Patent Description:
Conventional testing of CMOS (complementary metal-oxide-silicon) image scan devices makes use of a halogen lamp to provide light for testing. This arrangement has significant disadvantages, including large size, significant required maintenance and difficulty in reconfiguring test setups. These problems become especially acute in the context of wafer-scale device testing, where a light source having a diameter as high as <NUM> may be required. Accordingly, it would be an advance in the art to provide an improved light source/probe card for testing optoelectronic devices.

<CIT> discloses an optical test apparatus, related test method and method of operation, and related probe card adapted to optically test an image sensor. An illumination source of the optical test apparatus provides an optical test signal to the image sensor through the probe card. The optical test signal has a property variably defined by a feedback loop formed between a reference image sensor associated with the probe card and a control unit connected between the reference image sensor and the illumination source.

<CIT> relates to an apparatus for testing an image sensor is provided to selectively test the required number of chips and to improve test reliability by controlling a region of light incident into the image sensor according to the number of the chips for testing. A light source irradiates light to an image sensor. A diffusion filter is arranged between the light source and the image sensor. The diffusion filter controls an angle of the light incident into the image sensor. A probe card has a probe contacted to a pad of the image sensor. The probe card tests photoelectric transformation characteristic of the image sensor according to the light. A support supports a side of the diffusion filter. The support fixes the diffusion filter to the probe card. A fly-eye lens array is formed between the light source and the diffusion filter. The fly-eye lens array makes intensity of the light irradiated from the light source uniform.

<CIT> relates to a system to provide visible lighting of a selectable spectral characteristic (e.g. a selectable color combination of light) uses an optical integrating cavity or other diffuse mixing element to combine light of different colors from different color LEDs. Amplitude modulation of pulsed operation the light sources, e.g. pulse amplitude modulation added to a baseline forward bias current for each of the LEDs, controls the amount of each light color supplied to the diffuse mixing element and thus the amount included in the combined light output of the system. A color sensor may provide feedback as to a color characteristic of the combined light, for closed-loop control of one or more of the pulse amplitude modulations. Examples are also disclosed that utilize phosphor doping of one or more of the system's reflective elements, to add desired wavelengths of light to the combined output.

<CIT> discloses an inspection device, for each of at least two solid-state imaging elements: probes each electrically connected to a signal terminal of each solid-state imaging element; a shielding structure covering at least a light reception part of each solid-state imaging element; and a light source arranged inside the shielding structures. When light rays are emitted from the respective light sources at the same time, the light rays from the respective light sources are prevented from interfering with each other by the shielding structures. Accordingly, the inspection device can emit, to the plurality of solid-state imaging elements, stable light rays at the same time.

The present invention is defined in appended independent claim <NUM> to which reference should be made.

We provide a solution to the above-described problems based on the use of light emitting diodes (LEDs) as the light source for CIS device testing. The basic architecture is an LED array having one LED source per CIS device chip being tested. Arrays of LEDs are used because such testing is typically done wafer-scale, as opposed to being performed individually for single devices. The LEDs illuminate a phosphor which provides the light used for testing. An aperture + lens arrangement is used to provide telecentric light. In some embodiments, the aperture can be used as part of the cooling arrangement for the LEDs.

Uniformity filters can be used to improve illumination uniformity within a chip and/or from one chip to the next. Such filters can be regarded as pixelated neutral density filters. Operation under closed loop control can be used to correct for device to device variation among the LEDs. In one example, such control makes use of two calibration tables, one for high illuminance (i.e., over <NUM> lx) and low illuminance (i.e., <NUM> lx or less). Another feature of the feedback control, according to the present invention, is the use of two photodetectors, one photodetector having a neutral density filter on it, and the other photodetector not having a neutral density filter on it. Feedback control of LED intensity can make use of two feedback paths: <NUM>) LED to photodetector and <NUM>) LED to phosphor to photodetector. Feedback control can also be done with a single photodetector in the LED light source module, either with or without a neutral density filter on the photodetector. However, this option does not fall within the scope of the claims.

<FIG> show an exemplary embodiment of the invention. Here <FIG> is a top view and <FIG> is a corresponding side view of apparatus for testing an array of light-sensitive electronic devices. <FIG> shows an array of LED (light emitting diode) light sources <NUM>, four of which are referenced as <NUM>, <NUM>, <NUM>, and <NUM>. The array of sources need not be square as shown on <FIG>. Another possible variation is that gaps may be placed between the LED light sources so that the arrangement is not contiguous.

