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

Подождите немного. Документ загружается.

864 S.J. Cunningham a nd M. Kupnik

The advantage of IR imaging is its speed, simplicity, and low cost to implement and

perform. The increased resolution is obtained at the expense of cost and speed.

Ultrasound or acoustic microscopy uses the propagation properties of a sound

wave in the bonded materials. The measurements are made in water or other liquids

and are relatively expensive to make. The propagation and scattering of the acoustic

wave depends on the elastic properties of the materials, which distinguishes the

bonded wafers and any voids that are present. The measurement resolution of the

acoustic microscopy depends on the measurement frequency but it is in the range of

10 μm.

X-ray topography i s an expensive and time-consuming method when compared

to other techniques that are used to image cross-sections to examine distortion or

defects in lattice planes. It is applied to single crystalline materials and has a typical

resolution of 2–20 μm.

Two destructive techniques include interface etching [84] and cross-sectional

analysis. With interface etching, one wafer is sacriﬁced by etching until the interface

or selective etch stop is reached. Once the interface is reached, voids and defects can

be visually inspected. This is a common approach with SOI wafers. Cross-sectional

analysis is usually performed in conjunction with a scanning electron microscope

(SEM) or transmission electron microscope (TEM) to image the bond interface. For

access to the bond interface the wafers are cleaved to expose the bonded interface by

using a focused ion beam (FIB). The image of the bonded interface can be further

enhanced by a defect etch to enhance the void.

Bond strength characterization is measured by a number of techniques that

include the pressure burst test, tensile test, shear test, bending test, and the razor

blade test (knife edge test). The tensile and shear tests are performed on prepared

samples of a speciﬁc area. The tensile test uses mounts to the two wafer surfaces to

load the sample perpendicular to the bond interface. The maximum l oad at failure or

the maximum stress, which is the maximum load divided by the sample area, is used

as the failure criterion. The shear test is similarly constructed but the load is applied

to the two wafers so that it acts parallel to the bond interface. The shear failure is

characterized by the maximum shear load or by the maximum shear stress, which is

the maximum shear force divided by the bond area tested.

The tensile/shear test has certain challenges relative to implementation and char-

acterization. The ﬁrst challenge is the isolation of loading conditions. Without due

care and even with due care, it is possible to have torsion and bending loads that

are s uperimposed on the t ensile (shear) load. The superposition of these and other

loading stated causes the sample to fail differently and at a lower load compared

to the simple tensile load. The second challenge is stress intensiﬁers that increase

the stress locally during the tensile test. It is the local maximum stress rather than

the area averaged stress that leads to failure and propagation of an interface crack. A

third challenge is the characteristics of the failure that include the propagation of t he

failure. If the crack propagates into one of the wafers or travels some distance along

the bond interface then turns into one of the wafers, it is not directly measuring the

strength of the bond.

11 Wafer Bonding 865

The knife edge test uses a blade of speciﬁed thickness that is inserted directly

into the bond interface where a crack has been initiated. As the blade is inserted the

length of the opening between the two wafers (crack length) is measured using an

imaging technique to estimate the surface energy from the blade thickness, crack

length, and elastic modulus of the wafers. The surface energy scales as the fourth

power of the crack length; that is, uncertainties i n crack length are magniﬁed four-

fold. The crack length has been observed to be time- and humidity-dependent so

conditions must be carefully monitored. One challenge of the knife edge test is

using it to characterize a strong bond where it is difﬁcult to insert the knife edge

without a crack propagating into one of the wafers (chipping).

The four-point bending test is performed on a sample that has a precrack started

in one of the wafers. The precrack can be started by etching or sawing. When the

sample is loading under four-point bending conditions, the precrack is expected to

propagate into the weak interface of the bond. The load versus displacement is used

to calculate the elastic energy and the surface energy of the bond.

