devices

Process IP conﬁned to foundry

Requires complete process run

Custom process ﬂow Dedicated ﬂows

Inherent ramp to manufacturing

Unknown development cost

Standard unit process Lower cost

Qualiﬁed sequences

Work accomplished at multiple facilities

Custom unit process Outsourced development

Unit process-level qualiﬁcation

Higher development costs

Higher unit process costs

Industrial

laboratory

Technician or operator Fast processing

In-house expertise

Allows low-rate initial production

Personnel intensive

Capital intensive

High maintenance costs

Limited ramp to production

Engineering development Innovative processes

State-of-the-art materials

Internally developed IP

Personnel intensive

Capital intensive

High maintenance costs

Requires process transfer

University

facility

Industrial user Novel devices, new processes

Self-controlled IP

Minimal infrastructure

Embedded engineer or technician Established facilities

Direct control of personnel

Very fast

Simultaneous pilot and device runs

Requires personnel c ommitments

Costly budget item

478 D.W. Burns

Table 8.3 (continued)

Facility Mode Advantages Disadvantages

Local consultant Established facilities

Experienced personnel

Can be fast

Technology transfer assistance

External expenditure

Limited IP control

Student/faculty Established facilities

Novel devices, new processes

Experienced mentors

Internal IP

Training required

Time consumption

Equipment support

Equipment downtime

Remote user Established facilities

Experienced personnel

Flexible processes and materials

Can be slow

No production

External expenditure

Government

laboratory

Internal user Well-endowed facilities

Fixed or custom processes

Internal IP

Variable funding

Remote user Well-endowed facilities

Fixed or custom processes

High costs

Limited production capability

8 MEMS Wet-Etch Processes and Procedures 479

8.3 Evaluation and Development of Wet-Etch Facilities

and Procedures

Wet-etch procedures present concerns common to other aspects of semiconductor

and MEMS/NEMS device processing: facilities, equipment, services, wafer han-

dling, safety procedures, and training. Additional considerations may emerge due

to the presence of caustic and potentially harmful liquids, as well as the need to

control their source, use, and disposition. These concerns and considerations need

to be evaluated regardless of whether a wet-etch process is developed internally or

provided by outside services.

8.3.1 Facility Requirements

Wet-etch capability places certain requirements on facilities, maintenance, and oper-

ating personnel. One exemplary facility may be structured as multiple laboratories

with one lab dedicated to highly clean integrated circuit processing, a second lab

for CMOS-compatible MEMS and related semiconductor device processing, and

a general lab for exploring and developing nonstandard devices and processes.

Another facility may designate speciﬁc bays to standard (e.g., silicon) IC and clean

MEMS processing with other bays designated for compound semiconductor and

gold-compatible processing. Another facility may separate wet-chemical process-

ing within clean-room bays by chemical hood, with speciﬁc hoods designated, for

example, for oxide etching and PR stripping, nitride etching and PR stripping, alu-

minum etching and PR stripping, pre-furnace wafer cleaning (nonmetal), nongold

metal etching; anisotropic wafer etching; compound semiconductor etching, gen-

eral etching (including gold), and solvent processing. Still other facilities may have

no limitations on types of processing in cleanroom fume hoods, except for restric-

tions to use only preapproved chemicals and materials. Some special requirements

for IC, MEMS, and NEMS wet-chemical processing are listed below with brief

descriptions.

8.3.1.1 General Facilities

Gowning room – outﬁtted with lab coats, jump suits, smocks, bouffant caps,

hoods, facial protection, safety glasses, goggles, latex and vinyl gloves, and

booties

Chemical storage – cabinet with passthroughs for supplying new etchant and

for storing partially empty bottles, chemical in-use labels, and material safety

data sheets

Chemical disposition – temporary storage for empty and in-use bottles with

access to labels for identifying chemicals and bottle contents; containment

vessels for local disposition of etchants for heavy metals (e.g., gold etchant)

480 D.W. Burns

Cleanroom bays – set up by function and cleanliness control with local or lab-

oratorywide air ﬁltering and conditioning that is graded by particulate count

for class 10,000 (general gowned area), class 1000 (clean assembly area),

class 100 (general processing area), class 10 (lithography and wafer cleaning

areas), and better

Wet-etch stations – one or more stations with fume hoods that may be dedi-

cated for clean processing, semiclean processing, and general processing for

contamination control

Acid neutralization system – automated system to collect and neutralize acid

and base waste (not HF) from sinks and fume hoods before discharge

Fume scrubber – system to remove harmful vapors from hood exhaust with

water dilution

Test bays and benches – contains test and characterization equipment to validate

process steps and perform preliminary evaluation of devices

Facility alarms – gas, chemical, acid/base neutralizer, smoke, ﬁre detection, and

strobe alarms

Emergency exits – one or more emergency exits for fast laboratory evacuation.

