Mitsubishi Heavy Industries Technical Review Vol. 57 No. 3 (September 2020)
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Room-temperature Wafer Bonder Applicable toManufacturing of Semiconductor Devices inVarious Fields

TAKAYUKI GOTO*1

The room-temperature bonding technique enables strong bonding without applying heat. As
it can be used with diverse materials, this technique has recently been applied in various device
fields.
Mitsubishi Heavy Industries Machine Tool Co., Ltd. manufactures and markets
room-temperature wafer bonders that can be used for various purposes, ranging from research anddevelopment (R&D) to device mass production and is expanding the applicable fields forroom-temperature bonding.

|1. Introduction
The principle of room-temperature bonding has long been known, and related research has
been energetically conducted, including that by Honorary Professor Suga of the University ofTokyo(1). As a result, it has become a highly-competitive bonding technique that can be said to be aJapanese specialty. However, it was not until recently that the technique was actively applied in
actual industrial fields.
The room-temperature bonding technique, which holds bonding materials (wafers) together
by activating their surfaces in a vacuum, enables strong bonding without applying heat. The use ofroom-temperature bonding has primarily been used in the field of sealed packaging for Micro
Electro Mechanical Systems (MEMS). Because this technique can be applied to diverse materials,
it has recently been used in other device fields such as surface acoustic wave (SAW) device,
light-emitting device and power device.
We manufacture and market room-temperature wafer bonders that can be used for various
purposes corresponding to each wafer size, ranging from R&D to device mass production, and are
expanding the applicable fields of room-temperature bonding. These bonders are utilized in themanufacture of various devices due to their high-quality bonding characteristics. This paper
presents the principles and features of room-temperature bonding, fields of application, bonding
equipment and bonding examples.

|2. Principles and features of room-temperature bonding
2.1

Classification of wafer bonding techniques

Various processes have long been used for wafer bonding. Typical wafer bonding techniques
are roughly classified into bonding with an intermediate material and direct bonding as shown inFigure 1. In the case of all processes other than room-temperature bonding, it is necessary to heat
the wafer to a high temperature, and there are issues in terms of the reduction of temperature stresson the device and low thermal strain bonding of materials with different thermal expansion
coefficients.

2.2

Principle of room-temperature bonding

Figure 2 illustrates the principle of room-temperature bonding. Under normal circumstances,the bonding material surface is covered with an oxide film and an adsorption layer. In this state,joining cannot be made by contacting and pressurizing at room temperature. However, if these*1

Micro Machining System Group, Engineering Headquarters, Mitsubishi Heavy Industries Machine Tool Co., Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 57 No. 3 (September 2020)
2materials are placed in a high-vacuum chamber and neutral atoms or ions of an inert element suchas Ar are irradiated to remove the oxide film layer and adsorption layer on the bonding surface,
dangling bonds appear on the surface. At this time, the surface is in a high energy state and is called
an “activated surface.” When surfaces activated by such means are brought into contact with eachother in a high vacuum, the dangling bonds are bonded to each other and a strong bonding force isgenerated. In this way, room-temperature bonding is a process that proceeds entirely at room
temperature without heating, and is also known as “surface activated bonding,” because thesurfaces are activated and bonded thereby.

Figure 1

Classification of wafer bonding techniques

Figure 2 Principle of room-temperature bonding

When the degree of vacuum in the vacuum chamber is low, residual gas components in thechamber are adsorbed again on the activated surface and terminate the dangling bonds, so a degreeof vacuum of 10-6 Pa is generally required. For bonding, the dangling bonds need to be close toeach other at the atomic level. Therefore, the surface roughness of the bonding interface greatlyaffects the possibility of bonding. Specifically, the arithmetic average surface roughness Ra must
be 0.5 nm to 0.7 nm or less. In addition, particles existing at the bonding interface cause voids, sosufficient cleaning is required.

Figure 3

TEM image of

Si to Si bonding interface

Mitsubishi Heavy Industries Technical Review Vol. 57 No. 3 (September 2020)
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Figure 3 is a TEM(Transmission electron microscope) image of the bonded interface
between silicon materials. As shown in the figure, there is an amorphous layer with a thickness ofseveral nanometers at the bonding interface. An amorphous layer formed at the bonding interface inthis way allows the bonding of materials with different crystal orientations and lattice constant.