Quantitative approaches have been developed through shear and tensile testing of

bonded dies (Nese and Hanneborg [85], Obermeier [86], DeReus and Lindahl [87],

Kubair and Spearing [88], and Tatic-Lucic et al. [89]) or burst pressure testing of

bonded cavities (Stratton et al. [90]). Niklaus et al. [9] reviewed these techniques for

the evaluation of adhesive bonds. These tests usually result in the load or pressure at

failure, which is not broadly applicable as a failure criterion because it depends on

the specimen size, geometry, and type of loading. Although the loading conditions

are simple, the stress state at the interface corner where fracture initiates, is quite

complex. This complexity of the interfacial loading has been argued by Madou [91]

to be a negative attribute of the tests because they fail to “yield information about

the detailed nature of the bond.” Another approach, the blasé test, is used to deter-

mine the interface surface energy. It was argued by Madou [91] to be advantageous

because it creates a well-deﬁned interfacial loading.

In the blade test (Maszara et al. [92]), a thin blade is inserted into the interface

between two bonded wafers to propagate a crack. The blade test was approximated

by replacing the blade with patterned lines of varying pitch as demonstrated by

Tatic-Lucic et al. [89]. The elastic energy in the system is varied and correlated with

the surface energy required to produce a bond within the spaced lines. Based on the

extracted bond surface energy, the differences between different process variations

have been compared.

Elastic interface fracture mechanics applied to the fracture of silicon–glass

anodic bonds was attempted by Hurd et al. [93], and Go and Cho [94]. The appli-

cation of interface fracture mechanics is based on a crack propagating along the

interface. One challenge of interface fracture mechanics is the resulting crack does

not propagate along the interface, but turns away from the interface into the glass.

Another anodic bond fracture specimen was demonstrated by inserting a thin metal

blade between the silicon and glass prior to bonding [94]. In each case, an attempt

was made to apply interface fracture mechanics, but this was problematic and not

valid because often the crack does not propagate along the interface. Chen et al.

866 S.J. Cunningham a nd M. Kupnik

[95] have examined the effect of morphology on bond strength for copper wafer

bonding.

Another approach is to correlate fracture initiation at the silicon–glass interface

corner with a critical value of the stress intensity calculated from a linear elastic

stress analysis. In the spirit of the interface fracture mechanics this approach is

intended to be universal (independent of bond size) and can be used to design a

variety of different reliable bonds. It is guided by application to adhesively bonded

butt joints (Reedy and Guess [96–98]), homogeneous acrylic (Dunn et al. [99]), and

etched silicon microstructures (Suwito et al. [100]). This approach is valid for other

homogeneous or heterogeneous wafer bonded structures that are produced using any

of the processes described previously as proposed by Dunn et al. [101], Labossiere

and Dunn [102], and Labossierre et al. [103].

11.8 Existing Wafer Bonding Infrastructure

In his 1998 review, Schmidt [10] saw the primary driver for wafer-to-wafer bond-

ing to be the enablement of wafer-level packaging followed by the fabrication of

microstructures and the fabrication of heterogeneous starting wafer material. In the

meantime one might add the emerging need of 3-D IC integration, such as for high-

density memories. Schmidt [10] outlined several areas where continued progress

was needed in order to realize the full potential of wafer-to-wafer bonding. One of

these areas was to have a healthy infrastructure of equipment vendors and bonding

services that supply wafer bonding equipment solutions.

In the meantime, there is such a infrastructure available. For beginning R&D

purposes one can choose wafer bonding services to test the feasibility of a MEMS

fabrication process that requires wafer bonding. We provide a list for such services

in Section 11.8.1. Several tool vendors are available that provide state-of-the-

art wafer bonding tools. Most equipment offered provides an automatic/robotic

interface for handling wafers and automatic wafer-to-wafer alignment, the auto-

matic contacting of the wafers, and the recipe for a process in terms of pressure,

voltage, temperature, time (ramp up, hold, ramp down, anneal), and ambient envi-

ronment (vacuum, dry nitrogen or other inert gas). In Section 11.8.2 we provide an

overview of available wafer bonding tool vendors and discuss some aspects of their

systems.

