Environmental Sensor Test Methodology

Abstract

We disclose herein a method for testing a batch of environmental sensors to determine the fitness for purpose of the batch of environmental sensors, the method comprising: performing a plurality of electrical test sequences to the sensor inputs of the batch of environmental sensors to measure electrical responses of the sensor outputs of the batch of environmental sensors; correlating the measured electrical responses from the batch of environmental sensors to predetermined environmental parametric ranges of at least one environmental sensor so as to define correlated electrical test limits; and determining the fitness for purpose of the batch of environmental sensors if the measured electrical responses are within the correlated electrical test limits.

Claims

1. A method for testing a batch of environmental sensors to determine the fitness for purpose of the batch of environmental sensors, the method comprising: performing a plurality of electrical test sequences to the sensor inputs of the batch of environmental sensors to measure electrical responses of the sensor outputs of the batch of environmental sensors; correlating the measured electrical responses from the batch of environmental sensors to predetermined environmental parametric ranges of at least one environmental sensor so as to define correlated electrical test limits; and determining the fitness for purpose of the batch of environmental sensors if the measured electrical responses are within the correlated electrical test limits.

2. A method according to claim 1, wherein the predetermined environmental parametric ranges are determined by performing of a plurality of environmental tests of the at least one sensor under an environmental condition.

3. A method according to claim 1, wherein the testing of the environmental sensors to determine the fitness for purpose is performed by exclusively applying electrical impulses to the each sensor of the batch of environmental sensors and by exclusively measuring electrical responses of the batch of environmental sensors.

4. A method according to claim 3, wherein the batch of environmental sensors are not directly tested under an environmental condition.

5. A method according to claim 1, wherein the step of performing the plurality of electrical test sequences is performed by an automated test equipment and the electrical responses are measured by the automated test equipment.

6. A method according to claim 1, wherein the electrical responses provide calibration values that are stored within the environmental sensors.

7. A method according to claim 1, wherein the environmental sensors comprise gas sensors.

8. A method according to claim 7, wherein each gas sensor comprises: a dielectric membrane formed on a semiconductor substrate comprising an etched portion; a heater formed in the dielectric membrane; gas sensing electrodes formed on the dielectric membrane; and a gas sensitive layer formed on the gas sensing electrodes

9. A method according to claim 8, wherein said electrical impulses are applied to the heater of each gas sensor and the electrical response is measured across the gas sensing electrodes of each gas sensor.

10. A method according to claim 7, wherein the predetermined parametric ranges are determined by running a test in the presence of a gas.

11. A method according to claim 10, wherein the predetermined parametric ranges are determined from the sensor resistance variation in air and the sensor resistance variation in the gas to define said correlated electrical test limits.

12. A method according to claim 11, wherein the measured electrical responses from the batch of gas sensors are compared with said correlated electrical test limits.

13. A method according to claim 12, wherein the fitness for purpose of the gas sensors is determined when the measured electrical responses from the batch of gas sensors are within said correlated electrical test limits.

14. A method according to claim 7, wherein the gas sensors are metal oxide gas sensors.

15. A method according to claim 1, wherein the environmental sensors comprise humidity sensors.

16. A method according to claim 1, wherein the environmental sensors comprise pressure sensors.

17. A method according to claim 1 wherein the batch of sensors are tested in wafer form or any other form prior to packaging.

18. A method according to claim 1 wherein the batch of sensors are tested in wafer level package format.

19. A method according to claim 1 wherein the batch of sensors are tested in a package strip format.

20. A method according to claim 19 wherein the package strip is supported face down on a dicing tape which is further supported by a film frame.

21. A method according to claim 19, wherein the package strip comprises a plurality of environmental sensors which are electrically isolated from one another.

22. A method according to claim 19, wherein the plurality of environmental sensors are electrically isolated by using a conductor etching process such as an etch back process.

23. A method according to claim 19, wherein the plurality of environmental sensors are electrically isolated by using of a sawing process.

24. A method according to claim 23, wherein the sawing process only cuts through the metal conductors between environmental sensors and keeps the integrity of the strip intact.

25. A method according to claim 23, wherein the sawing process cuts through the full package structure including the metal conductors between sensors to leave an array of separate sensors.

26. An environmental sensor test system to determine the fitness for purpose of a batch of environmental sensors, the test system comprising: said batch of environmental sensors; an automated test equipment to perform a plurality of electrical test sequences to the sensor inputs of the batch of environmental sensors and to measure electrical responses of the sensor outputs of the batch of environmental sensors; a data analysis tool to correlate the measured electrical responses from the batch of environmental sensors to predetermined environmental parametric ranges of at least one environmental sensor so as to define correlated electrical test limits; and wherein the data analysis tool is configured to determine the fitness for purpose of the batch of environmental sensors if the measured electrical responses are within the correlated electrical test limits.