2.3

Features of room-temperature bonding

Based on the aforementioned bonding principle, room-temperature bonding has thefollowing features.
(1) Provides bonding strength equivalent to that of the base material even when bonded at roomtemperature
The resulting bonding strength depends on the material, but in the case of bonding siliconmaterials, for example, it is possible to easily obtain a bonding strength similar to that of thebase material.
(2) No thermal strain due to bonding
Due to room-temperature bonding, even materials with greatly different thermal
expansion coefficients can be bonded without being affected by thermal distortion. This greatly
expands the range of applicable material combinations and contributes to device
miniaturization.
(3) Achieves high productivity by eliminating the need for a heating/cooling process.
(4) Capable of bonding a wide range of materials, such as silicon-based materials, compound
semiconductors, oxides and metals, as well as bonding different kinds of materials to eachother.

|3. Application fields of room-temperature bonding
Use of room-temperature bonding began in the MEMS field. However, the areas ofapplication have been expanding in recent years as the technique has been noted for its capability tobond a wide range of materials and to provide solid-state direct bonding at room temperature. In
this section, the applications of room-temperature bonding are categorized below and discussed.

3.1

Wafer-level packaging in MEMS and other fields

Wafer-level packaging is a method used to perform seal-packaging at the wafer level bybonding a cover wafer to another wafer on which MEMS structures are formed.
In MEMS manufacturing, fine MEMS structures are formed on a wafer by bulk
micromachining(2). Depending on the wafer size, hundreds to tens of thousands of devices areformed on a single wafer. Seal-packaging is performed after dicing, but at that time, it is inefficientto perform packaging separately for each singulated device. Therefore, sealing is performed
collectively by bonding at the wafer level, and then dicing is performed to singulate the devices.
Compared to separate packaging of singulated devices, wafer-level packaging has significant
advantages in terms of cost and yield. Heating during bonding causes thermal stress and thermal
distortion, leading to a decrease in yield and device performance. On the other hand, by bonding atroom temperature using the room-temperature bonding technique, a high yield and improvement indevice performance can be realized. Figure 4 is a schematic diagram of wafer-level packaging. It ismainly used for the mass production of acceleration sensors and pressure sensors. The use ofwafer-level packaging is gradually expanding in the MEMS field, as well as the field of quartz
crystal devices.

Figure 4

Wafer-level packaging of MEMS

Mitsubishi Heavy Industries Technical Review Vol. 57 No. 3 (September 2020)
43.2

Production of engineered substrate

A engineered substrate is produced by bonding different kinds of materials, such as oxides,dielectrics and optical materials. The resultant engineered substrate is then subjected to micro
fabrication such as patterning to obtain the desired devices. Room-temperature bonding is well
suited for bonding materials with significantly different thermal expansion coefficients.
As shown in Figure 5, SAW filters used for smartphones are manufactured by bonding asilicon or sapphire wafer, which is a supporting substrate, to a lithium tantalate (LiTaO3) wafer,
which is a engineered substrate, and then performing patterning and thinning. A SAW filter allows
necessary frequencies to pass and blocks unnecessary frequencies. Several tens of SAW filters areused for a single smartphone.

Figure 5 Manufacture of engineered substrate for SAW
filterIn recent years, as smartphones have become more multifunctional and sophisticated, themounting density of components has increased, and higher internal temperatures in smartphones
have become an unavoidable issue. Lithium tantalate is used as a engineered substrate for SAW
filters, but its thermal expansion coefficient is large (16.1 ppm/°C), so it is necessary to reduce thefrequency shift associated with the increased temperatures. Therefore, a method was devised toimprove the frequency temperature characteristics of SAW filters by bonding a wafer with a low
thermal expansion coefficient such as silicon (3.4 ppm/°C), sapphire (7.7 ppm/°C), etc., as asupport substrate on a lithium tantalate wafer to suppress the thermal expansion. This type of filter
is known as a temperature compensated-surface acoustic wave (TC-SAW) filter.
Since there is a significant difference between the thermal expansion coefficients of lithium
tantalate and silicon or sapphire, heat bonding of these materials causes a large amount of warpage
or cracking. However, room-temperature bonding makes it possible to perform high-quality
bonding.