8.3.1.2 Wet-Bench Services

Electricity – adequate power for lights, ventilation, alarms, hot plates, stirrers,

temperature controllers, timers, and solenoids as needed for bench operation

Water supply – plumbed deionized water with suitable faucets, taps, and

sprayers

Drains – plumbed with separate systems for HF acid drainage and for acid/base

neutralization

Nitrogen – plumbed with hoses and nitrogen guns

Compressed air – plumbed for pneumatic actuators

Fume exhaust – adequate draw and external venting for wet benches and user

safety

Emergency alarms – nearby push-button switches for laboratory alerts

PA system and phones – nearby public address system for lab announcements.

8.3.1.3 Wet-Bench Equipment

Wet bench with fume hood – ﬂame-retardant polyvinyl chloride and polypropy-

lene [33] wet bench for acids and stainless steel for solvents; proper exhaust

and gauges; optional laminar ﬂow and local HEPA ﬁltration; optional deep

sinks, gooseneck DI water tap, and industrial water for plenums

Etch tanks – PFA, PTFE, or quartz tanks for acids; stainless steel or polypropy-

lene for most solvents; lids

Hot plates – integrated into etch tanks or set into sinks with temperature control;

may be placed on hood work surface; optional stirrers

8 MEMS Wet-Etch Processes and Procedures 481

Rinse tanks – PFA, polypropylene, or PTFE tanks for acids; stainless steel or

polypropylene for most solvents; dump rinsers or cascade rinsers for wafer

cleaning

Timers and controllers – preferably built into the wet bench

Nozzles – spray nozzles for DI water and ﬁltered blowoff guns for nitrogen

Wands – vacuum wands for individual wafer transfers

Aspirator – ﬂexible tube and nozzle with switch for pumping acids/bases into

acid neutralization system

Spin-rinse dryer – chamber that accommodates wafer cassettes; low spin speed

for spraying with DI water and high spin speed with heated nitrogen for

drying wafers; generally placed near the wet bench

Labware – color-coded or marked PFA, polypropylene, or PTFE cassettes,

cassette handles, beakers, graduated cylinders, quartz or Teﬂon



holders,

stirrers, thermometers, and special ﬁxturing

Wipes – lint-free cleanroom wipes for drops, small spills, and blow-drying

wafers

Bench space – sufﬁcient room for safe transfer of wafers with adequate counter

space

Floor space – sufﬁcient room for operators and additional aisle space for safe

passage

Chemical carts – pails or wheeled carts for transporting bottles

Solid waste disposal – wastebaskets for solid waste such as soiled cleanroom

wipes; may be covered and vented for solvent or photoresist waste; zip-

lock bags for wet cleanroom wipe disposal; dedicated containers for broken

substrates and labware

Carboys – polypropylene tanks for collecting discarded solvents.

8.3.1.4 Safety

Personal protection – goggles, face shields, chemical gloves, and chemical

aprons

Operating manuals – accessible from near wet bench

Safety procedures – written and accessible from near wet bench

Phones – with numbers for major emergencies and staff contact information for

minor emergencies

Emergency alarms – alerts for lab personnel and for emergency response team

Emergency showers – accessible and within reach of wet bench, even with

lights out

Emergency eye washes – accessible and within reach, even with lights out

First aid kits – includes tubes of calcium gluconate gel for HF exposure

Spill cart – cart with emergency equipment to contain and treat chemical spills

Materials and chemicals – procedures for chemical storage, use, disposal, and

chemical control to minimize contamination risks and ensure safety.

482 D.W. Burns

When establishing new facilities or determining which facility to use for wet

chemical etching, it is generally advisable to have all process sequences performed

in the same facility. Important peripheral considerations are given to optical and e-

beam lithography requirements; furnace operations; deposition and material growth

processes; wet and dry etching capabilities; and inspection, test, and characteriza-

tion needs. Wet-etching and wet-processing steps are prominent in most process

ﬂows. Because most devices or projects require processing steps beyond that of wet

etching, the integration of all processes and wafer transport between steps requires

careful consideration.

