Wafer-to-Wafer Bonding for
Microstructure Formation
MARTIN A.SCHMIDT
Invited Paper
Wafer-to-wafer bonding process for microstructure fabrica-
tion are categorized and described.The process have an
impact in packaging and structure design.Process are catego-
rized into direct bonds,anodic bonds,and bonds with intermediate
layers.Reprentative devices using wafer-to-wafer bonding are
prented.Process and methods for characterization of a range
of bonding methods are discusd.Opportunities for continued
development are outlined.
Keywords—Anodic bonding,MEMS,micromachining,silicon
语音英文direct bonding,silicon fusion bonding,wafer bonding.
I.I NTRODUCTION
Micromachining encompass a broad range of technolo-
gies anchored in the core technology of microlithographic
pattern transfer.A large fraction of the micromachining
technologies are specific to the silicon material system
principally due to the origins of thefield,namely,the
silicon integrated circuit(IC)industry.In the silicon micro-
machiningfield,there have been two dominant fabrication
四诗
methods,broadly classified as bulk micromachining(etch-
ing deep features into a wafer)and surface micromachining
(depositing,patterning,and lective etching offilms on a
wafer).Fundamentally,both of the techniques rely on
some form of etching or material removal.A complemen-
PROCEEDINGS OF THE IEEE,VOL.86 tary,additive,micromachining technology is the wafer-to-
wafer bonding of patterned substrates,often simultaneously
involving alignment of the substrates.
Historically,some of the earliest us of wafer-to-wafer
外贸邮件范文
bonding were for packaging of pressure nsors.The
wafer-to-wafer bonds were performed at low temperatures
(less than450
脏话大全越毒越好
Fig.1.A Motorola pressure nsor using glass frit wafer bonding for packaging[9].
lower wafer is etched to form an inlet port,and thus the
role of the lower wafer bond reverts from that of a vacuum
al to afirst-level pressure inlet manifold.In either ca,
the primary function of the wafer bonding is to effect a package function.
This rais an important point,namely,the concept of
wafer-level packaging.It is readily acknowledged,partic-
ularly in the nsors industry,that packaging can be a dominant component in thefinal manufactured cost of a device.By achieving package function at the wafer level,
it is possible to realize tremendous overall savings in cost
since this enables the packaging of a multitude of nsors
or actuators simultaneously,eliminating costly individual
chip-packaging steps.
Another example of the u of wafer bonding in the manufacture of pressure nsors is shown in Fig.2[10].
Here,the pressure nsor is formed by a high-temperature
silicon direct bond,followed by thinning of a wafer and fabrication of the piezoresistors.In comparison with the conventional bulk micromachined pressure nsor of Fig.1,
this pressure nsor has veral interesting features that are
enabled by the wafer bonding.The wafer bonding produces
a reference cavity for the absolute pressure nsor as in
the previous ca;however,the cavity depth can be made
considerably smaller(1–10
500
200
四头狮子(a)
(b)
Fig.2.A silicon wafer-bonded pressure nsor.(a)Die photo.(Courtesy of Lucas NovaSensor [10].)(b)Cross ction of process.
III.C ATEGORIES OF W AFER-TO -W AFER B ONDING
The types of wafer bonding that are most commonly employed in microstructure fabrication can be placed in three categories:
A.Direct Bonds
Wafers are directly contacted without the assistance of significant pressure or any intermediate layers or fields.The bonding schemes rely on the tendency for smooth surfaces to adhere,and always utilize some form of thermal cycling after the contacting to increa the bond strength.
The maximum temperature of the anneal defines the bond-ing as being low temperature
(800
C.The bond is typically performed between a
sodium-baring glass wafer and a silicon wafer.
C.Intermediate-Layer Bonds
This category includes all bonding mechanisms that re-quire an intermediate layer to promote the wafer bond.This could include eutectic bonds,polymers,solders,or thermo-compression bonds.Many of the wafer-level bonds are a scale-up of bonds that have been routinely performed at the die level in the packaging of IC’s.
The next ctions will discuss each of the bonding methods in detail.The emphasis will be placed on silicon direct bonding.IV.D IRECT B ONDING
A.Background
The direct-bond method relies on forces that naturally attract surfaces together when they are very smooth and flat.A range of mechanisms have been propod to explain this initial contact attractive f
orce.It is well known that smooth metal surfaces,if atomically clean,will bond together.This process is often referred to as “cold welding”and is typically achieved by cleaning and contacting the metal surfaces in vacuum to maintain cleanliness.This bond usually relies on plastic deformation of the metal to bring the atoms in clo contact.In the ca of most direct bonds that have been performed for microstructures,some surface treatment (e.g.,hydration,oxygen plasma exposure)is conducted prior to the contacting to promote the surface attraction and bonding process.This is sometimes assisted by a modest pressure to expel air from between the wafers and to initiate the contact.The bond is usually followed by a thermal cycle,which increas the strength of the bond.In the following ctions,we will begin with a description of the silicon direct bonding process,discuss characterization methods for the silicon bond that generalize to all bonding methods,and investigate bonding methods for other types of substrates.
B.Silicon Bonding Process
Extensive review articles have been written on the silicon wafer-bonding process,particularly as it pertains to elec-tronic device fabrication [2],[4],[20]–[22].This process has also been referred to as silicon fusion bonding.This c-tion will simply summarize the major points of the process for the bonding of silicon or silicon dioxide surfaces.The silicon wafer-bonding process consists of three bas
ic steps:surface preparation,contacting,and annealing.The starting wafers must be smooth and flat.There have been studies
SCHMIDT:WAFER-TO-WAFER BONDING
1577
Fig.3.A microaccelerometer using wafer bonding [12].(Courtesy of Lucas
花样人间
NovaSensor.)