11 Wafer Bonding 867

11.8.1 Wafer Bonding Services

Table 11.5 Wafer bonding service providers

Company information Principle services

Applied Microengineering, Ltd.

173 Curie Ave

Didcot Oxfordshire OX11 0QG

United Kingdom

www.aml.co.uk

Wafer Size: 50–200 mm

Processes: FB, AN, AB, DS, MM, TC

Materials: Silicon, CW, Glass

Dalsa Semiconductor

18 Airport Blvd.

Bromont, Quebec, Canada

J2L 1S7

www.dalsasemi.com

Wafer Size: 100–150 mm

Processes: FB, AB, DS, MM, TC

Materials: Silicon, Glass, CW, CMOS

Innovative Micro Technology

75 Robin Hill Road

Santa Barbara, CA 93117

www.imtmems.com

Wafer Size: up to 150 mm

Processes: FB, AN, AB, TC, Eutectic, Glass Frit

Materials: Silicon, Glass

TWV: Yes

Integrated Sensing Systems

(ISSYS)

391 Airport Industrial Drive

Ypsilanti, MI 48198

www.mems-issys.com

Wafer Size: 100–150 mm

Processes: FB, AN, DS, Glass Frit

Materials: Silicon, Glass

Silex Microsystems

Bruttovägen 1, SE-175 26

Järfälla, Sweden

www.silexmicrosystems.com

Wafer Size: 150 mm, 200 mm

Processes: FB, AN, AB, DS, TC

Materials: Silicon, Glass

TWV: Yes

Ziptronix Inc.

800 Perimeter Park Dr., Ste B

Morrisville, NC 27560

www.ziptronix.com

Wafer Size: to 300 mm

Processes: FB,

Materials: Silicon, CW

11.8.2 Bonding Tool Vendors

In addition to existing wafer bonding services (Table 11.5), there is a healthy

infrastructure of wafer bonding vendors available from which to choose. We have

identiﬁed four companies that sell state-of-the-art bonding tools. Thus, there is

a multifaceted product portfolio available. Depending on the needs for academia

or industry, one can choose from manually operated tabletop systems up to fully

automated systems that perform cleaning, surface activation, wafer alignment, and

bonding including postbonding inspection (IR), in a closed environment inside the

bonding tool itself. Almost every tool supports all bonding methods that we have

868 S.J. Cunningham a nd M. Kupnik

discussed in the previous sections for various substrate sizes, but there are different

aspects in specialties of each of these tools. To give a ﬁrst overview, we brieﬂy

introduce these companies (in alphabetical order) with some examples and aspects

of their tools. Each company provided photos for the shown ﬁgures, company

descriptions, and tool speciﬁcations for that purpose.

11.8.2.1 Applied Microengineering Ltd (AML), UK

The company AML offers wafer bonding machines (Fig. 11.25a) capable of in situ

alignment under vacuum and elevated temperature, surface activation, and bonding.

The target markets are the MEMS, IC, and III-V industries. The tools offer high

throughput, because pumpdown, heating, and alignment are performed in parallel,

speeding up the bonding time per wafer.

The supported wafer sizes range from 2 to 8 in. The alignment tolerance is spec-

iﬁed to be 1 μm and the unique XYZ-Theta alignment mechanism (Fig. 11.25b,d)

allows a large distance between the bonding surfaces before the bond (surface

activation and outgasing) without contacting the surfaces with ﬂags or spacers.

