Abstract:
A method and device are disclosed for calibrating sensors, which sensors are arranged on semiconductor chips and are e.g. to be used for detecting a substance in a fluid. The sensors are calibrated while they are still assembled on a semiconductor wafer by exposing the wafer to a calibration fluid containing a known amount of the substance to be measured. Hence, rather than first cutting the wafer, the sensors are calibrated at an early stage. For this purpose, they are placed on a chuck below a lid. The calibration fluid with known parameters is introduced between the wafer and the lid. This allows to test and calibrate a large number of sensors quickly.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the priority of European patent application 04019445.8,. filed Aug. 17, 2004, the disclosure of which is incorporated herein by reference in its entirety.  
       BACKGROUND OF THE INVENTION  
       [0002]     The invention relates to a method and device for calibrating sensors, which sensors are e.g. to be used for detecting a substance in a fluid and are integrated on semiconductor chips. In a particularly advantageous embodiment, the invention relates to the calibration of sensors measuring humidity in gases.  
         [0003]     One known type of humidity sensors uses a layer of a humidity sensitive material arranged on a semiconductor chip, as it is described in WO 01/42776. Other types of sensors e.g. use metal oxide technologies and be adapted to measure various types of substances in gases or liquids. Typical substances that can be measured are e.g. CO, CO 2 , NO x , volatile organic compounds (VOC), in particular any type of gaseous organic compounds, and any other types of compound.  
         [0004]     Semiconductor chips are usually manufactured in wafers, where each wafer may comprise hundreds or more chips. After manufacturing, the wafers are cut to separate the chips, the chips are placed in a suitable housing and are then calibrated by exposure to fluids of known composition, as it is e.g. described in WO 01/40784.  
         [0005]     However, manufacturing a large number of sensors in this manner is cumbersome and expensive.  
       BRIEF SUMMARY OF THE INVENTION  
       [0006]     Hence, it is a general object of the invention to provide a method and device for simplifying the above process.  
         [0007]     Now, in order to implement these and still further objects of the invention, which will become more readily apparent as the description proceeds, the method of a first aspect of the invention comprises the steps of  
         [0008]     exposing a semiconductor wafer comprising a plurality of said sensors to a fluid with an amount of said substance and  
         [0009]     performing calibration measurements on the sensors on the wafer while said wafer is exposed to said fluid.  
         [0010]     Accordingly, calibration measurements on the sensors are carried out while the sensors are still assembled in the wafer by exposing the wafer to a fluid with a known amount of the substance to be measured. Rather than first cutting the wafer, housing the sensors and then calibrating them, the sensors are calibrated at an early stage. This allows to calibrate a large number of sensors quickly and allows to eliminate those sensors that cannot be calibrated from the further manufacturing steps. Furthermore, it requires only a small volume of calibration fluid for calibrating a large number of sensors.  
         [0011]     One embodiment of a suited apparatus comprises  
         [0012]     a support for receiving a semiconductor wafer with a plurality of said sensors integrated thereon,  
         [0013]     a lid arranged at a distance from said sensor for forming a gap between a surface of the wafer mounted on said support and a surface of said lid,  
         [0014]     a fluid feed for introducing fluid with an amount of said substance into said gap, and  
         [0015]     a probe for contacting said sensors while said fluid is in said gap.  
         [0016]     This type of arrangement allows to calibrate the sensors on the wafer.  
         [0017]     In a further aspect, the apparatus for calibrating sensors comprises  
         [0018]     a support for receiving a semiconductor wafer with a plurality of said sensors integrated thereon,  
         [0019]     a lid arranged at a distance from said sensor for forming a gap between a surface of the wafer mounted on said support and a surface of said lid,  
         [0020]     a probe for contacting said sensors while said wafer is in said gap and  
         [0021]     a cooler and/or heater for maintaining said lid and said support at given temperatures.  
         [0022]     Hence, both the chuck and the waver are temperature controlled for generating a substantially homogeneous temperature distribution around the wafer. This type of arrangement is suited for the calibration of substance sensors as well as of temperature sensors.  
         [0023]     The method and apparatus are advantageously used for humidity sensors. In that case, the apparatus is preferably equipped with a humidity generator for preparing a gas with a known concentration of water. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings, wherein:  
         [0025]      FIG. 1  is a schematic view of an apparatus for the on-wafer calibration of sensors,  
         [0026]      FIG. 2  is a partially sectional view of a first embodiment of part of such an apparatus, and  
         [0027]      FIG. 3  is a partially sectional view of a second embodiment of part of such an apparatus. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]     Attached  FIG. 1  shows the basic set-up of an apparatus for calibrating humidity sensors. In the present embodiment the sensors are humidity sensors that detect the amount of water in air or in another gas.  