Fig.4.Flow channels formed by silicon–glass bonding.
of the necessary surface quality for wafer bonding [23],[24],and in general,it has been experimentally obrved that the wafers should have a roughness of no greater than
about 10˚A
and a bow of less than
5wafer).Also,protrusions from the surface (resulting from previous
processing)of greater than 10˚A
can produce problems in the bonding.All of the process steps are conducted in a cleanroom environment,although G¨o le has propod a powerful “microcleanroom”concept that does not require a cleanroom [25].The surface-preparation step involves cleaning the mirror-smooth,flat surfaces of two wafers to form a hydrated surface.There have been studies of the differences between hydrophobic and hydrophilic surfaces on the final bond interface [22].Following this preparation,the wafers are contacted in a clean environment by gently pressing the two surfaces together at one central point.The surfaces come into contact at this point and are bound by a surface attraction of the two hydrated surfaces.A contact wave is initiated at this point and sweeps across the wafer surfaces,bringing them into intimate contact over the entire surface.The contacting process is critical to prevent trapping of particulate or air between the surfaces.The exact origin of the attractive force that promotes the contact wave is not universally agreed upon [22]and depends to a
certain extent on whether the bond is Si–Si or Si–SiO OH groups on the opposing surfaces.
The final step in the bonding process is an elevated tem-perature anneal of the contacted pair at temperatures any-where from room temperature to 1200
C).
Measurements of the bond strength as a function of an-neal temperature indicate three distinct regions.The first region,for anneal temperatures less than 300
C,the bond strength increas and then levels out.It is presumed that an Si–O–Si bridging bond is formed between the surfaces and a water molecule is liberated.At temperatures greater than 800
C or greater,
the bond strength is in the range of the strength of the silicon crystal itlf.Kinetically,the bond strength at high temperatures
(
erally lumped into two categories:extrinsic and intrinsic. The extrinsic voids are tho created by particles,protru-sions on the wafer surface,or trapped air.The voids are usually obrved on contact and do not change significantly during annealing.Fig.6shows infrared(IR)transmission images of wafers with various forms of extrinsic voids(the imaging methods are described in the next ction).Intrinsic voids are voids that are generated during the anneal cycle [4].After contact,the wafer pair will appear void free. However,as the anneal temperature is incread,voids begin to appear above400
C.The voids usually are only en when bond-ing silicon to silicon without an intermediate oxide,and they are often attributed to hydrocarbon contamination, although there is not a connsus on their origin.G¨o le has provided some detailed discussion of this issue[67]. It has been obrved that cavities in the wafers can rve to“getter”the microvoids,thus minimizing the problem
[27],[28].
D.Bond Characterization
Several nondestructive and destructive techniques exist for mechanical characterization of the bonding process. In this ction,we will discuss the methods as applied to silicon direct bonding,but 好的反义词是什么
many of the same tech-niques can be applied to other methods of bonding.The most common techniques are bond imaging,cross-ctional analysis,and bond-strength measurement.The imaging methods are nondestructive and can be ud as in-process monitors,while the cross-ctional analysis and bond-strength measurements are destructive and require control wafers for characterization.
The three dominant methods for imaging a bonded pair of silicon wafers are infrared transmission,ultrasonic,and X-ray topography.Examples of the images obtained by the methods for two bonded4-in silicon wafer pairs are shown in Fig.6.A simplified schematic of an infrared imaging system is shown in Fig.7.It consists of an IR source(typically an incandescent light bulb)and an IR-nsitive camera.A silicon charge-coupled device camera has sufficient nsitivity in the near-IR range that it can be ud when outfitted with afilter for visible light. The bonded wafer pair is located between the source and camera.Any imperfection in the bond shows up as changes in contrast in the IR image.Large unbonded regions(“voids”)appear with a characteristic“Newton’s Rings”pattern.This imaging method generally cannot image voids with a paration of surfaces less than one-quarter of the wavelength of the IR source.Bad on a typical particle void,this translates to a spatial resolution of veral millimeters[24].Fig.8,which compares the three imaging methods using
迪士尼乐园英语the same wafer,clearly illustrates voids not prent in the IR image,which do show up in the other methods.Also,this technique works for silicon wafers of moderate doping level with smooth surfaces. Highly doped layers,IR absorbingfilms,or rough surfaces (backside of wafer)can limit the image quality.In spite of this resolution limit,the IR method has the
advantage Fig.6.IR transmission image of two bonded wafer pairs.The
pair on the right has veral voids at the bonded interface due to particles and trapped
air.
Fig.7.Schematic illustration of the IR imaging system.
of being simple,fast,and inexpensive.It can be ud directly in the cleanroom to image the wafers before and after anneal.The other two imaging methods offer higher resolution at the expen of speed,cost,and incompatibility with cleanroom processing.
Cross-ctional analysis can be performed at the bonded interface by cleaving the sample.Scanning electron micro-scope(SEM)and transmission electron microscope tech-niques have been ud to image the bonded interface at a submicrometer scale.The studies have helped to understand the composition of the bonded interface[22]. Additionally,it is possible to gain a great deal of informa-tion about the bonded interface by simply defect etching the cross-ctioned sample.Several groups have demonstrated
SCHMIDT:WAFER-TO-WAFER BONDING1579