Top lid of bonding

chamber

(a)

(b)

(d)

Optics for

alignment

During In-Situ

Radical Activation

XYZ-Theta

manipulation for in

chamber alignment

Large gap between top

and bottom chuck possible

(c)

Fig. 11.25 (a) AML’s versatile wafer bonding platform (2–8 in.), which features alignment,

activation, bonding, in situ UV curing, all in the same chamber without any handling steps

between. (b) Bonding chamber from inside. (c) Tool during in situ radical activation for room

temperature bonding; (d) Alignment optics (cameras) (Photos and drawings courtesy of Applied

Microengineering Ltd. (AML), UK)

11 Wafer Bonding 869

Upgrades for in situ radical surface activation, featuring room temperature direct

bonding, are available (Fig. 11.25c), as well as the capability of in situ high-accuracy

alignment with UV cure using UV LED array inside the chamber. Another upgrade

is available for polymer embossing, imprinting, NIL, and other pattern transfer

techniques. The machines have the ﬂexibility for R&D and the throughput and

automation for volume production as well. All wafer bonding types are supported.

Stacks of up to 8 mm can be bonded.

11.8.2.2 EV Group (EVG), Austria

The company EVG offers several types of wafer bonding tools for the MEMS,

3-D–IC integration, and advanced packaging, as well as the compound semiconduc-

tor industries. The systems accommodate the demanding applications by bonding

under high vacuum, precisely controlled ﬁne vacuum, temperature or high-pressure

conditions. EVG’s tools support a wide variety of bonding processes such as anodic,

thermocompression, glass frit, eutectic, diffusion, fusion, solder, adhesive, and UV

bonds. Based on a modular bond chamber design, the systems can be conﬁgured for

R&D, pilot line, or high-volume production.

A maximum level of automation and process integration is achieved by EVG’s

GEMINI



platform (Fig. 11.26). The automated production wafer bonding systems

Bond module

EVG

s production

wafer bonding

Fig. 11.26 The GEMINI



is EVG’s production wafer bonding platform with up to four bond

chambers for high throughput. The system is also available with more than four bond chambers

and it supports wafer sizes up to 300 mm. The wafers ﬁrst go through wafer surface preparation

modules (plasma and cleaning), then to the SmartView



face-to-face-bond alignment module

before the wafers are loaded into the bond chamber (bond module) (Photos courtesy of EV Group

(EVG), Austria)

870 S.J. Cunningham a nd M. Kupnik

for high-volume applications support wafer-to-wafer alignment and wafer bonding

processes with up to four bond chambers in parallel for high throughput. EVG’s

patented SmartView



face-to-face-bond aligner and various preprocess modules

for wafer sizes up to 300 mm are available. Furthermore, for postbonding inspection

compatible metrology equipment, such as IR inspection, is offered as well. EVG

introduced and focuses on the concept of separation between wafer alignment and

bonding.

11.8.2.3 Mitsubishi Heavy Industries Ltd. (MHI), Japan

The target market of Mitsubishi’s l atest 200 mm bonding tool (Fig. 11.27a)is

the integration of CMOS and MEMS wafers. The tool allows performing direct

bonding at room temperature for various material combinations, as illustrated by

the examples shown in Fig. 11.27b–i.

+ +

Dangling bonds

Activation Activated Surface After Bonding

Ion Beam

Oxide

(f) (g) (h)

(i)

(d) (e)

(b) (c)

(a)

(j)

Fig. 11.27 (a) Mitsubishi’s MWB-08A fully automated room temperature bonding machine for

8 in. wafer. Examples: (b)Si–Si;(c) Quartz–quartz; (d) GaAs–GaAs; (e) MEMS device; (f)Au–

Au; (g) LiNbO3–Si; (h) vacuum leak test sample; and (i) Al TSV bonding. (j) Operation principle

of room temperature bonding with ion beam under high vacuum conditions (Photos courtesy of

Mitsubishi Heavy Industries Ltd. (MHI), Japan)

11 Wafer Bonding 871

The achieved bonding strengths are comparable to those of the base material

with the advantage that no heat treatment is required. The technology is based on a

sophisticated surface activation step under high vacuum conditions. The disclosed

main steps (Fig. 11.27j) are as follows. Oxide and other absorbed substances are

removed from the wafer surface under high vacuum conditions by means of ion

irradiation. This creates a highly reactive surface (dangling bonds), which then can

be bonded at room temperature to another wafer.

