Infrared sensor package

An optical sensor package with a substrate that supports a membrane carrying an optical sensor and through which radiation passes to impinge the sensor. The substrate has a first surface in which a cavity is defined, a second surface opposite the first surface, and a wall between the cavity and the second surface. The optical sensor is supported on the membrane, which is bonded to the substrate and spans the cavity in the substrate. A window is defined at the second surface of the substrate for enabling infrared radiation to pass through the wall of the substrate to the optical sensor.

BACKGROUND OF INVENTION

1. Field of the Invention

The present invention generally relates to packages for containing electronic devices, and more particularly to a package having a infrared sensor mounted to a silicon base through which infrared radiation passes before impinging the sensor.

2. Description of the Related Art

Infrared (IR) sensors have been used to measure the temperature of thermal sources, such as hot materials, humans, etc. To accurately detect heat radiated from a target, interference from ambient light, particularly visible light, should be filtered out. This can be done by adding a filter in front of the IR sensor. For example, in commonly-assigned U.S. Patent No. 6,844,606 to Logsdon et al., a chip formed of silicon—which allows only wavelengths longer than about 1.1 μm to pass through—is individually mounted to a chip carrier on which an infrared sensor is mounted so that the silicon chip is between the sensor and the target being sensed. In the automotive applications, an IR sensor package equipped with such a silicon “window” allows a targeted subject, such as the driver or passengers of a car, to be monitored with minimum background interferences.

SUMMARY OF INVENTION

The present invention is directed to an optical sensor package in which a substrate supports a membrane carrying an optical sensor, and radiation passes through the substrate to impinge the sensor.

Generally, the optical sensor package includes a substrate, at least a portion of which is formed of silicon. The substrate has a first surface in which a cavity is defined, a second surface opposite the first surface, and a wall between the cavity and the second surface. An optical sensing element is supported on a membrane bonded to the substrate and spanning the cavity in the substrate. A window is defined at the second surface of the substrate for enabling infrared radiation to pass through the wall of the substrate to the optical sensing element. The wall preferably has a bandgap of about 1.1 eV so as to absorb impinging radiation with wavelengths shorter than 1.1 micrometers, such that only radiation of wavelengths longer than 1.1 micrometers pass therethrough to the optical sensing element.

From the above, it can be seen that a significant advantage of the invention is that the filter required for the sensor to be active only in the desired optical range is an integral part of the sensor structure.

DETAILED DESCRIPTION

The present invention provides an optical sensor package that employs a silicon-containing substrate as the supporting material for an IR sensor, which is preferably made by standard integrated circuit (IC) fabrication processes. A preferred sensor employs a thermopile as the IR sensor, such as the thermopiles disclosed in commonly-assigned U.S. Pat. Nos. 6,793,389 and 6,828,172, the contents of which relating to thermopile construction are incorporated herein by reference. In addition, a suitable thermocouple structure for the thermopile is disclosed in commonly-assigned U.S. patent application publication No. 2005/0016576to Jiang et al., the contents of which relating to thermocouple construction are incorporated herein by reference.

According to the present invention, the substrate is configured to define a wall that serves as a filter to filter out visible light, allowing only infrared wavelengths of interest to reach the IR sensor. According to a preferred aspect of the invention, integrated circuitry for performing logic functions and signal processing required for the IR sensor can also be fabricated on or in the substrate.

FIG. 1is a cross-sectional view of an IR sensor package10in accordance with a first embodiment of the invention. The package10comprises a silicon substrate12in which a cavity14has been formed (e.g., etched), a multilayer membrane16bonded to the substrate12and enclosing the cavity14, and a thermopile sensor18formed in a layer20of the membrane16. Standard IC processes and micromachining techniques can be used to fabricate the sensor18, including its structure and elements, on the membrane16. One or more layers22of infrared absorbing and reflecting materials are shown as being formed on the membrane16, thereby defining the outer surface of the membrane16to enhance infrared absorption and heat generation within the sensor18. Suitable materials for the infrared absorbing and reflecting layers22include oxynitride, tetra-ethyl-ortho-silicate (TEOS) based oxides, low-temperature deposited oxides, and aluminum.

The membrane16is bonded to the silicon substrate12, which is preferably a monocrystallographic silicon chip. According to a preferred aspect of the invention, the package10is one of any number of packages that are micromachined and assembled at wafer-level and subsequently singulated into individual packages. Integrated circuitry32for performing logic functions and signal processing required for the thermopile sensor18is represented as being fabricated in the silicon substrate12, such as located in the surface to which the membrane16is bonded. A wall24is defined by and between the cavity14and the backside surface of the substrate12, and serves as a silicon window through which infrared radiation is permitted to pass to impinge the sensor18while filtering out undesired radiation. For this purpose, the backside surface of the substrate12has an antireflection coating26, a portion of which is exposed within an opening28formed in an outer coating30to define the window region of the wall24. The antireflection coating26minimizes the amount of infrared radiation reflected by the silicon substrate12. Suitable materials for the antireflection coating26include a single layer of silicon nitride, organic layers, and other custom composite layers of appropriate materials at appropriate thicknesses to meet the required radiation spectrum sensing regime. The outer coating30on the substrate12is preferable opaque to the desired range of infrared radiation wavelengths, so that radiation of the desired wavelengths impinges the location on the membrane16corresponding to the location of the thermopile sensor18, but radiation of other wavelengths are otherwise reflected to minimize thermal energy absorption by the sensor package10.

In view of the package configuration shown inFIG. 1, use of the sensor package10involves facing the backside of the silicon substrate12toward the intended target, such as the driver or passengers of a vehicle, so that infrared radiation passes through the silicon wall24of the substrate12to the sensing elements of the sensor18on the membrane16. In this manner, visible light with wavelengths shorter than about 1.1 μm is filtered out by the silicon wall24before reaching the sensor18.

FIG. 2shows an IR sensor package40similar to that ofFIG. 1(with the same reference numbers used to identify essentially the same features), but with a filter material42implanted and driven-in or epitaxially grown on the backside of the substrate12to provide a filtering layer. Depending on the filter material42different wavelengths of light can be filtered. Candidate materials include germanium (Ge), PbS, InAs, and PbTe, which allow only wavelengths longer than about 1.88, 3.02, 3.44 and 4.0 μm, respectively, to pass through the window24to the sensor18.

FIG. 3shows a package50that is a variation ofFIG. 2, in which filtering is achieved with a filter chip54that has been bonded to a silicon chip52to form the substrate12. The chip52is represented as having been etched from backside to frontside (opposite that ofFIGS. 1 and 2), such that the cavity14extends completely through the chip52. The filter chip54can be, for example, a silicon or germanium chip bonded to the silicon chip52. In this embodiment, wafer-level packaging of the additional filter material, as opposed to chip-level packaging, has the ability of lowering manufacture costs.

FIG. 4shows another variation ofFIG. 2, in which a triple stack of wafers is used to form a package60that encloses the membrane16, thereby protecting the membrane16and its thermopile sensor18from the surrounding environment. The package60is represented as having a third chip62bonded to the membrane16, with a cavity64defined in the chip62so as to protectively enclose the membrane16. Suitable materials for the chip62include silicon, the use of which permits the package60to be formed by silicon-to-silicon wafer bonding techniques.

While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. Accordingly, the scope of the invention is to be limited only by the following claims.