As shown in the side view of <FIG>, array <NUM> is configured to have an LED light source corresponding to each of the electronic devices under test. In this example, sources <NUM>, <NUM>, <NUM>, <NUM> correspond to devices under test <NUM>, <NUM>, <NUM>, <NUM>, respectively. More specifically, sources <NUM>, <NUM>, <NUM>, <NUM> provide output light <NUM>, <NUM>, <NUM>, <NUM> to devices under test <NUM>, <NUM>, <NUM>, <NUM>, respectively. Here devices under test <NUM>, <NUM>, <NUM>, <NUM> are schematically shown as being disposed in substrate <NUM>, which would be the case for wafer scale testing.

An important advantage of the present approach is that the small size of the LED light sources simplifies integration of optical illumination and electrical probing to provide a true optoelectronic probe card for testing optoelectronic imaging devices. In the example of <FIG>, this preferred feature is shown by electrical probes 132a,b, 134a,b, 136a,b, 138a,b which make electrical contact to electrical terminals of devices under test <NUM>, <NUM>, <NUM>, <NUM>, respectively. For ease of illustration, two electrical probes per device under test are shown, but in practice any number of electrical probes can be used per device under test.

<FIG> is a detailed view of an exemplary LED source suitable for use in embodiments of the invention. In this example of a preferred embodiment, light emitting diode <NUM> provides LED light (solid arrows), and a phosphor <NUM> is disposed to receive the LED light and to provide phosphorescence light (dashed arrows). An aperture <NUM> is configured to provide point source illumination from the phosphorescence light, as shown. A lens <NUM> is configured to provide collimated light from the point source illumination. Lens <NUM> is preferably an aspheric lens designed for this collimation function over the wavelength range provided by phosphor <NUM>. A uniformity filter <NUM> is disposed to receive the collimated light and to provide output light having improved uniformity of illumination.

The example of <FIG> also shows further features that are present in preferred embodiments and/or in complete designs. Here LED <NUM> is mounted on a heat sink member <NUM>, and the package includes a first photodiode <NUM>, a second photodiode <NUM>, and a neutral density filter <NUM> disposed on the second photodiode. As explained in more detail below, these photodiodes are used in preferred embodiments to provide signals for feedback control of the array of LED light sources. Heat sink <NUM> and photodiodes <NUM> and <NUM> are disposed on substrate <NUM>. Spacer members <NUM> are used to support the phosphor <NUM> and aperture <NUM> at the appropriate vertical height. A lens housing <NUM> positions lens <NUM> at the correct distance from aperture <NUM> to provide collimated (i.e., telecentric) light as shown. Optional components that can be disposed in the path of the collimated light include infra-red (IR) filter <NUM> and/or diffuser <NUM>. Here the IR filter blocks IR radiation from reaching devices under test to provide purely visible light to the device under test, and the diffuser helps improve the uniformity of light provided by the light source to the device under test.

As shown on <FIG> the first photodetector <NUM> and the second photodetector <NUM> will typically receive light within the LED light source from both LED <NUM> (solid arrows) and phosphor <NUM> (dashed arrows). The phosphor for each LED light source is preferably configured as a thin film of uniform thickness disposed on the light emitting diode of the LED light source, as shown on <FIG>. Preferably phosphor <NUM> is a mixture of phosphors chosen to provide substantially white illumination (e.g., by combining red, green and blue emitting phosphors). Alternatively, the phosphor or phosphor mixture can be selected to emphasize a particular desired spectral distribution.

An important aspect of preferred embodiments of the invention is feedback control of each LED light source in the emitting array, to provide uniformity of illumination to all devices under test. <FIG> shows a control approach suitable for use in embodiments of the invention. Here a feedback control system <NUM> receives measured optical signals (dotted lines) from each LED light source <NUM>, <NUM>, <NUM>, <NUM> and is configured to control operation (solid lines) of the array of LED light sources using the measured optical signals to provide uniform illumination to the array of light-sensitive electronic devices. A calibration of the system can be performed to relate measured optical signals to emitted light intensity for each LED light source. This data can then be used to create a lookup table in control system <NUM> such that each LED light source is driven so that its measured optical signals are in accordance with the lookup table for the desired array intensity. Interpolation in the lookup table can be performed for desired intensity values that are intermediate between calibration data points. Such calibration can be done initially and then repeated occasionally if or as needed to account for drift in device performance over time. For ease of illustration, only four LED light sources are shown in the example of <FIG>, but such feedback control can be practiced for arrays having any number of LED light sources.

