JP2010171274A - METHOD OF BONDING Si WAFERS - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of bonding Si wafers, by which second and third Si wafers each having a diameter almost equal to that of a first Si wafer and protecting plural MEMS functional elements are bonded to both sides of the first Si wafer with the plural MEMS functional elements such that the centers of the diameters are almost identical, and at the same time to provide the method of bonding the Si wafers with high productivity and high reliability in a hermetic sealing manner. <P>SOLUTION: Notches are provided on the perimeters of the third Si wafer, and the first to third Si wafers are all bonded by melting a low fusing point metal. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

The present invention relates to MEMS (Micro Electro Mechanical Sys).
tem) The present invention relates to a semiconductor sensor wafer bonding technique provided with a functional element.

In recent years, WLP (Wafer level packa) is used for hermetic sealing of MEMS functional elements.
ge) A method using a technique has been actively tried. As one of the WLP techniques, a technique is known in which another cap wafer is bonded to a wafer on which functional elements are arranged, and each functional element is hermetically sealed in the wafer state. In the prior art, the chips on which the functional elements are formed are separated into pieces and sealed in a packaging container or the like. However, if the WLP technology is used, all the functional elements arranged in the wafer are collectively sealed in the wafer state. Can be stopped. If the WLP technique is used, the packaging container used in the conventional technique becomes unnecessary, and the manufacturing cost can be reduced. Further, the package itself can be reduced in size.

One MEMS functional element to which the WLP technology can be applied is a MEMS acceleration sensor. A capacitance type, a piezoresistive type, and the like are known for each type, and in each type, a sensor includes a movable part that deforms according to applied acceleration and a support part that supports the movable part.
In this structure, the entire acceleration sensor element is hermetically sealed in a packaging container so that the movement of the movable part is not affected by the ambient temperature, humidity, or other atmosphere, or both front and back surfaces of the part including the movable part in the element Alternatively, it is necessary to provide a cap or the like on either side to seal and seal the movable part. WLP technology can be used for the latter encapsulation. When sealing from both front and back surfaces using WLP technology, it is necessary to join three substrates.

Examples of sealing the movable part of the MEMS functional element by the WLP technique are disclosed in Patent Document 1, Patent Document 2, and the like. Patent Document 1 discloses a MEMS assembly in which a MEMS chip having a movable portion is sealed with a cap chip, and these MEMS assemblies are separated into individual pieces after the MEMS substrate and the cap substrate are brought into close contact with each other. It is supposed to be obtained. Patent Document 2
This example discloses an example in which a package substrate is bonded to the front and back surfaces of a substrate on which an acceleration sensor element is formed by MEMS technology.

Examples of joining three substrates are disclosed in Patent Document 2, Patent Document 3, and Patent Document 4. In an example of Patent Document 2, an example is disclosed in which a package substrate is bonded to a front surface and a back surface of a substrate on which an acceleration sensor element is formed by a MEMS technique by Si-Si room temperature bonding. Patent Documents 3 and 4 disclose examples in which a plurality of substrates constituting an inkjet head are joined. Patent Document 3 discloses an example in which the nozzle plate and the electrode substrate are bonded to the upper and lower surfaces of the cavity plate by anodic bonding. In Patent Document 4, the upper and lower surfaces of the glass component substrate having through holes are An example in which a Si component substrate having a through hole and a notch and a Si groove forming substrate are simultaneously bonded by anodic bonding is disclosed.

JP 2008-91845 A JP 2007-201196 A Japanese Patent Laid-Open No. 11-192712 JP 2008-62414 A

Si wafer provided with MEMS functional elements and Si protecting the MEMS functional elements from the front and back surfaces
A fine pattern is formed on the surface of the two wafers by photolithography. These wafers are to be manufactured on the same manufacturing line in terms of manufacturing cost, and are preferably Si wafers having the same diameter. Further, these three Si wafers need to be joined by aligning fine patterns with high accuracy. As described above, none of Patent Documents 1 to 3 discloses a Si wafer bonding method for positioning and bonding three Si substrates having the same diameter with high accuracy.

