Mitsubishi Heavy Industries Technical Review Vol. 48 No. 1 (March 2011)48

Room-temperature Wafer Bonders
Applicable to Devices in Various Fields

KENSUKE IDE*1

TAKAYUKI GOTO*2

JUN UTSUMI*2

TAKENORI SUZUKI*3

The room-temperature bonding technique, which holds bonding materials (wafers)
together by activating their surfaces in a vacuum, can provide strong bonding without applying
heat. The use of room-temperature bonding has primarily been used in the field of sealed
packaging for MEMS (Micro Electro Mechanical Systems), but because this technique can beapplied to diverse materials, it has recently been used in fields other than MEMS, such as
light-emitting and radio-frequency (RF) devices. Mitsubishi Heavy Industries, Ltd. (MHI) has
released a line of room-temperature bonders applicable to different wafer sizes for various
purposes, ranging from research and development (R&D) to device mass production. Thecompany is also trying to expand the application fields of room-temperature bonding byproviding various kinds of bonding support services.

|1. Introduction
Although the principle of room-temperature bonding has long been known, it was not until
recently that the technique was actually applied in industry. Because it enables strong bonding
without the application of heat, it was originally used in wafer-level packaging in the field of
MEMS (Micro Electro Mechanical systems) devices. However, the application ofroom-temperature bonding for device fabrication has expanded into various fields over the last
couple of years.

|2. Principles and Features of Room-temperature Bonding
Room-temperature bonding is a technique in which wafers are irradiated with atoms or ionsin a vacuum to remove the oxide film and absorption layers on the bonding surfaces so that theactivated surfaces can bond (Figure 1). This method, which enables strong bonding without
applying heat, has the following features.

Figure 1 Principle of room-temperature bondingWafers are bonded by irradiating the bonding surfaces with ions or neutral atoms in a vacuum.
*1

Engineering Manager, Engineering Department, Machine Tool Division*2

Research Manager, Advanced Technology Research Center, Technical Headquarters*3

Engineering Department, Machine Tool DivisionMitsubishi Heavy Industries Technical Review Vol. 48 No. 1 (March 2011)49(1) It provides bonding strength equivalent to that of the base material even when bonded at roomtemperature.
(2) It does not cause thermal strain due to bonding.
(3) It achieves high productivity by eliminating the need for a heating/cooling process.
(4) It can bond a wide range of materials, such as silicon-based materials, compound
semiconductors, oxides, and metals, and is capable of bonding different kinds of materials toeach other.

|3. Mitsubishi Heavy Industries’ (MHI) Room-temperature WaferBonders
MHI provides a line of various types of room-temperature 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 2).

Figure 2 Lineup of MHI’s room-temperature wafer bonders

The bonders support three wafer sizes (4, 6, and 8 inches) and are also adaptable to othersizes and nonstandard wafer shapes.
Equipped with built-in wafer transfer and precision alignment mechanisms (to position twowafers for bonding), all models can be used for production shortly after installation. Additionally,
the bonders allow precise alignment (2 μm) and high-load application (100 kN), which are the most
crucial features of bonding machines.
The fully automatic bonder, used for device mass production, is designed to successively
bond 10 sets (20 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 of the 10 sets. Specifications for two fully
automatic models, respectively designed for R&D prototyping and mass production (compatiblewith 8-inch wafers), are listed in Table 1.
Table 1 Specifications of bonders for R&D prototyping and mass production

Type
Unit of operation
Wafer size
Operation mode
Alignment accuracy
Surface activation
Press unit
Alignment method

Semiautomatic bonder
Fully automatic bonder(For prototyping, low- to medium-volume(For mass production: 200-mm compatible)
production)
MWB-04/06 R
MWB-06/08 AX
1 set
10 sets (max.)
100/150 mm
150/200 mm
Semiautomatic
Fully/semiautomatic
±2 μm
±2 μm
Ion gunIon gunMax. load: 20 kN
Max. load: 100 kN
Infrared transparent image
Infrared transparent image

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 1 (March 2011)50|4. Application Fields and Examples of Room-temperature Bonding
As discussed above, the use of room-temperature bonding began in the field of MEMS.
However, the areas of application are expanding recently as the technique has been noted for its
capability to bond a wide range of materials and to provide solid-state direct bonding at roomtemperature. In this paper, the applications of room-temperature bonding are categorized below anddiscussed using examples.
(1) Wafer-level packaging in MEMS and other fields
Wafer-level packaging is a method used to seal packaging by bonding a cover wafer toanother wafer on which MEMS devices are formed. Compared to separate packaging of
singulated devices, wafer-level packaging has advantages of lower cost and higher yields.
Figure 3 is a schematic diagram of wafer-level packaging. Figure 4 shows an example ofacceleration sensors packaged on the wafer level. In this example, packaging is performed bythree-layer bonding, which is a process that bonds cover wafers on the upper and lower surfacesof a wafer with sensors formed thereon. The use of wafer-level packaging is gradually
expanding in the MEMS field as well as the field of quartz crystal devices.

Figure 3 Schematic diagram ofwafer-level packaging
Cover and device wafers are bonded on thewafer level and then diced into chips.

