JP4966902B2 - Glass wafer with built-in optical filter - Google Patents

Description

  The present invention relates to a structure and a manufacturing method of a cap glass wafer incorporating an optical filter such as an infrared cut filter mounted when a solid-state imaging device is assembled. In the wafer level package of the solid-state imaging device, the warpage of the cap glass wafer due to the optical filter such as the infrared cut filter on the surface is reduced, and the processing accuracy in the subsequent process is improved.

  As described in Patent Document 3, an infrared cut filter is mounted on a solid-state imaging device such as a CCD and a CMOS for high sensitivity. As a filter constituent material and structure, as described in Patent Document 1, it is common to use a laminated structure of various metals and oxides thereof. As the forming method, physical and chemical vapor deposition are the mainstream.

Wafer level chip size packages (hereinafter referred to as WLCSP) have been implemented in order to reduce the size of the solid-state imaging device package and reduce the manufacturing cost. In the WLCSP of a solid-state image sensor, a cap wafer is bonded to the surface of the image sensor wafer for the purpose of protecting the image sensor. In view of the function as an image pickup body, the cap wafer is made of a translucent material such as glass. In consideration of the influence on the imaging characteristics due to refraction of incident light, it is a mainstream to provide a gap of several tens of μm between the cap wafer and the imaging element.
Patent Document 2 and the like have been proposed as a method for creating a WLCSP of a solid-state imaging device. In this method, a cap wafer made of glass is bonded to a wafer on which a solid-state imaging device is formed using a photosensitive adhesive.

In order to incorporate the infrared cut filter into the above-described WLCSP, the above-described process is performed using a cap wafer on which an infrared cut filter has been formed in advance.
JP 07-061835 JP 2003-092394 A JP-A-2005-338395

When an optical filter such as an infrared cut filter is formed on the surface of the cap wafer, the wafer after the film is greatly warped. This is due to the difference in the thermal expansion coefficient between the optical filter and the cap wafer and the large film stress of the optical filter. When using a cap wafer with such warpage in a WLCSP of a solid-state imaging device,
1. The alignment accuracy between the cap wafer and the image sensor wafer decreases.
2. It becomes difficult to post-process a bonded body of a cap wafer and an image sensor wafer (hereinafter referred to as a bonded wafer).
There is a problem.

  In an actual process, the warpage amount of a bonded wafer on which an optical filter is formed may be twice or more as compared with a bonded wafer using a cap wafer on which an optical filter is not formed.

  In order to solve the above problems, the present invention uses a cap wafer in which an optical filter such as an infrared cut filter is sandwiched between glass wafers. The upper and lower glass wafers sandwiching the optical filter are preferably made of the same material. Preferably, the upper and lower glass wafers are made equal in thickness.

  In this way, by using a cap wafer having a geometric line object structure centered on an optical filter, a cap wafer having an optical filter and less warpage can be obtained. Further, by performing the WLCSP process on the solid-state imaging device using the cap wafer described above, it is possible to obtain a bonded wafer having a small amount of warp while having an optical filter. As a result, the alignment accuracy is improved and the post-process can flow without problems.

  The present invention provides a WLCSP process with less warpage while having a cap filter with an optical filter such as an infrared cut filter. As described above, the conventionally used cap wafer with an optical filter has a large amount of warpage and has various problems in the WLCSP process. In order to avoid these problems, it is only necessary to carry out optical filter film formation on the surface of the cap wafer constituting the bonded wafer after completion of the WLCSP process of the solid-state imaging device. This method cannot be selected because the temperature exceeds the heat resistance temperature of the solid-state imaging device including the element.

  Therefore, the present invention uses a cap wafer in which an optical filter is sandwiched between glass wafers of the same material and the same thickness.

  FIG. 1 is a schematic cross-sectional view of a cap wafer proposed in the present invention. 1, 3 is a glass wafer, and 2 is an optical filter. A feature of the present invention resides in a cap wafer structure in which the optical filter 2 is sandwiched between the glass wafers 1 and 3. The glass wafers 1 and 3 sandwiching the optical filter 2 are preferably made of the same material. This makes the thermal expansion coefficients of the upper and lower glass wafers 1 and 3 equal (in the figure, the thermal expansion coefficient (h) of the upper glass wafer 1 = the thermal expansion coefficient h ′ of the lower glass wafer 3), and its thermal expansion coefficient. This is to suppress the occurrence of warping of the cap wafer due to the difference. Therefore, the important point is the coefficient of thermal expansion, which is not necessarily the same glass. Preferably, the difference in thermal expansion coefficient between the upper glass wafer 1 and the lower glass wafer is 20% or less. Exceeding this value is undesirable because the amount of warpage increases and exceeds an allowable level (a state that causes no problem in the process).