Further, the residual stress generated at the bonding interface between the Si substrate on which the MEMS functional element is arranged and the two wafers bonded to the front and back surfaces is as small as possible and does not affect the characteristics of the MEMS functional element. It is preferable that When a difference in residual stress occurs at the upper and lower bonding interfaces of the MEMS functional element, the movable part of the MEMS functional element is deformed in the initial state, and at the same time, there is a possibility that characteristic fluctuations due to temperature changes occur. These are not preferable from the viewpoint of reliability.

In Patent Document 3, the substrates are joined by anodic bonding. However, after joining the middle substrate and the lower substrate, the middle substrate and the upper substrate are joined. In this method, it is difficult to equalize the state of the upper and lower joint interfaces and balance the residual stress at the upper and lower joint interfaces. In the case of an inkjet head,
Even if the residual stress generated at the bonding interface is not balanced, the effect on the ink ejection characteristics is small. However, in a functional element that detects a minute movement of the movable part such as a MEMS acceleration sensor, such a residual of the bonding interface is present. The effect of stress balance on properties cannot be ignored. In Patent Document 4, three substrates are bonded by anodic bonding. In this case, substrates of different materials are bonded, and generation of residual stress is inevitable. Further, in Patent Document 4, parts having notches are joined. The purpose of the notches is to take electrode contacts for anodic joining, and the purpose of fixing the entire wafer is mentioned. Absent.

In the method of Patent Document 2, since no heat is applied to the bonding, the residual stress difference generated at the bonding interface between the sensor substrate and the front and back package substrates can be reduced.
It is necessary to perform surface activation treatment using many processes such as degassing treatment, ion beam irradiation, and plasma irradiation. In addition, since the bonding state is very sensitive to the surface state of the bonded portion before bonding, the quality of the bonded state may vary unless the bonding surface state before bonding is sufficiently managed. Therefore, industrially, it is more preferable to employ another joining method that is highly productive and highly reliable for hermetic sealing.

In the present invention, a first Si wafer provided with a plurality of MEMS functional elements, a second Si wafer having a diameter substantially equal to that of the first Si wafer and protecting the MEMS functional elements, and the first Si wafer, A Si wafer bonding method in which the diameter of the Si wafer is substantially equal to that of the Si wafer and the third Si wafer protecting the MEMS functional element is bonded so that the centers of the diameters substantially coincide with each other. Therefore, the present invention provides a method for bonding Si wafers, which has a high reliability and high hermetic sealing performance.

In the Si wafer bonding method of the present invention, the first Si wafer provided with a plurality of MEMS functional elements has a diameter substantially equal to that of the first Si wafer and protects the functional elements.
The wafer and the first Si wafer have substantially the same diameter, have a notch on the outer periphery, and the MEM
A Si wafer bonding method in which a third Si wafer for protecting an S functional element is bonded so that the centers of diameters thereof substantially coincide with each other, wherein the first and second Si wafers have an approximately center of diameter. A step of clamping and fixing a part of the outer periphery so as to match, a step of providing a cutout portion on the outer periphery of the third Si wafer, and a cutout portion provided in the third Si wafer, A step of fixing the outer periphery by aligning the first and third Si wafers so that the centers of the diameters substantially coincide with each other while allowing the clamp fixing portions of the first and second Si wafers to escape; and an external force And a step of collectively bonding the first to third Si wafers.

The Si wafer bonding method of the present invention is characterized in that a plurality of notches of the third Si wafer are arranged symmetrically with respect to the center of the wafer diameter.

The Si wafer bonding method of the present invention is characterized in that an SOI wafer is used as the first Si wafer.

The Si wafer bonding method of the present invention is characterized in that the first and second Si wafers and the first and third Si wafers are bonded by melting a low melting point metal.

According to the present invention, on the front and back of the first Si wafer provided with a plurality of MEMS functional elements, the second Si wafer having a diameter substantially equal to that of the first Si wafer and protecting the MEMS functional elements, The third Si wafer having a diameter substantially equal to that of the first Si wafer and protecting the MEMS functional element is bonded so that the centers of the diameters substantially coincide with each other, and at the same time, the productivity is high and the hermetic sealing property is high. In addition, a highly reliable Si wafer bonding method can be provided.