27. A test system according to claim 26, further comprising an environmental test equipment which is configured to determine the predetermined environmental parametric ranges by performing of a plurality of tests of the at least one sensor under an environmental condition.

28. A test system according to claim 26, wherein the environmental sensors each store calibration values of the electrical responses.

29. A test system according to claim 26, wherein the environmental sensors comprise gas sensors.

30. A test system according to claim 29, wherein each gas sensor comprises: a dielectric membrane formed on a semiconductor substrate comprising an etched portion; a heater formed in the dielectric membrane; gas sensing electrodes formed on the dielectric membrane; and a gas sensitive layer formed on the gas sensing electrodes

31. A test system according to claim 30, wherein said automated test equipment is configured to apply electrical impulses to the heater of each gas sensor and to measure the electrical response across the gas sensing electrodes of each gas sensor.

32. A test system according to claim 30, further comprising a gas testing equipment which is configured to determine predetermined parametric ranges by running a test in the presence of a gas.

33. A test system according to claim 32, wherein the gas testing equipment is configured to determine the predetermined parametric ranges from the sensor resistance variation in air and the sensor resistance variation in the gas to define said correlated electrical test limits.

34. A test system according to claim 29, wherein the gas sensors are metal oxide gas sensors.

35. A test system according to claim 26, wherein the environmental sensors comprise humidity sensors.

36. A test system according to claim 26, wherein the environmental sensors comprise pressure sensors.

37. A test system according to claim 26 wherein the batch of sensors are in wafer form or any other form prior to packaging.

38. A test system according to claim 26 wherein the batch of sensors are packaged in wafer level package format

39. A test system according to claim 26, wherein the batch of sensors are packaged in a package strip format.

40. A test system according to claim 39, further comprising a film frame and a dicing tape which is supported by the film frame, wherein the package strip is supported face down on the dicing tape.

41. A test system according to claim 40, wherein the package strip comprises a plurality of environmental sensors which are electrically isolated from one another.

42. A test system according to claim 41, wherein the plurality of environmental sensors are electrically isolated by using a conductor etching technique such as an etching back technique.

43. A test system according to claim 41, wherein the plurality of environmental sensors are electrically isolated by using of a sawing technique.

44. A test system according to claim 43, wherein the environmental sensors are configured such that the sawing technique only cuts through the metal conductors between the environmental sensors and keeps the integrity of the strip intact.

45. A test system according to claim 43, wherein the environmental sensors are configured such that the sawing technique cuts through the full package structure including the metal conductors between sensors to leave an array of separate sensors.

46. A test system according to claim 41, further comprising a film frame prober to test the sensors.

Description

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0051] Some preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

[0052] FIG. 1 illustrates an electrical test system for an environmental sensor such as a metal oxide (MOX) gas sensor;

[0053] FIG. 2 illustrates correlation test results according to one embodiment of the present invention;

[0054] FIG. 3 illustrates a top view of a wafer film frame in which package strips of sensors are provided; and

[0055] FIG. 4(a) to (c) illustrate exemplary electrical isolation techniques according to the embodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0056] Embodiments of the invention provide a method for screening gas sensors for their suitability to perform in the end application (or fitness for purpose) by means of applying electrical impulses and measuring the sensors electrical responses. Embodiments of this invention makes use of standard semiconductor production automatic test equipment (ATE) to perform electrical test sequences, the results of which are correlated to gas test performance so can be used to screen good units from reject units.

[0057] FIG. 1 illustrates an electrical test system for an environmental sensor such as a metal oxide (MOX) gas sensor. The sensor includes a heater across which electrical impulses are applied to generate the heat. The heater can be an IR emitter which emits IR radiation to the sensor electrode on which MOX is deposited. Electrical responses (or resistances) are then measured across sensor outputs (or across sensor electrodes). Electrical responses from the sensor output are then correlated with the separate gas testing results.

[0058] The following are examples of electrical test sequences that could be used to screen gas sensors. These can be used as standalone tests, in combination with each other or with alternative test solutions not listed here since an exhaustive list is not practical. Furthermore, the results from these tests can be used to store calibration values within the sensor product under test, e.g. in one time programmable (OTP) or flash within a digital control device (not shown).

EXAMPLE 1

[0059] Apply low voltage (eg 500 mV) to heater and measure heater current and sensor resistance (R1)

[0060] Apply higher voltage (eg 1.2V) to heater and measure heater current and sensor resistance (R2)

[0061] Calculate the ratio or difference of R1 and R2 and apply test limits to all results

EXAMPLE 2

[0062] Apply voltage (eg 1V) to heater and measure heater current and sensor resistance after a delay (eg 100 ms) (R1)

[0063] Continue to apply same voltage and measure heater current and sensor resistance after a further delay (eg 1 s) (R2)

[0064] Calculate the ratio or difference of R1 and R2 and apply test limits to all results