For research and development, a semiautomatic model is available as well. The

fully automated room temperature bonding machine for 8 in. wafers (Fig. 11.27a)

features an alignment tolerance of 2 μm and a powerful bonding unit that features

up to 100 kN force for metal–metal bonding. The degree of vacuum is 1 e–5 Pa.

11.8.2.4 SUSS MicroTec AG, Germany

SUSS MicroTec provides wafer bonding solutions for the R&D and low- and high-

volume production needs of customers in the MEMS, semicompound/LED and

Cool plate

module

Plasma

module

Align module

Bond module

Inspection module

SUSS MicroTec s

CBC200 wafer

bonding cluster

system for the

MEMS market

Cleaning module

Fig. 11.28 SUSS MicroTec’s CBC200 wafer bonding cluster system for MEMS market with

process modules available. Listed are the modules the wafers run through from surface cleaning

and conditioning, through aligning and bonding, ﬁnishing with cooling and postbond metrology

(Photos courtesy of SUSS MicroTec AG, Germany)

872 S.J. Cunningham a nd M. Kupnik

3-D industries. The CBC200, featuring interchangeable modules to accommodate

evolving process and production requirements, is SUSS MicroTec’s 200 mm fully

automated production wafer bonder platform for customers in the MEMS and 3-

D industries (Fig. 11.28). SUSS MicroTec’s bond modules have a closed bond

chamber design for a contamination-free bonding environment. Wafers are loaded

through a slot door and then the chamber is sealed throughout the bond process. The

chamber is purged with nitrogen to keep moisture and particles out, reducing cycle

time and cost. A gas overpressure up to 3 bars can be used to dampen the motion of

accelerometers or other sensitive MEMS devices. The tool can be conﬁgured with a

bond chamber that is ideal for glass frit, anodic, and direct bonding, or with a bond

chamber that is ideal for providing high forces necessary for thermo-compression,

eutectic, metal–metal bonding, and so on.

Further features of the bond module are: a pressure column technology (patent

pending) that evenly distributes the force across the bond interface to ensure force

uniformity, and multizone vacuum isolated heaters (patent pending) for temperature

uniformity.

In addition to its wafer bonding capabilities, the CBC200 has highly advanced

bond aligner modules, which utilize face-to-face ISA, TSA (topside), or BSA

(backside) alignment methods for ﬂexible and precise alignment capability.

11.9 Summary and Outlook

In the last decades, wafer bonding has served as one of the dominant tools in the ﬁeld

of microfabrication. Not only can it be seen as an enabling technology for the ﬁeld

of MEMS, it also pushes 3-D–IC integration technology and the compound semi-

conductor industry forward, and, thus, will be an integral part of the semiconductor

industry in the future.

Wafer bonding is best known for advanced wafer-level packaging applications

for various ﬁelds, such as sensors. It is accepted that often the packaging cost on a

chip level is the highest cost of the product.

Wafer bonding is often the only approach to fabricate speciﬁc MEMS devices,

mainly because bridging large gaps with sacriﬁcial release methods is often imprac-

tical if not impossible. A good example is the fabrication of micromachined pressure

sensors or capacitive micromachined ultrasonic transducers for lower frequencies

needed for airborne applications, just to name two examples.

It is essential that the MEMS product engineer is ﬂuent in all aspects of wafer

bonding. Only then can the best fabrication process for a certain problem statement

be found and the best equipment chosen for a certain fabrication task. The infras-

tructure and knowledge are available in the form of commercially available bonding

equipment, wafer bonding service, and the literature.

Wafer bonding is still being heavily researched to improve the technology. One

good example is the recent progress in providing commercial wafer bonding tools

that allow reliable direct wafer bonding without any heat treatment. The value

11 Wafer Bonding 873

in such technology lies in the fact that heat treatment can cause problems when

different materials are direct bonded due to different thermal expansion coefﬁcients

or thermal budget limits such as found in IC circuitry. Such tools will extend the

range of possible material combinations, and, thus, open the door to new applica-

tions. It will be only a matter of time that all wafer bonding tool vendors provide

such additional features in their product palette.