In an Example, it is an upper surface schematic diagram which shows the positional relationship of the fixed 1st to 3rd Si wafer. In an Example, it is a side surface schematic diagram which shows the fixation procedure of the 1st to 3rd Si wafer. In an Example, it is a side surface schematic diagram which shows the state which piled up the wafer fixing jig | tool which enables a press on the fixed 1st-3rd Si wafer. In an Example, it is a cross-sectional schematic diagram which shows the joining apparatus which presses and heat-processes to the fixed 1st-3rd Si wafer.

Hereinafter, the present invention will be described in detail based on examples with reference to the drawings. In order to make the explanation easy to understand, the same reference numerals are used for the same parts and parts.

(Example)
As the first Si wafer 1 of the present example, an SOI wafer having an outer diameter of 6 inches φ and a thickness of 0.4 mm provided with a piezoresistive acceleration sensor element of 1.18 mm × 1.32 mm was used. The piezoresistive acceleration sensor element is a MEMS functional element in which a frame part and a movable part are formed by a dry etching technique. Since the structure of a piezoresistive acceleration sensor element (hereinafter referred to as an acceleration sensor element) is disclosed in many technical literatures, detailed description thereof is omitted here. Au plating with a thickness of 2 μm was applied to the front and back surfaces of the frame portion of the acceleration sensor element. The second Si
As the wafer 2 and the third Si wafer 3, Si wafers having an outer diameter of 6 inches φ and a thickness of 0.4 mm were used. These Si wafers were provided with a Au-20Sn solder plating pattern, which is a low melting point metal, at a position facing the Au plating pattern of the first Si wafer 1. In this example,
These Au-20Sn solder platings were heated and melted, and the first to third Si wafers were joined by a diffusion reaction with the Au plating applied to the first Si wafer 1.

In the present embodiment, an SOI wafer provided with an acceleration sensor element is used as the first Si wafer 1, but a gyro sensor element, a pressure sensor element, or the like can also be used as the MEMS functional element. In addition, a Si substrate can be used instead of the SOI wafer. An acceleration sensor element can also be formed using a Si wafer in which a thin portion is formed by etching and a metal is bonded to the weight portion.

In addition, the first to third Si wafers of this example were wafers having the same outer diameter of 6 inches φ. By using wafers having the same outer diameter, the first to third Si wafers can be manufactured on the same manufacturing line, which is very preferable in manufacturing. Also, the first to third Si
By making the outer shape of the wafer the same, the acceleration sensor element can be formed on the entire surface of the first Si wafer 1 where the element can be formed. The number of devices that can be removed per wafer can be increased.
This is preferable in terms of manufacturing cost.

In this example, as the low melting point metal for connecting the first to third Si wafers, Au—
Although 20Sn solder plating was used, other low melting point metals can also be used. For example, Au
-20 to 37.6Sn (wt%), Au-90Sn, Sn-9Zn, Sn-3.5Ag,
Sn-3Ag-0.5Cu, Pb-5Sn, Pb-10Sn, Sn-37Pb, Sn-5
A solder material widely used for electronic component mounting, such as 7Bi, can be applied. The composition of the solder is not limited to these, and the solder may contain a trace amount of alloy elements or may have a slightly different composition.

In the bonding of Si wafers using Au-20Sn solder as in this embodiment, the Au-20Sn solder is melted by heating and spreads on the bonding interface, and diffuses and reacts with Au.
Wafers are bonded together. By adopting this method, the hermetic sealing property required in the present embodiment can be easily confirmed by the spread of the wetness of the solder. Therefore, it can be said that this is a preferable process in terms of product quality assurance. Moreover, this method does not require special pretreatment such as surface activation treatment at the bonding interface, and can realize a high quality and stable bonding state. Therefore, joining using solder as in this embodiment has high productivity, and can be said to be a preferable method for joining Si wafers.