  Preferably, the upper and lower glass wafers 1 and 3 are equal in thickness. However, it is not necessary to have the same thickness, and the difference in thickness between the upper glass wafer 1 and the lower glass wafer is preferably 20% or less. If the difference in thickness exceeds 20%, even if the material of the glass wafer is the same, the amount of warpage due to the difference in thickness becomes large and exceeds the allowable level (state in which there is no problem in the process). This is to suppress the warpage of the cap wafer due to the dimensional difference between the upper and lower glass wafers 1 and 3.

  In this way, by using a cap wafer having a geometric line object structure centered on an optical filter, a cap wafer having an optical filter and less warpage can be obtained. Further, by performing the WLCSP process of the solid-state imaging device using the cap wafer having the structure in which the optical filter 2 is sandwiched between the upper and lower glass wafers 1 and 3, a bonded wafer having a small amount of warp while having the optical filter is obtained. Obtainable.

As a method for producing the cap wafer, an optical filter is first formed on one glass wafer by using existing PVD (physical vapor deposition) or CVD (chemical vapor deposition). Thereafter, the other glass wafer is bonded to the optical filter to obtain the cap wafer of the present invention. As a bonding method, SAB (Surface Activated Bonding), fusion bonding, or a transparent adhesive can be used.
The bonding method will be specifically described below.

Example 1 Production of Cap Wafer by Fusion Bonding First, an optical filter is formed on one glass wafer using existing PVD and CVD. Thereafter, the other glass wafer is brought into contact with the optical filter surface. At this time, it is desirable to work in a reduced-pressure atmosphere in order to suppress air bubbles from entering the contact interface. Heating is performed on both bonded wafers. The temperature at this time needs to be set to be not less than the melting point of the optical filter resurface layer and not more than the melting point of the glass wafer. Therefore, in order to widen the process window, it is desirable to select an optical filter material having a low melting point and a glass material having a high melting point.

Example 2 Production of Cap Wafer by SAB First, an optical filter is formed on one glass wafer using existing PVD and CVD. Subsequently, the glass wafer and the other glass wafer are irradiated with plasma such as O 2 and H 2 to activate the surface. Thereafter, both wafers are brought into contact with each other without taking time. At this time, the atmosphere is preferably decompressed, and it is desirable to apply a pressure at the time of contact.

Example 3 Production of Cap Wafer with Adhesive FIG. 2 is a schematic view of a cross section of a cap wafer when an adhesive is used. 1 and 3 are glass wafers, 2 is an optical filter, and 4 is an adhesive layer. The glass wafer 1 to be bonded to the optical filter 2 is bonded via the adhesive layer 4. As the adhesive, polystyrene, polymethyl methacrylate, polycarbonate, and other acrylic ester resin materials have high visible light transmittance.

  An example of this structure will be described. First, the optical filter 2 is formed on one glass wafer 3 using existing PVD and CVD. Then, the adhesive layer 4 is formed on the surface of the optical filter 2 or the surface of the glass wafer 1 to be joined. As the forming method, spin coating, dip coating, and spray coating can be used if the adhesive is liquid, and roll lamination or press laminating is used if it is a film. The thickness of the adhesive layer 4 is desirably as thin as possible in order to avoid impairing the imaging accuracy of the solid-state imaging device.

  As the bonding method, a photochemical, thermochemical or thermophysical method can be selected. The choice of these methods depends on the physical properties of the adhesive material. Many acrylic resins are active to ultraviolet light, and the polymerization reaction proceeds upon irradiation with ultraviolet light. When these materials are used, a cap wafer can be produced by irradiating one glass surface with an arbitrary amount of ultraviolet light after contacting both glass wafers.

If the adhesive material is photochemically inert, bonding by heating is performed. If the adhesive is a kind of material that causes a polymerization reaction at a high temperature, the cap wafer is produced by holding both glass wafers and holding them at the polymerization temperature for an arbitrary time.
If the adhesive material is a material that is photochemically inactive and does not cause a polymerization reaction below its melting point or decomposition temperature, an adhesion by fusion is attempted. That is, both glass wafers are brought into contact and then heated to the melting point of the adhesive, and the optical filter and the adhesive layer are fusion bonded to produce a cap wafer.