3.3

Application to high value-added devices enabled by direct bonding

For example, room-temperature bonding is used to improve the efficiency of semiconductormaterials by direct bonding or to enhance cooling efficiency by directly bonding a heat source anda cooling layer. Figure 6 depicts a method to manufacture a engineered substrate for power
semiconductors or light emitting devices by bonding a functional substrate of single crystal SiC,GaN, etc., to a supporting substrate of polycrystalline SiC, sapphire, etc., and thinning it using
Smart cutTM (3). This method makes it possible to manufacture several hundreds of bonded
substrates from a single functional substrate by Smart cutTM the functional substrate andperforming bonding repeatedly, and to greatly reduce the manufacturing cost as a result.

Figure 6 Manufacture of engineered substrate for high value-added devicesMitsubishi Heavy Industries Technical Review Vol. 57 No. 3 (September 2020)5|4. Room-temperature wafer bonders
Our product lineup features various types of room-temperature wafer bonders compatible
with different diameters of wafers made from a variety of materials that satisfy their intended
purposes, ranging from R&D and prototyping to device mass production. Figure 7 presents ourlineup of room-temperature wafer bonders.

Figure 7

Our lineup of room-temperature wafer bonders

The bonders support four wafer sizes (φ100 mm (4 inches), φ150 mm (6 inches), φ200mm (8 inches) and φ300 mm (12 inches)) and are also adaptable to other sizes and non-standard
wafer shapes.
Equipped with built-in wafer transfer and alignment mechanisms (to position two wafers forbonding), all models can be used for production shortly after installation. In addition, by making
full use of the digital simulation technology cultivated in machine tools, the bonders provide bothprecision alignment (bonding accuracy after bonding of 2 μm or less) and high load application atbonding (up to 200 kN in the case of a bonder capable of handling 300 mm wafers), and realize
uniform activation of the wafer surface using activation simulation. As a result, high-qualityroom-temperature bonding can be achieved, which enables stable production quality even in massproduction.
The fully-automatic bonder, used for device mass production, is designed to successively
bond 12 sets (24 pieces) of wafers. All processes, including wafer transfer and alignment, areautomated. Furthermore, it can be adapted to various styles of small-lot production because specificbonding conditions can be set separately for each bonding batch. The specifications for twomodels, one for R&D and prototyping and the other for mass production (capable of handling 200mm wafers), are listed in Table 1.
Table 1 Main specifications of room-temperature wafer bonders

Type4.1

Semiautomatic bonder(For R&D prototyping, low- tomedium-volume production)
MWB-04/06R

Fully-automatic bonder(For mass production: 200-mmcompatible)
MWB-04/06/08AX

Unit of operation

1 set

12 sets (max.)

Wafer diameter

100 mm/150 mm

100 mm/150 mm/200 mm

Operation mode

Semiautomatic

Fully/semi-automatic

Alignment accuracy±2 μm±2 μm

Press unit

Max. load: 20 kN

Max. load: 100 kN

Alignment method

Infrared transparent/reflection image

Infrared transparent/reflection image

Bonder for R&D and prototyping

This bonder consists of a load lock chamber in which two wafers—upper and lower—are
inserted and a process chamber in which bonding is performed. The two wafers to be bonded are
set on a tray in a station in the load lock chamber.
The rough bonding flow is as follows.

Mitsubishi Heavy Industries Technical Review Vol. 57 No. 3 (September 2020)
6(1)
(2)
(3)
(4)

The upper wafer is carried into the process chamber and held by the electrostatic chuck.
The lower wafer is carried into the process chamber.
An Ar beam is irradiated to activate the bonding surface.
Alignment is performed with the alignment marks formed on the left and right sides of thewafer.
(5) Bonding is performed.
(6) The bonded wafer is returned to the load lock chamber.
All these operations are displayed on a PC screen along with the flow and device status, andthe operator can confirm the flow and proceed with the work simply by clicking a button on thescreen. The image processing system also supports labor-intensive alignment, so it can becompleted in a short amount of time. As for the Ar beam irradiation conditions and other bonding
conditions, both the conditions specified as a “recipe” and the actually-measured conditions are
saved as a process log, which can be analyzed by commercially available software.