FIG. 1 shows the positional relationship between a first Si wafer 1, a second Si wafer 2 (not shown), and a third Si wafer 3 fixed on a circular plate-like carbon wafer fixing jig 4A. FIG. The first to third Si wafers having substantially the same diameter are aligned so that the centers of the diameters substantially coincide, and the first wafer fixing claws 5A and 5B and the second wafer fixing claws 6A and 6 are aligned.
B was clamped on the wafer fixing jig 1A. The third Si wafer 3 is provided with notches 7A and 7B so that the first wafer fixing claws 5A and 5B can clamp and fix the first Si wafer 1 and the second Si wafer 2 directly. .

On the wafer fixing jig 4A, the first Si wafer 1, the second Si wafer 2, and the third S
The procedure for fixing the i-wafer 3 is shown in FIG. First, (a) the second Si wafer 2 is placed on the circular plate-like carbon wafer fixing jig 4A, and then (b) the first Si wafer 1 is placed on the second Si wafer 2. Arranged. At this time, the first Si wafer 1 and the second Si wafer 2 are
The center of the diameter of the wafer is made to substantially coincide with the Au plating pattern of the first Si wafer 1 and the second Si
The solder plating patterns of the wafer 2 were aligned and overlapped. Next, (c) the first Si wafer 1 and the second Si wafer 2 were clamped and fixed to the wafer fixing jig 4A by the first wafer fixing claws 5A and 5B. Next, (d) the third Si wafer 3 was arranged on the first Si wafer 1 so that the notches 7A and 7B provided in advance avoid the first wafer fixing claws 5A and 5B. At this time, the first Si wafer 1 and the third Si wafer 3 have substantially the same diameter center,
The solder plating pattern of the third Si wafer 3 was aligned and overlapped with the Au plating pattern of the first Si wafer 1. Finally, the first to third Si wafers were clamped and fixed on the wafer fixing jig 4A by the second wafer fixing claws 6A and 6B.

The alignment of the Si wafers was performed by reading an alignment mark formed on the Si wafer surface with an infrared microscope and finely adjusting the positional relationship of the wafers. In order to perform the alignment operation smoothly, it is preferable to use a mechanism that performs image processing on the read alignment mark and automatically adjusts the positional relationship of the Si wafer. Further, in this embodiment, after the first Si wafer 1 and the second Si wafer 2 are aligned, the first Si wafer 1 and the second Si wafer 2 are fixed, and further the third Si wafer. 3 is fixed and the first to third Si wafers are fixed. However, the first to third Si wafers are simultaneously aligned and the first to third Si wafers are fixed simultaneously. May be. In this case, the second Si with respect to the first Si wafer 1
It is preferable to use an apparatus provided with a mechanism for aligning the wafer 2 and a mechanism for aligning the third Si wafer 3 with respect to the second Si wafer 2.

Without the notches 7A and 7B, the first Si wafer 1 and the second Si wafer 2 cannot be fixed after being aligned, and the first Si wafer 1 and the third Si wafer 3 are aligned. In this case, the positional relationship between the first Si wafer 1 and the second Si wafer 2 may be shifted. After the first Si wafer 1 and the second Si wafer 2 are aligned and fixed,
A method of joining these two Si wafers and then joining the first Si wafer 1 and the third Si wafer 3 by positioning is also conceivable. However, in this method, the joining is performed twice. 1
There is a possibility that a difference occurs in the state of the bonding interface between the upper and lower Si wafers 1, and an unbalance of residual stress generated at the bonding interface may affect the characteristics of the acceleration sensor, which is not preferable. If the notches 7A and 7B are provided in the third Si wafer 3 as in this embodiment, the first to third Si wafers can be easily joined together.