         [0029]     The apparatus comprises a control unit  1 . Control unit  1  controls the operation of x-, y- and z-actuators of a testing device  2  adapted to contact each individual sensor on a wafer by means of a probe head  3 . Control unit  1  further contains the circuitry and software for operating the sensors contacted by probe head  3  and for calibrating the same, e.g. by storing calibration data on a memory device integrated with each sensor. Control unit  1  also controls the operation of a humidity generator  4 , which is basically a device that adds and/or removes humidity to/from a volume of gas until the same has a given level of humidity.  
         [0030]      FIG. 2  shows a more detailed view of testing device  2 . Testing device  2  comprises a stationary frame or stand  10  carrying an x-y-positioning device  12 .  
         [0031]     X-y-positioning device  12  carries a housing  13  and is able to accurately position the same along the horizontal directions x and y. Direction x is illustrated by an arrow in the figure while direction y is perpendicular to the image plane.  
         [0032]     Housing  13  has an e.g. circular bottom wall  14  and a cylindrical side wall  15 . Arranged in housing  13  and substantially rigidly connected thereto is a substantially cylindrical chuck  16  acting as a support for a wafer  17 . Wafer  17  contains a two-dimensional matrix of sensors that are basically ready for operation but that still need to be calibrated, cut and, where applicable, packaged. Wafer  17  rests on a flat top surface  18  of chuck  16 . Chuck  16  can optionally be equipped with suction ducts (not shown) ending in top surface  18  and being used to hold wafer  17  stationary on chuck  16  as known by the person skilled in the art.  
         [0033]     Housing  13  has an opening at its top end, which is covered by a lid  11  at a distance of e.g. 5 mm or less from wafer  17 . The top edge of housing  13  is abutting against the bottom side of lid  11  but not mechanically connected thereto, such that housing  13  can follow the motions of x-y-positioning device  12 . Optionally, lid  11  and housing  13  may be mutually displaceable along the z-direction for slightly separating the two parts while x-y-positioning device  12  moves the housing.  
         [0034]     Probe head  3  is arranged in a recess at the bottom side of lid  11  and rigidly connected to rods  19  extending through holes  20  of lid  11 . Rods  19  are embedded in a positioning ring  21 , which in turn is rigidly connected to a z-positioning device  22 , the latter being arranged stationary on frame or stand  10 . The rods  19  extending through the holes  20  hold lid  11  in x- and y-direction.  
         [0035]     Probe head  3  comprises a carrier plate  24 , with probe electrodes  25  mounted at the bottom side thereof. The probe electrodes  25  are arranged such that their tips can contact the contact pads of the sensor chips on wafer  17  as it is known to a person skilled in the art.  
         [0036]     A central hole  26  extends through positioning ring  21 , lid  11  and carrier plate  24 , making it possible to view the contacting of an individual sensor chip by means of probe head  3  through a microscope.  
         [0037]     A support cooler and/or heater  27  (see  FIG. 1 ) is provided for heating and/or cooling chuck  16  to a given temperature, thereby substantially controlling the temperature of the wafer arranged on top of chuck  16 . Support cooler and/or heater  27  may e.g. consist of a thermostat keeping a water reservoir at a given temperature and pumping water from the reservoir through ducts (not shown) in chuck  16  and/or it can comprise an electrical heater in chuck  16 .  
         [0038]     Similarly, a lid cooler and/or heater  28  is provided for heating and/or cooling lid  11 . Preferably, it is set to the same temperature as support cooler and/or heater  27  and may use the same water reservoir.  
         [0039]     For calibrating sensors with the device of  FIGS. 1 and 2 , a wafer comprising a plurality of sensors to be calibrated is placed on chuck  16 , either manually or automatically. Humidity generator  4  is activated to generate a gas with a known humidity level. The gas may e.g. be air or nitrogen. For this purpose, humidity generator  4  can heat a dry gas to a first given temperature and add water to it until its relative humidity reaches a certain level. A pump (not shown) in humidity generator  4  then feeds the humid gas through a tube  30 , from which it enters the bottom of housing  13  through an opening  31 . As indicated by the arrows in  FIG. 2 , the gas passes through a gap between the bottom of chuck  16  and the bottom wall  14  of housing  13 , then through a cylindrical gap between the vertical surfaces of chuck  16  and vertical wall  15  housing  13 , to finally enter radially into the gap  33  formed between the bottom side of lid  11  and the top side of wafer  17 . During its passage through these gaps, the gas is in close contact with chuck  16  and lid  11 , both of which are heated to a second given temperature by means of their respective coolers and/or heaters  27 ,  28 . For this reason, when arriving at the location of probe head  3 , the gas has substantially the second given temperature and therefore a defined relative humidity.  