Acknowledgments We thank Prof. Roger Howe, Prof. Pierre Khuri-Yakub, and Dr. Eric

Perozziello, Stanford University, for many fruitful discussions and their support. We also thank

Dr. Robert Rhoades, Entrepix Inc., Dr. Aaron Partridge, SiTime Corporation, and Dr. Steve Vargo,

ST Systems USA Inc., for valuable feedback on CMP, direct hand bonding, and HF vapor etching,

respectively.

In addition, the authors acknowledge the employees from the bonding tool vendors that pro-

vided technical speciﬁcations, feedback, and/or provided photo material for Section 11.8.2. These

are Rob Santilli, from Applied Microengineering Ltd; Paul Maciel, from Optical Associates Inc.;

Kensuke Ide, from Mitsubishi Heavy Industries Ltd.; Garret Oakes and Renae Bellah, from

EV Group Inc.; and Kristin Connors, Jim Hermanowski, and Sabine Radeboldt, from SUSS

MicroTec AG.

References

1. F. Lärmer, A. Schilp: Method for anisotropic etching of silicon, US patent 5501893 (1996)

2. Y. Huang et al.: Fabricating capacitive micromachined ultrasonic transducers with wafer-

bonding technology, J. Microelectromech. Syst. 12, 128–137 (2003)

3. M.W. Messana et al.: Packaging of Large Lateral Deﬂection MEMS Using a Combination of

Fusion Bonding and Epitaxial Reactor Sealing, to appear in Proceedings Solid-State Sensors,

Actuators, and Microsystems Workshop, June 6–10, 2010, pp. 336–339 (Hilton Head Island,

South Caroline, 2010)

4. S. Li et al.: Fabrication of micronozzles using low-temperature wafer-level bonding with

SU-8, J. Micromech. Microeng. 13, 732–738 (2003)

5. J.A. Dziuban: Bonding in Microsystem Technology, Springer Series in Advanced

Microelectronics (Springer, Heidelberg, 2006)

6. Q.-Y. Tong, U. Gösele: Semiconductor Wafer Bonding: Science and Technology (Wiley,

New York, NY, 1999)

7. M. Alexe, U. Gösele: Wafer Bonding: Applications and Technology (Springer, Berlin, 2004)

8. A. Plößl, G. Kräuter: Wafer direct bonding: Tailoring adhesion between brittle materials,

Mater. Sci. Eng. R25, 1–88 (1999)

9. F. Niklaus, G. Stemme, J.-Q. Lu, R.J. Gutmann: Adhesive wafer bonding, J. Appl. Phys. 99,

1–27 (2006)

10. M.A. Schmidt: Wafer-to-wafer bonding for microstructure formation, Proc. IEEE 86, 1575–

1585 (1998)

11. K.T. Turner: Wafer Bonding: Mechanics-Based Models and Experiments, Doctoral thesis,

MIT (2004)

12. D. Tabor: Macroscopic properties and interatomic forces, Phys. Educ. 10(7), 487–490 (1975)

13. K. Kendall: Adhesion: Molecules and mechanics, Science 263, 1720–1725 (1994)

14. G. Binning et al.: Atomic force microscope, Phys. Rev. Lett. 56, 930–934 (1986)

15. N. Miki, S.M. Spearing: Effect of nanoscale surface roughness on the bonding energy of

direct-bonded silicon wafers, J. Appl. Phys. 94, 6800–6806 (2003)

16. C. Gui et al.: Fusion bonding of rough surfaces with polishing technique for silicon

micromachining, Microsyst. J. Technol. 3(3), 122–128 (1997)

17. W. Kern: Handbook of Semiconductor Wafer Cleaning Technology – Science, Technology,

and Applications (Noyes Publications, Westwood, NJ, 1993)