The cutout portions 7A and 7B of the third Si wafer 3 have the diameter centers of the first to third Si wafers substantially coincided with each other so that the Au plating pattern of the first Si wafer 1 has the second Si wafer. When the solder plating patterns of the second and third Si wafers 3 are aligned and overlapped, the first
The first wafer fixing claws 5A and 5B for fixing the Si wafer 1 and the second Si wafer 2 are provided. In order to fix each wafer with equal force, the first wafer fixing claw 5A is used.
5B and the second wafer fixing claws 6A and 6B are preferably arranged symmetrically with respect to the center of the diameter of the wafer. Therefore, the third Si corresponding to the position of the first wafer fixing claws 5A, 5B.
The positions of the cutout portions 7A and 7B of the wafer 3 are also preferably arranged symmetrically with respect to the center of the wafer diameter. Further, in this embodiment, the first wafer fixing claws and the second wafer fixing claws are arranged in two places, respectively, but three or more places may be provided. At this time, it is preferable that the wafer fixing claws are arranged symmetrically with respect to the center of the diameter of the wafer, and a plurality of notches are formed on the third Si wafer so as to correspond to the positions of the first wafer fixing claws. Needless to say, a part is provided.

Further, the shape of the notches 7A and 7B provided on the third Si wafer 3 is such that the interference of the first wafer fixing claws 5A and 5B is caused when the third Si wafer 3 is stacked on the first Si wafer 1. Any shape can be used as long as it can be avoided, but a linear shape as in the present embodiment is preferable because it can be formed by easy processing such as dicing. Notch 7A using dicing,
When 7B is formed, substrate contamination due to dicing affects the reliability of the acceleration sensor, and thus it is necessary to perform a sufficient cleaning process before bonding.

Further, it is preferable that the size of the notches 7A and 7B is set so as to fit in the wafer handling part (the part where the transfer arm of the apparatus touches the wafer). The wafer handling portion is a portion where elements cannot be formed in the process. If the notches 7A and 7B are formed in this portion, the element can be formed using the entire element formable region of the first Si wafer 1, which is very preferable in terms of manufacturing cost.

In the procedure of FIG. 2, the first to third Si wafers fixed on the wafer fixing jig 4A are
A wafer-fixing jig 4B that enables pressing as shown in FIG. The wafer fixing jig 4B is made of the same carbon as the wafer fixing jig 4A, and is subjected to relief processing so as not to interfere with the wafer fixing claws when being in surface contact with the third Si wafer 3.

FIG. 4 is a schematic diagram showing a state in which the first to third Si wafers fixed to the wafer fixing jig are set in the bonding apparatus used in this embodiment. First to third S fixed on a wafer fixing jig
The i-wafer was further heated by being sandwiched between plate-like carbon heaters 8A and 8B, and pressed together by pressing rods 10A and 10B via quartz plates 9A and 9B that insulate heat and electricity. The atmosphere for bonding the first to third Si wafers was adjusted by introducing the atmosphere gas after evacuating the chamber 11.

In this embodiment, first, the first to third Si wafers fixed to the wafer fixing jigs 4A and 4B are set between the carbon heaters 8A and 8B, and the inside of the chamber 11 is evacuated and then N _2. Gas was introduced to create an N ₂ atmosphere. After that, through the carbon heaters 8A and 8B,
The first to third Si wafers are pressed with a load of 1000 kg with the pressing rods 10A and 10B,
By energizing the heaters 8A and 8B made of carbon, the temperature was raised to 305 ° C. above the melting point of Au-20Sn solder of 278 ° C. and held at 305 ° C. for 5 minutes, and then the wafer was cooled while N ₂ gas was replaced in the chamber. To complete the joining process.

In this embodiment, N ₂ gas is used as the atmospheric gas in the chamber 11, but an inert gas other than N ₂ such as Ar may be used. In addition, when a solder other than Au-20Sn is used as the low melting point metal, the pressing amount and the heating temperature may be adjusted depending on the type of solder.

After separating the first to third Si wafers bonded in this example for each acceleration sensor element,
When the sensor characteristics were evaluated by mounting on a board for characteristic evaluation, initial offset deviation of the output voltage caused by deformation of the element movable part and abnormal temperature coefficient of the output voltage when temperature change was recognized. I couldn't. This result means that by using the bonding method of the present embodiment, the influence of the residual stress generated at the wafer bonding interface on the characteristics of the acceleration sensor element can be extremely reduced.