8.3.2 Wafer Handling Considerations

Wet-etch development and execution requires consideration of wafer handling. For

small wafer pieces, a pair of Teﬂon-coated metal or plastic tweezers or an ETFE bas-

ket may sufﬁce. Single wafers may be placed in a PFA or PTFE lollipop-style dipper

holder. Multiple wafers beneﬁt from the use of a quartz wafer holder with multiple

slots, a standard polypropylene cassette, or a dedicated Teﬂon cassette. Vacuum

wands with conductive PEEK, PFA, polyimide, PTFE, or quartz tips may be used

for transferring and blow-drying wafers. Gravity or push-fed cassette-to-cassette

transfers are encouraged to avoid direct handling of each wafer. Good handling pro-

cedures should prevent wafers from touching each other and minimize contact with

any appliance.

Each wafer-handling apparatus is typically allocated to a particular wet bench

and restricted to that location. Wet benches may have imposed restrictions to the

type of substrate, such as silicon-based substrates in one hood and compound

semiconductor substrates in another. For example, wet benches may be set up

for extremely clean processes such as wafer cleaning before furnace or epitax-

ial growth operations, oxide etching and photoresist stripping, nitride etching,

standard metal etching, nonstandard metal or general etching, gold-contaminated

processing, anisotropic silicon etching, GaAs processing, acid etching, base pro-

cessing, solvent processing, and specialty processing. In limited facilities, one

or two wet benches may sufﬁce: acid/base/alkalis and solvents, or acids and

base/solvents. The processing history of the substrates may further dictate restric-

tions on wet bench accessibility. For example, a facility’s procedures may restrict a

wafer from any wet bench or equipment dedicated to clean processing for furnace

operations.

Wet-etch operations should ensure that movements of all wafers, holders, and

etchants be obstruction-free to help prevent wafer damage and to provide for oper-

ator safety. Provisions for hand-drying of wafers with a nitrogen gun should ensure

an open, unobstructed work area away from the user so that the user may hold

the wafers downward and away from the user’s face. Inspection of wafers should

be done with minimal movement and transport to avoid inadvertent breakage. The

8 MEMS Wet-Etch Processes and Procedures 483

wafers should be kept close to countertop surfaces at all times to minimize break-

age, and wafer transfers along aisles to spin-rinse dryers or to another work station

should use protective cassettes to avoid breakage.

8.3.3 Safety Concerns

Wet-etch processes have exposed ﬂuids that, by their nature, may be caustic and

potentially harmful to a user. Care always needs to be taken or a user could be

exposed accidentally to an etchant at any time between the opening of an etchant

bottle to the ﬁnal rinsing and discarding of the etchant. Special precautions should be

observed while setting up or using any wet-etch process. Those who can recall holes

in clothing from sulfuric acid residues on a wet bench, inﬁltration of a full-shield

face mask with hydrochloric–nitric acid fumes from a tube washing, the dry scent

of concentrated hydroﬂuoric acid, or view of a hydrazine cloud from an overheated

hotplate will have a healthy respect for standard, common-sense precautions when

wet etching. Operator safety is paramount in performing a wet etch. If the user

cannot be safe, then the etch procedure is inappropriate and should not be used. A

procedure should be written up and taught for each etch sequence, explaining each

apparatus and movement in detail including the order of mixing etchants, operating

temperature, holders to be used, and storage or disposal of the etchants and rinse

liquids.

Exposure to chemical etchants can cause burns, rashes, eye damage, lung prob-

lems, and chronic health issues. Electricity in the wet benches can create shock

hazards. The invariable broken glassware and broken wafers can cause cuts to the

operator and to others who unwittingly become exposed to the sharp edges if bro-

ken glass or wafers are not disposed of properly. An operator must know what to do,

where to go, whom to call, and how to clean up or to get assistance in case of a spill,

breakage, or incident. At least one layer of protection such as a transparent shield

should be located between the user and the etchant at all times so that the user is

protected against splashes of liquid or s pitting of hot etchant. A practical reminder

is this: always have protective clothing and eyewear on, because when accidents

happen, it is too late to use any steps of precaution. Experienced users carefully step

backwards from equipment while protecting their wafers, and move steadily and

predictably down lab aisles.

8.3.4 Training

Training of operators and users is a requirement for safe successful execution of

a wet-etch process. Training is best done with an experienced user, along with

written or multimedia materials that contain all relevant procedures and precau-

tions. Trainers should oversee a new user during at least one actual etch sequence.

Experienced users should be observed by other lab users and politely corrected if