In preferred embodiments like the example of <FIG>, two feedback signals are used from each LED light source, a first signal from a first photodetector that has no neutral density filter disposed on it, and a second signal from a second photodetector that has a neutral density filter disposed on it. The purpose of having two detectors for feedback configured this way is to improve dynamic range because a single detector can only provide useful feedback over a limited range of optical intensity. So in this example, second detector <NUM> will be able to provide useful feedback when the optical intensity is high enough to saturate first detector <NUM> because of neutral density filter <NUM>. Preferably neutral density filter <NUM> is configured to provide a continuous feedback range for the two detectors combined (e.g., signal from detector <NUM> is at <NUM>% of its saturation value for the lowest optical intensity that saturates detector <NUM>). Additional feedback detectors plus neutral density filters can be added to further increase dynamic range according to these principles if necessary. Alternatively, however, not falling within the scope of the appended claims, a single detector (with or without a neutral density filter on it) can be used to provide the optical signals for feedback.

Uniformity is also improved by making use of uniformity filters <NUM> in each LED light source. These uniformity filters can be neutral density filters individually customized for their corresponding LED light sources according to calibration measurements of output light uniformity of the array of LED light sources prior to installation of the uniformity filter. In other words, uniformity from one LED light source to another is provided using both active measures (feedback control) and passive measures (the uniformity filters) in combination.

<FIG> show first exemplary experimental results for array uniformity. Here <FIG> is a table of emission values from an LED array with all drive currents being the same, and <FIG> is from the same array after installation of suitable customized uniformity filters for each source in the array. In <FIG>, the emission values have a maximum of <NUM>, a minimum of <NUM>, an average of <NUM> and a standard deviation of <NUM>. In <FIG>, the emission values have a maximum of <NUM>, a minimum of <NUM>, an average of <NUM>, and a standard deviation of <NUM>. Clearly the array of <FIG> provide much more uniform illumination than the array of <FIG>, and closed loop control can be expected to further improve illumination uniformity from one LED light source to another.

<FIG> show second exemplary experimental results for array uniformity. <FIG> are results for one tester type with the apparatus, and <FIG> are results for another tester type with the same or similar apparatus. Here <FIG> is a table of emission values from an LED array with all drive currents being the same, and <FIG> is from the same array after installation of suitable customized uniformity filters for each source in the array. In <FIG>, the emission values have a maximum of <NUM>, a minimum of <NUM>, an average of <NUM> and a standard deviation of <NUM>. In <FIG>, the emission values have a maximum of <NUM>, a minimum of <NUM>, an average of <NUM>, and a standard deviation of <NUM>. Clearly the array of <FIG> provide much more uniform illumination than the array of <FIG>, and closed loop control can be expected to further improve illumination uniformity from one LED light source to another.

LED source arrays as in the examples of <FIG> and <FIG> have been compared to conventional halogen light sources for testing applications, and the LED source arrays provide comparably uniform illumination to the conventional halogen light sources.

The preceding examples show how uniformity filters can be used to improve uniformity from one device under test to another. It is also possible to use the uniformity filters to improve uniformity of illumination from each LED light source, alternatively to or in addition to improving uniformity between LED light sources.

Claim 1:
Apparatus for testing an array of light-sensitive electronic devices, the apparatus comprising:
an array of LED, light emitting diode, light sources, wherein the array of LED light sources (<NUM>) is configured to have a respective LED light source (<NUM>, <NUM>, <NUM>, <NUM>) corresponding to each of the electronic devices (<NUM>, <NUM>, <NUM>, <NUM>);
wherein each LED light source (<NUM>, <NUM>, <NUM>, <NUM>) includes a light emitting diode configured to provide LED light;
wherein each LED light source (<NUM>, <NUM>, <NUM>, <NUM>) includes a phosphor (<NUM>) disposed to receive the LED light and to provide phosphorescence light;
wherein each LED light source (<NUM>, <NUM>, <NUM>, <NUM>) includes an aperture (<NUM>) configured to provide point source illumination from the phosphorescence light;
wherein each LED light source (<NUM>, <NUM>, <NUM>, <NUM>) includes a lens configured to provide collimated light from the point source illumination;
wherein each LED light source (<NUM>, <NUM>, <NUM>, <NUM>) includes a uniformity filter disposed to receive the collimated light and to provide output light having improved uniformity of illumination;
a feedback control system configured to measure optical signals from each LED light source (<NUM>, <NUM>, <NUM>, <NUM>) and configured to control operation of the array of LED light sources (<NUM>, <NUM>, <NUM>, <NUM>) using the measured optical signals to provide uniform illumination to the array of light-sensitive electronic devices(<NUM>, <NUM>, <NUM>, <NUM>) ;
wherein each LED light source further comprises a first photodetector and a second photodetector with a neutral density filter disposed on the second photodetector;
wherein the optical signals are provided by both the first photodetector and the second photodetector.