         [0040]     It must be noted that the relative humidity of the gas at probe head  3  is constant even if the gas temperature is allowed to deviate from its initial value during the passage of the gas through tube  30  as long as the absolute humidity of the gas is not changed. The absolute humidity of the gas is not changed as long as its temperature does not fall below its dew point and as long as the components it passes do not absorb water in significant amounts.  
         [0041]     Hence, because the gas is cooled or heated, in test device  2 , to the known second temperature prior to or during entry into gap  33 , it is not necessary to maintain its temperature accurately while it is being fed from humidity generator  4  to test device  2 .  
         [0042]     Preferably, the first given temperature used in humidity generator  4  is equal to the second given temperature in test device  2 , thereby establishing the same relative humidity in gap  33  as in humidity generator  4 .  
         [0043]     The gas in gap  33  will finally leave the same through central opening  26  and the holes  20  as well as any further openings in lid  11  or housing  13 . However, new gas is continuously fed from humidity generator  4  and the pressure in gap  33  is always kept slightly above ambient pressure, which prevents ambient air from entering gap  33  and affecting the humidity level of the calibration gas.  
         [0044]     While wafer  17  is exposed to the calibration gas, the sensors on it can be calibrated by displacing housing  13  and chuck  16  and by contacting each one of them by means of the electrodes  25  of probe head  3 . Calibration can consist of a calibration measurement and subsequent storage of calibration data in the sensor. Preferably, the calibration data is stored in the sensor immediately after calibrating it.  
         [0045]     During calibration, the general functionality of each sensor can be tested as well, and non-functional sensors can be discarded after cutting wafer  17  as known to a person skilled in the art.  
         [0046]     Depending on the nature of the humidity sensors and the desired accuracy, one or more calibration steps at different relative or absolute humidities and/or temperatures have to be carried out. A multi-step calibration can be run quickly by first carrying out the first calibration step at a first temperature and humidity for all sensors, then change the temperature and/or humidity, then run a the second calibration step for all sensors at a second temperature and humidity, etc. If only a single calibration step is required, the wafer is exposed to a given humidity and temperature and then the calibration measurements are carried out for the sensors on the wafer.  
         [0047]     Since the volume of calibration gas in tube  30  and the gaps around chuck  16  is small, the humidity and temperature can be changed quickly, which allows to carry out a large number of calibrations in a given amount of time.  
         [0048]     Once the calibration of the sensors on wafer  17  is complete, wafer  17  can be removed from test device  2 , either manually or automatically. It then can be cut for separating the individual sensors.  
         [0049]      FIG. 3  shows a second embodiment of a test device, which is particularly suited for a fully automatic calibration. In this embodiment, x-y-positioning device  12  comprises a robot arm  40  carrying chuck  16 . Robot arm  40  can be used for displacing chuck  16  in respect to probe head  3  and also for bringing chuck  16  to a transfer position remote from probe head  3  for unloading and loading a wafer  17 .  
         [0050]     In contrast to the first embodiment, no housing is provided close to chuck  16 . Therefore, the gas from tube  30  is introduced into a circular feed duct  41  in lid  11 , from where it enters gap  33  through small openings  42  located radially between central opening  26  and a peripheral edge  43  of a centered wafer  17 . From openings  42 , a first part of the calibration gas flows radially outwards to exit gap  33  at its periphery, while a second part flows radially inwards to exit gap  33  through central opening  26 . In order to fill the whole of gap  33  continuously and reliably with calibration gas, the amount of gas exiting through the periphery of gap  33  and the amount of gas exiting through central opening  26  should advantageously be of the same order of magnitude. To ensure this if central opening  26  has a large diameter, a plug  44  with one or more smaller openings  45  may be provided for blocking central opening  26  partially. The diameter of the openings  45  defines the ratio between the amount of gas exiting through plug  44  as compared to the amount of gas exiting radially from gap  33 . Plug  44  can be removed for viewing probe head  3  through opening  26 .  
         [0051]     A further function of plug  44  is to prevent light from entering through central opening  26  during calibration because such light can lead to erroneous signals from the sensor chips.  
         [0052]     Plug  44  can also be used in the embodiment of  FIG. 2 .  
         [0053]     For contacting the individual sensors with the electrodes  25  of probe head  3 , lid  11 , probe head  3  or robot arm  40  of  FIG. 3  can again be mounted to a suitable z-positioning device.  
         [0054]     A calibration with the device of  FIG. 3  comprises substantially the same steps as the calibration with the device of  FIG. 2 . Again, gas of a given absolute humidity is fed through tube  30  and enters feed duct  41 , where it is brought to a known temperature. It enters gap  33  where it creates a defined environment for testing the sensors on wafer  17 . Once the wafer is exposed to the calibration gas, each sensor is contacted by probe head  3 . This operation can be repeated for several temperatures and/or humidities.  