Further, when the bonding cross section of the first to third Si wafers was observed by SEM, the bonding interface between the first Si wafer 1 and the second Si wafer 2 and the first Si wafer 1 and the third Si wafer were observed. The Au-20Sn solder is sufficiently wetted and spread at both of the bonding interfaces of the first Si wafer 1.
It was confirmed that the acceleration sensor element provided in the above was sufficiently hermetically sealed. Further, the bonding interface between the first Si wafer 1 and the second Si wafer 2, and the first Si wafer 1 and the third Si wafer 1
It was also confirmed that Au-20Sn solder and Au plating were diffused almost equally at the bonding interface of the Si wafer 3. Furthermore, when the joint strength between the first Si wafer 1 and the second Si wafer 2 and between the first SI wafer 1 and the third Si wafer 3 was evaluated by a shear test, the joint strength was found at any joint interface. Was sufficient, and the Si wafer did not peel off from the bonding interface.

As a comparative example, after aligning the first Si wafer 1 and the second Si wafer 2, first, only two Si wafers are pressed and heated, and then bonded to the first Si wafer 1. Three Si wafers 3 were aligned, and a sample was manufactured by pressing and heating the first to third Si wafers. Similarly to the example, the Si wafer of this comparative example was singulated for each acceleration sensor element and evaluated for characteristics. As a result, the initial offset of the output voltage was large, and at the same time, the temperature coefficient of the output voltage when the temperature was applied was It varied widely from device to device. Also, the bonding interface is SEM
As a result of observation, the first Si wafer 1 and the third Si wafer 3 are compared with the bonding interface in the first.
It was confirmed that the diffusion of Au-20Sn solder and Au plating was progressing at the bonding interface between the Si wafer 1 and the second Si wafer 2. The reason why the above results were obtained in the acceleration sensor element of the comparative example was that the first to third Si wafers were not collectively bonded, so that a difference occurred in the bonding state on the upper and lower surfaces of the acceleration sensor element, It is considered that the unbalance of the residual stress has deformed the movable part of the acceleration sensor element.

From the above, the first Si wafer 1 can be obtained by using the Si wafer bonding method as in the present embodiment.
And the second Si wafer 2, and the first SI wafer 1 and the third Si wafer 3 can be bonded with substantially equal bonding interfaces, have good hermetic sealing properties, and are provided on the first Si wafer 1. It was confirmed that the first to third Si wafers can be bonded without affecting the characteristics of the acceleration sensor element.

1 first Si wafer,
2 second Si wafer,
3 Third Si wafer,
4A, 4B Wafer fixing jig,
5A, 5B First wafer fixing claw,
6A, 6B Second wafer fixing claw,
7A, 7B Notch,
8A, 8B heater,
9A, 9B quartz plate,
10A, 10B pressing rod,
11 Chamber.

Claims (4)

A first Si wafer provided with a plurality of MEMS functional elements has a diameter substantially the same as that of the first Si wafer and protects the functional elements, and a diameter of the first Si wafer and the diameter of the first Si wafer. In the Si wafer bonding method, the first Si wafer is bonded to a third Si wafer having substantially the same diameter and having a cutout portion on the outer periphery and protecting the MEMS functional element so that the centers of the diameters are substantially coincident with each other. And a second Si wafer are aligned so that the centers of their diameters are substantially coincident, and a part of the outer periphery is clamped and fixed, a step of providing a notch on the outer periphery of the third Si wafer, The center of the diameter of the first and third Si wafers are substantially coincided with each other while allowing the clamp fixing parts of the first and second Si wafers to escape into the cutouts provided in the three Si wafers. A process of aligning and fixing the outer periphery; Bonding method of the Si wafer, wherein the step of joining in bulk third Si wafer from the first by an external force. 2. The Si wafer bonding method according to claim 1, wherein a plurality of cutout portions of the third Si wafer are arranged symmetrically with respect to the center of the diameter of the wafer. 2. The Si according to claim 1, wherein the first Si wafer is an SOI wafer.
Wafer bonding method. 2. The method for bonding Si wafers according to claim 1, wherein the first and second Si wafers and the first and third Si wafers are bonded together by melting a low melting point metal.

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