EXAMPLE 3

[0065] Apply a sinusoidal voltage (eg from 0.8V to 1.2V at a frequency of 200 Hz) to heater for a fixed duration (eg 5 s) and measure the heater current and the resistance response of the sensor

[0066] Apply phase shift, amplitude and jitter test limits to the resulting resistance profile

EXAMPLE 4

[0067] Apply multiple voltages to the heater and measure the heater current and the sensor resistance in each case (eg 0V, 0.5V, 0V, 1.0V, 0V, 1.4V, 0V, 1.8V, 0V, 1.4V, 0V, 1.0V, 0V, 0.5V, 0V)

[0068] Calculate resistance ratios or differences and apply test limits to all results

EXAMPLE 5

[0069] Apply low voltage (eg 500 mV) to heater, sweep voltage on sensor (eg 1V to 5V) and measure heater current and sensor current at each voltage step

[0070] Apply high voltage (eg 1.4V) to heater, sweep voltage on sensor (eg 1V to 5V) and measure heater current and sensor current at each voltage step

[0071] Calculate sensor resistances and apply test limits to all values as well as ratios and differences.

[0072] As mentioned above, one or more of the above electrical tests can be applied to a plurality of batches of sensors to determine the fitness for purpose of the sensors. The fitness for purpose is determined for a large batch of production line sensors by using some or all of the electrical test sequences described above and, potentially, other electrical tests. Therefore the list of electrical tests above is not exhaustive as other electrical tests are also possible.

[0073] FIG. 2 illustrates correlation test results according to the embodiments of the present invention. The correlation test results of FIG. 2 are directed to a batch of environmental sensors such as gas sensors. Each sensor from the batch has gone through both the electrical test and environmental (gas) test separately. The ‘X’ axis represents the electrical test response and the ‘Y’ axis represents the gas test response for the same sensors of the batch of sensors. Each filled grey circle of FIG. 2 represents the correlated electrical and gas test result of each sensor of the batch of sensors. In this example, the environmental response lower specification limit (or predetermined environmental parametric range) is set to at least 2 (dotted line). Then, the electrical response lower specification limit (or correlated electrical test limit) is set, in this example, to at least 40 (dashed line) to ensure all sensors have an environmental response ≧2. It can be seen from this example data that all sensors which have an electrical response greater or equal to 40 also have an environmental response greater or equal to 2. Therefore, a lower electrical test limit of 40, in this case, guarantees the sensor will also have an environmental response of greater or equal to 2. Therefore, it is no longer necessary to perform an environmental test on further sensor batches since the electrical test is sufficient to guarantee environmental performance by applying the lower specification limit of 40 to the electrical test result. This data is given by way of example only as these limits apply to this set of data for these particular tests and this particular sensor type. Other environmental tests and/or electrical test will most likely result in different limits for those tests.

[0074] In the correlation test results of FIG. 2, the top right quadrant includes sensors which fulfil the fitness for purpose requirement. For any further batches of sensors which will be mass produced for shipment will only go through the electrical test sequences without any gas or environmental test sequences. From these further batches of sensors, those sensors achieve electrical test response above the electrical response lower specification limit, e.g. ≧40, would be marked as passing the fitness for purpose test. There is no need to do the gas test result for each sensor anymore, because it is already understood from the correlation test results of FIG. 2 that the environmental response lower specification limit is already ≧2 for those sensors which have achieved electrical response lower specification limit of ≧40. Advantageously this enables to reduce the requirement of performing an environmental test sequence on each gas sensor in a very large batch of mass producible sensors.

[0075] Furthermore, embodiments of the invention can include the use of wafer film frame handling formats to further reduce cost and increase throughput. FIG. 3 illustrates a top view of a wafer film frame in which package strips of sensors are provided. The package strips are face down onto a dicing adhesive tape attached to a standard dicing film frame. If the packages are electrically isolated, e.g. by use of an etch back substrate, then they can proceed straight to test. In FIG. 3, each strip is made up of a plurality of gas sensors which are at least electrically isolated from one another.

[0076] FIG. 4(a) to (c) illustrate exemplary electrical isolation techniques according to embodiments of the present invention. In FIG. 4(a), the individual packages are electrically isolated by etching of the conductors. In order to provide isolation between packages, in FIG. 4(b), the strips are half cut, i.e. sawn through the conductors of the substrate or lead-frame only to isolate devices. In FIG. 4(c), the strips are full cut, i.e. sawn all the way through the substrate or lead-frame to fully singulate or isolate the devices. Once devices are electrically isolated they can be tested using a film frame prober such for example a Accretech FP3000, and a standard ATE such as a Teradyne J750.

[0077] In two embodiments, the plurality of gas sensors may be electrically isolated by using of a sawing process. The sawing process will only cut through the metal conductors between gas sensors whilst leaving the integrity of the strip intact. The sawing process may cut through the full package structure including the metal conductors between gas sensors to leave an array of singulated gas sensors.

[0078] Although the invention has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.