Cover cap for semiconductor wafer devices

Details Cover cap for semiconductor wafer devices Cover cap for semiconductor wafer devices

1998-11-18

2003-03-18

Talbott, David L. (Department: 2827)

Semiconductor device manufacturing: process

Packaging or treatment of packaged semiconductor

Making plural separate devices

C438S118000, C438S458000, C438S460000

Reexamination Certificate

active

06534340

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to semiconductor devices, and more particularly, to a patternable protective cover for a semiconductor wafer substrate, wherein one or more devices reside on and may be fabricated from the semiconductor substrate.
BACKGROUND OF THE INVENTION
A variety of semiconductor devices may be implemented on a substrate according to several known techniques. The semiconductor devices may perform, for example, electrical, mechanical, optical, or other functions, or combinations of such functions.
Often, a semiconductor wafer serves as a substrate for such devices. The devices may be fabricated from the semiconductor material of the substrate wafer itself using a variety of known processes, such as growth of various material layers on a surface of the substrate, ion implantation, diffusion, oxidation, photolithography, etching and many other processes. During fabrication, typically at least some portion of each device is formed “within” the substrate wafer, below the surface of the substrate, and may additionally include particular topographic or structural features near the substrate surface.
Alternatively, semiconductor devices may be fabricated from a first semiconductor wafer, and subsequently mounted on a second semiconductor wafer or other type of material that serves as a substrate. The semiconductor devices fabricated from the first wafer may be mounted on the substrate either as individual devices or groups of devices. In this case, the devices “reside on” the substrate, as opposed to being “fabricated from” the substrate, as discussed above.
For purposes of the present invention, either of the foregoing examples of substrates, namely, substrates on which semiconductor devices reside, and/or from which semiconductor devices are fabricated, is referred to as a “device” substrate. A device substrate may include one or a large number of devices.
Many known semiconductor devices are extremely fragile and/or sensitive to environmental hazards. Some examples of such hazards include contamination by dust or other particulates, moisture, and inadvertent scratching or other damage to portions of the surface of the devices that include “active” areas. An active area of a device generally refers to a functional region such as an electrical contact, a semiconductor junction, an optically sensitive area, or a micro-mechanical structure.
Functional defects may result from one or more environmental hazards, as discussed above, and are a major cause of low device yield and other malperformance characteristics. Device damage due to any number of such hazards may occur, for example, during the process of “dicing” (separating the device substrate into individual devices), as well as during packaging of devices. Often, the number of functioning devices remaining after dicing and packaging is markedly reduced due to defects resulting from environmental hazards.
Various techniques are known in the art for protecting semiconductor devices on a substrate from such hazards. Some of these techniques include bonding a protective semiconductor cap wafer to a device substrate before dicing the substrate into individual devices. This technique has been employed particularly with wafer substrates of semiconductor devices that include micro-machined parts or microscopic mechanisms fabricated on the surface of the substrate, such as micro-electrical-mechanical systems or MEMS.
According to one known technique for protecting semiconductor micro-mechanical devices, an entire device substrate wafer is capped with another wafer using a pattern of glass-like “posts” or “frit glass” as a bonding agent. In this technique, the micro-mechanical devices are hermetically sealed inside an open cavity formed by the frit glass pattern, the device substrate and the cap wafer. Accordingly, any micro-mechanisms are free to move while simultaneously being protected from various environmental hazards, such as particulate contamination.
According to the technique discussed above, the cap wafer is typically another semiconductor wafer of the same type as that used for the device substrate (for example, silicon or gallium arsenide). As a result, the cap wafer has essentially identical thermal characteristics to that of the device substrate. This choice of cap wafer avoids most mechanical stresses that may result from a thermal mismatch between the cap wafer and the device substrate. For example, extreme mechanical stress can occur during a high temperature heat treatment necessary to ensure adequate bonding of the frit glass to the cap wafer and the device substrate. Any mechanical stress can severely damage or even destroy the devices on the substrate, and would especially degrade the accuracy of many delicate micro-machined devices, such as microscopic mechanical sensors or other MEMS.
It is also noteworthy in the technique discussed above that any processing steps required for fabrication of the cap wafer are performed before the cap wafer is bonded to the device substrate. For example, in some instances, one or more holes which extend completely through the cap wafer are provided by drilling or anisotropic etching, so as to allow for electrical connections to the devices on the device substrate. These drilling or etching steps are performed before assembly of the cap wafer and the device substrate, so as to avoid damage to the delicate micro-mechanical devices on the substrate.
Another known protective packaging technique has been employed with semiconductor image sensors. This technique differs from the protective capping technique used for micro-mechanical devices, as discussed above. In the technique for protecting micro-mechanical devices, an entire device substrate of micro-mechanical devices is protected with a cap wafer before dicing. In contrast, in the technique for protecting image sensors, each discrete image sensor is individually equipped with a protective glass cap after the image sensor has been separated from the device substrate. The image sensor protection technique also differs from the micro-mechanical device protection technique in that the protective cap must be transparent to a variety of radiation wavelengths, and more specifically, to a particular radiation wavelength range of interest so that the image sensors are not blocked from receiving the radiation of interest.
According to one technique for protecting semiconductor image sensors, each individual image sensor is bonded in a cavity package, such as a ceramic package, and the protective glass cover is attached to the package with an optically compatible adhesive that substantially underfills the glass cover.
There are a number of drawbacks to this approach. First, the image sensor surface is exposed during the entire packaging operation, and therefore it is still vulnerable to damage from several types of environmental hazards, including particulate contamination. Second, any particles trapped in the package after the glass cover is attached can lead to unpredictable in-use failure of the image sensor at some later time. Third, the protective glass caps must be individually fabricated and individually attached, which is expensive. In addition, this technique cannot be readily implemented with micro-mechanical devices, because the adhesive underfills the protective glass cap and leaves essentially no open cavity to allow useful movement of a micro-mechanism.
SUMMARY OF THE INVENTION
In view of the foregoing, it would be advantageous to protect semiconductor devices, such as image sensors, on a “wafer” level; namely, simultaneously protecting one or more devices residing on or fabricated from a device substrate, as opposed to protecting individual devices after the device wafer has been diced. While such a wafer level technique has been employed with micro-mechanical devices by using a semiconductor cap wafer, as discussed above, the capping technique employed for micro-mechanical devices does not permit exposure of the devices to various radiation wavelength ranges of interest, as would be necessary

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