4.2

Bonder for mass production

A bonder for R&D and prototyping opens the load lock chamber and performs toolingchange every time a batch (bonding) ends. On the other hand, a bonder for mass production stores
12 upper wafers and 12 lower wafers in the wafer cassettes and automatically carries out all
operations of wafer transfer, alignment, surface activation and bonding. Since vacuum exhaustionis performed once for the bonding of 12 batches, the cycle time per a batch can be reduced. As thebonding conditions for each batch can be registered as individual “recipes,” it is possible to handle
wafers of different parts in a mixed manner and to flexibly deal with the production of many kindsof small quantities.

|5. Bonding examples
5.1

Silicon-to-silicon bonding

The photograph in Figure 8 shows the tensile test results of silicon-to-silicon bonding. For atensile test, the bonded test piece is attached to the tensile test tool using an adhesive. When the test
is conducted, however, since the adhesion strength of the adhesive is smaller than the bonding
strength of room-temperature bonding, peeling at the bonding interface does not occur and the test
piece comes off the tool. Therefore, the tensile test was performed using a mesa-shaped(trapezoidal-shaped) test piece as shown in Figure 8 prepared so that the room-temperature bonded
area fractures first. As a result, the fracture occurred at the base material rather than the bonding
interface as shown in the photograph, indicating that the bonding strength was equivalent to thebase material.

Figure 8 Si to Si bonding (after tensile test)

5.2

Metal-to-metal bonding

Figure 9 shows the tensile test of bonding between Au films formed on silicon substrates. Asin the case of silicon-to-silicon bonding described above, a mesa-shaped test piece was used. It is
indicated that the fracture occurred at the silicon base material, rather than the Au-to-Au bonding
interface.

Mitsubishi Heavy Industries Technical Review Vol. 57 No. 3 (September 2020)
7Figure 9 Bonding between Au films (after tensile test)

In a similar manner, Figure 10 presents the results of a tensile test of bonding between Cu
films formed on silicon substrates using a meta-shaped test piece. Since the adhesion strength
between the SiO2 film and the Ti film, which are formed in order to improve the adhesion between
the silicon substrate and the Cu film, is smaller than the bonding strength between the Cu films,
peeling occurred first at the interface between the SiO2 film and the Ti film. The adhesion strength
between the SiO2 film and the Ti film measured at that time was 8 MPa. Therefore, it can be seenthat at least the bonding strength between the Cu films was higher than 8 MPa.

Figure 10

Bonding between Cu films (after tensile test)

When wafers with Cu electrodes are bonded as shown in Figure 11, there is an issue with theelectrical characteristics at the interface. Figure 12 gives the results of measuring the electricalcharacteristics (I-V characteristics) after bonding the Cu films. As a result, it is found that theelectric resistance at the bonding interface is 20 mΩ or less and good electrical characteristics areobtained.

Figure 11 Bonding between wafers
with Cu electrodes

Figure 12 Electrical characteristics at Cu
bonding interface|6. Conclusion
Room-temperature bonding is a unique bonding technique that can provide strong directbonding of a wide range of materials at room temperature, and its applicable fields are expected tofurther expand. A large number of our room-temperature wafer bonders have been delivered in thefields of MEMS and SAW filters and used for mass production. We will continue to contribute to
the manufacture of a wide variety of semiconductor devices.

Mitsubishi Heavy Industries Technical Review Vol. 57 No. 3 (September 2020)
8References
(1)
(2)
(3)

T.Suga et al, Direct bonding of ceramics and metal by means of a surface activation method in ultrahighvacuum, MRS International Meeting on Advanced Material Research Soc., Vol.8 (1989) p.257
General incorporated foundation Micromachine Center, MEMS Reader, (2017)
http://www.mmc.or.jp/info/cafe/talk/ibeans/beans21.html
Junichi Sato, How-nual visual guide book Yokuwakaru Saishin Basics and Mechanism of Semiconductor
Nanoprocesses, Shuwasystem, (2012), p.54