Figure 4 Acceleration sensors made by wafer-levelpackaging
An example in which cover wafers are bonded on the upper andlower surfaces of a wafer with sensors formed thereon.

(2) Production of functional wafers
A functional wafer is produced by bonding different kinds of materials, e.g., oxides,dielectrics, and optical materials. The resultant functional wafer is then subjected to micro
fabrication to obtain the desired devices. Room-temperature bonding is well suited for bonding
materials with significantly different thermal expansion coefficients. Functional wafers can beused for optical and radio-frequency (RF) devices. Figure 5 shows an example of bonding
between lithium niobate (LiNbO3), a dielectric material, and a silicon wafer. Despite thesignificant difference between their thermal expansion coefficients, high-quality bonding can beachieved between lithium niobate and silicon using the room-temperature bonding technique.
(3) Application to high-value devices enabled by direct bonding
Room-temperature bonding is used to improve the efficiency of semiconductor materials
by direct bonding or to enhance cooling efficiency by directly bonding a heat source and acooling layer. The method is primarily applicable to light-emitting devices. Figure 6 shows anexample of bonding between gallium nitride (GaN) and silicon. GaN has recently beenhighlighted as a material used in light-emitting, RF, and power devices. Figure 7 shows anexample of bonding between silicon carbide (SiC) and silicon. SiC is used as a material in
high-efficiency power devices.
(4) Application to three-dimensionally stacked devices
Several layers of wafers, with a circuit provided thereon, are stacked by bonding throughsilicon vias (TSVs) that form through the wafers. The method is applicable to high-integration
MEMS, image sensors, and large-capacity memory devices. Figure 8 shows an example ofaluminum TSV bonding.

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 1 (March 2011)51

As discussed above, room-temperature bonding is currently being applied in various fields,depending on the materials and structures of wafers to be bonded. High expectations for futureapplications of this technique are anticipated, especially to light-emitting and power devices.

Figure 5 Example of lithium
niobate (LiNbO3) andsilicon (Si) bonding

An example in which a 3-inch
LiNbO3 wafer is bonded to a 4-inch
Si wafer.

Figure 8Figure 6
Example ofgallium nitride (GaN)and silicon (Si) bonding

An example in which a GaN chipis bonded to a 4-inch Si wafer.

Figure 7 Example of siliconcarbide(SiC)
andsilicon (Si) bonding
An example in which a 3-inch SiC
wafer is bonded to a 4-inch Siwafer.

Example of aluminum through silicon via (TSV) bonding

An example in which bonding pads are formed and bonded to aluminum TSVs with diameters of 120 μm(scanning electron micrograph).

|5. Device Development Support System
MHI has a line of bonders applicable to various wafer types and production styles, as
described above. To enable more users to utilize the room-temperature bonding technique, thecompany offers a development support program called the “Mitsubishi Bonding Support Program”(MBSP) to assist device development on the user level (Figure 9).

Figure 9

Mitsubishi Bonding Support Program (MBSP)

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 1 (March 2011)52

In general, the device development phase on the user level can be classified into the (1)device concept designing phase, (2) function trial phase, (3) trial mass production phase, and (4)mass production phase. In each of these phases, MHI offers support as outlined below.
(1) Device concept designing phase: In this phase, the user examines the materials of the device
and establishes the basic concept, including the fundamental structure. MHI evaluates thebonding condition and quality of the materials adopted by the users and performs preliminary
bonding tests using simplified devices, thereby determining the feasibility ofroom-temperature bonding.
(2) Function trial phase: This phase prototypes a device that actually functions. Among a series ofuser-level processes, MHI takes charge of the bonding process and provides support forbonding the prototyped samples. In some cases, several prototypes are required asengineering samples, and the company assists the production of bonding samples as well.
(3) Trial mass production phase: This phase prototypes and evaluates the device for the primary
purpose of productivity and yield improvement, thereby determining whether the prototyped
device can be mass produced. MHI enables the improvement of productivity and yield by
optimizing the bonding conditions. The company also assists the sample production in thisphase.
(4) Mass production phase: In this phase, the user actually begins mass producing the device. AnMHI bonder has already been delivered, and maintenance support, after-sales, and overhaul
services are provided to assist the stable device production and productivity enhancement at
the user level.
To ensure that these services of the MBSP function effectively, MHI has three demonstration
machines to bond user sample wafers on a daily basis. Additionally, the company has ample
facilities for testing and evaluating the bonding quality as well as an adequate staff to handle usertechnical problems. Currently, the company has assisted more than 3,000 cases of bonding, and theprogram helps many users accelerate the development of their devices.

|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 application fields are expected tofurther expand. This paper presents some examples of bonding and explains that the technique isexpected to be applied to a broader range of fields, including MEMS and RF, light-emitting, power,and three-dimensionally stacked devices. In the meantime, providing high-quality bonders and asystem to support user-level device development is essential to expand the application fields. MHI
intends to gain more experience, especially in the development of bonders with higher bonding
quality and the provision of bonding support programs for users, so that room-temperature bonding
will be used in a wider range of fields.