  As described above, the cap wafer having the structure in which the optical filter of the present invention is sandwiched between glass wafers can significantly reduce the amount of warpage as compared with the conventional cap wafer. By using the cap wafer of the present invention for the WLCSP process of a solid-state imaging device on which an optical filter is mounted, the alignment accuracy is improved and accurate positioning is possible. Further, the processing difficulty in the WLCSP process is reduced, post-processing is facilitated, and the processing yield is significantly improved. Further, since the optical filter is built in the cap wafer, the optical filter can be prevented from being damaged during the WLCSP process.

  The optical filter is not limited to the infrared cut filter. The present invention can be applied to various optical functional thin films such as a visible light cut filter, an ultraviolet cut filter, a cold mirror, a dichroic mirror, and a dichroic filter. Alternatively, the optical filter may be a filter that transmits only a specific wavelength and cuts (does not transmit) light other than that wavelength. The material used for the infrared cut filter is a material that has high visible light transparency and a high light absorption rate in the infrared region (wavelength of 800 nm or more). Examples thereof include inorganic materials such as ITO (indium / tin / oxide) film and ATO (aluminum / titanium / oxide) film, and various polymer materials such as phthalocyanine. In addition, the material used for the ultraviolet cut filter is a material that has high visible light transparency and a high light absorption rate in the ultraviolet region (wavelength of 400 nm or less). Examples thereof include inorganic materials such as ZnO (zinc oxide) film and GaN (gallium nitride) film, and various polymer materials such as polycarbonate and polyester. The glass wafer used for the cap wafer may be made of other materials as long as it has translucency. For example, there are transparent polymer materials such as acrylic, polycarbonate (PC), polyethylene terephthalate (PET), and polypropylene (PP).

  The present invention can be used for a wafer level package process using a bonded substrate used in the semiconductor industry.

FIG. 1 is a schematic cross-sectional view showing a cap wafer using the present invention. FIG. 2 is a schematic cross-sectional view showing another embodiment of a cap wafer using the present invention.

Explanation of symbols

1 ... Glass wafer,
2 ... Optical filter 3 ... Glass wafer 4 ... Adhesive layer

Claims (10)

In a wafer level chip size package (WLCSP) in which a cap wafer having an optical filter is bonded, the optical filter is sandwiched between two glass wafers, and the thermal expansion coefficients of the two glass wafers are the same. A cap wafer having an optical filter , characterized in that the difference between these thermal expansion coefficients is 20% or less . 2. The cap wafer according to claim 1, wherein the two glass wafers have the same thickness or a difference between the thicknesses of 20% or less. The cap wafer according to claim 1, wherein the two glass wafers are made of different materials . Forming an optical filter on the first glass wafer using PVD (physical vapor deposition) or CVD (chemical vapor deposition); and
A step of bonding a second glass wafer having a thermal expansion coefficient equal to or less than 20% on the optical filter formed on the first glass wafer. A method for producing a cap wafer having an optical filter, comprising: 5. The method for manufacturing a cap wafer according to claim 4, wherein the two glass wafers have the same thickness or a difference between the thicknesses of 20% or less. 6. The method of manufacturing a cap wafer according to claim 4, wherein the two glass wafers are made of different materials. 7. The bonding according to claim 4, wherein the bonding is any one of a fusion bonding method, a SAB (Surface Activated Bonding) method, and a method using a transparent adhesive. A method for producing a cap wafer according to the item . 8. The method according to claim 7, wherein when the bonding is a fusion bonding method or a SAB method, the step of bonding the second glass wafer onto the first glass wafer is performed in a reduced-pressure atmosphere. Of manufacturing a cap wafer. When the bonding is a fusion method, the melting point of the optical filter material is lower than the melting points of the first glass wafer and the second glass wafer, and the heating temperature is higher than the melting point of the optical filter material. The method for producing a cap wafer according to claim 7 or 8, wherein the method is lower than a melting point. When the bonding is the SAB method, the surface of the second glass wafer is irradiated with plasma such as O 2 and H 2 to activate the surface, and the first glass wafer is brought into contact with the first glass wafer. The method for producing a cap wafer according to claim 7, wherein the second glass wafer is bonded.

Images