         [0055]     The diameter of the openings  42  of the device of  FIG. 3  should be chosen such that the pressure drop of the incoming gas over the openings  42  is much larger than the pressure drop that the gas experiences while flowing through gap  33 . This ensures that, if chuck  16  is positioned to measure a peripheral chip on waver  17  and therefore part of the openings  42  are not directly above chuck  16 , the amount of gas streaming through this part of the openings is not substantially larger than the amount of gas steaming through those openings  42  that are still above chuck  16 .  
         [0056]     In the example of  FIG. 2 , the calibration gas is heated and/or cooled to a given temperature primarily by chuck  16  and partially by lid  11 , which therefore form a feed cooler and/or heater for adjusting a temperature of the gas prior to and during entry into gap  33 . In the embodiment of  FIG. 3 , the role of the feed cooler and/or heater is primarily assumed by lid  11 . However, depending on how the gas is introduced into gap  33 , a feed cooler and/or heater separate from chuck  16  and lid  11  could be used as well.  
         [0057]     In the previous embodiments, a humidity generator  4  has been used for preparing a gas having known, well-defined humidity. Alternatively, if the humidity of the gas is not well known in advance, it is possible to place a reference humidity sensor adjacent to the sensors to be calibrated. In such an embodiment, the reference humidity sensor can measure the humidity of the gas during the calibration process. The reference humidity sensor can e.g. be arranged on plug  44  or, as indicated under reference numeral  47  in  FIG. 3 , on probe head  3 . If a reference humidity sensor is used, it is not necessary (even though it may be advantageous) to use a humidity generator  4  for generating the gas.  
         [0058]     In a very simple embodiment, there is even no need to have a gap  33 . Rather, the gas can be blown onto the wafer at the location of probe head  3 . In that case, using a reference humidity sensor is recommended because it is more difficult to accurately control the humidity level of the gas.  
         [0059]     In a further embodiment, the whole apparatus, of  FIG. 2  or  3 , including at least part of x-y-positioning device  12  and chuck  16 , can be placed into a chamber containing a gas with a known humidity, e.g. in a climate controlled cabinet. In that case, again, having gap  33  is not required, nor feed duct  41 . A possible location of such a chamber is indicated under reference numeral  48  in  FIG. 3 .  
         [0060]     In addition or alternatively to a reference sensor  47 , one or more other monitoring sensors can be located adjacent to probe head  3  for monitoring the situation at the location of calibration.  
         [0061]     In particular, a pressure sensor  49  and/or a temperature sensor  50  can be provided, e.g. in plug  44 , as indicated in  FIG. 3 .  
         [0062]     Pressure sensor  49  is advantageously a differential pressure sensor for measuring the pressure difference between gap  33  and the environment—during calibration, the pressure in gap  33  should exceed the environmental pressure by a given amount in order to ensure that no environmental air can enter gap  33 . Pressure sensor  49  allows to monitor this condition and to issue a warning if it is not maintained.  
         [0063]     Temperature sensor  50  measures the temperature in gap  33  adjacent to the chips being calibrated. It allows a more accurate calibration and a monitoring of the condition of the gas.  
         [0064]     In the above examples, the invention has been explained in the context of an advantageous application, namely the calibration of humidity sensors. As explained above, however, the invention can also be used for calibrating other types of sensors detecting a substance in a fluid. In particular, it can be used for sensors detecting substances in gases or the composition of a gas mixture, in which case humidity generator  4  is replaced by a suitable device for preparing a mixture of gases with a defined ratio. Typical substances are CO, CO 2 , NO x , volatile organic compounds (VOC), any type of gaseous organic compounds, and any other types of compound.  
         [0065]     The invention could even be used for sensors adapted to measure a substance in a liquid, as long as the liquid allows the operation of probe head  3 . In that case, testing device  2  is preferably arranged in a bath of the liquid.  
         [0066]     Advantageously, when being used for calibrating sensors detecting a substance in a fluid, the apparatus of the invention should be provided with a suitable fluid feed for feeding the calibration fluid to gap  33 . The fluid is advantageously fed continuously into gap  33 , thereby maintaining a somewhat increased pressure therein and preventing ambient fluid from entering.  
         [0067]     The types of apparatus described here can also be used for calibrating temperature sensors on the wafer. In particular, having a temperature controlled lid  11  and a temperature controlled support or chuck  16  allows to generate a highly homogeneous temperature distribution in gap  33 , in particular if lid  11  and chuck  16  are kept at the same temperature. To calibrate temperature sensors on a wafer, the wafer is placed on chuck  16  and exposed to the temperature in gap  33 . Calibration measurements can be carried out by means of probe  3 .  
         [0068]     If the apparatus is used for temperature sensor calibration, it is not necessary to provide a fluid feed as it is used for the calibration of substance sensors.  
         [0069]     While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practised within the scope of the following claims.