Abstract:
1. Method for testing the tightness of capacitive sensors 
     2.1. Known methods of seal testing cannot be used because of the extremely small volume of the sensor cavity or on wafer level. Other known methods of seal testing are only possible with high expenditure on safety because, for example, radio isotopes are used. 
     2.2. Method for testing the tightness of capacitive sensors arranged in a hermetically sealed enclosure, in which the processed sensors are arranged in the form of a wafer, wherein the already sawn wafer bearing the sensors is immersed in a test fluid under defined conditions, the capacitance of each sensor is then measured and compared with the capacitance of reference sensors. 
     2.3. The invention is especially suitable for testing the tightness of capacitive, hermetically sealed sensors.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method for seal testing capacitive sensors, arranged in a hermetically sealed enclosure, in which the processed sensors are arranged in the form of a wafer. 
     Acceleration sensors manufactured by surface micromechanical engineering frequently work according to the capacitive measuring principle. Such an acceleration sensor is shown in FIG. 4 a.  In the capacitive measuring principle, a moveable test mass, with electrode combs (so-called moving fingers) attached to the sides, is suspended on tiny silicon springs in such a manner that it is deflected by accelerations in the sensitive direction. In so doing, the capacitance between the fixed and the moveable electrodes is increased on one side of the test mass and decreased on the other side. The sensor fingers, which are electrically isolated from one another, thus form a differential capacitor with a capacitance of around 600 fF. 
     It is absolutely essential to protect the processed wafers from moisture, particles and other mechanical influences by means of a cover in the form of a hermetically sealing lid made of silicon or a similar material. 
     Known leak and seal testing methods for testing the hermeticity according to MIL or EN standards (for example the “bubble test” or “coarse leak” according to MIL method 1014) cannot be used because of the extremely small volume of the sensor cavity. Other known methods of seal testing, either cannot be used at wafer level or can only be used with high expenditure on safety because of the use of radio isotopes, such as krypton 85, and therefore cannot be used for series production because the costs are too high. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide a testing method, with which capacitive sensors can be tested for tightness cost-effectively, reliably and in an environmentally compatible manner. 
     According to the invention, there is a method for testing the tightness of capacitive sensors arranged in a hermetically sealed enclosure, in which the processed sensors are arranged in the form of a wafer, in which the already sawn wafer bearing the sensors is immersed in a test fluid under defined conditions, the capacitance of each sensor is then measured and compared with the capacitance of reference sensors. 
     The method according to the invention possesses the advantages that the test fluid, with defined dielectrical properties, penetrating into the cavity enables the finest leaks, up into the so-called coarse leak range (10 −3  to 10 −5  mbar·dm 3 ·s −1 ), to be detected by clearly measurable capacitance changes. A further advantage lies in the fact that the test method can be applied as an “inline test method”, that is the seal test can be integrated into the normal wafer test in the form of 100% testing at wafer level without incurring additional testing time. Moreover, no negative effects (damage by oxidation, corrosion etc.) on the test piece are to be expected as a consequence of using the test fluids. Furthermore, no reaction is expected with the wafer clamped on foil. 
     The invention is particularly suitable for leak and sealing tests for micromechanically constructed and hermetically sealed capacitive sensors, in particular acceleration sensors. 
     Advantageous embodiments of the method according to claim 1 are stated in the subclaims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained in the following by means of an embodiment and with the aid of the drawings. 
     They show: 
     FIG.  1 : the typical curve of capacitance against voltage in the case of a tight acceleration sensor, 
     FIG.  2 : the typical curve of capacitance against voltage in the case of an untight acceleration sensor with a high leak rate, 
     FIG. 3 a:  the typical curve of capacitance against voltage in the case of an untight acceleration sensor with a low leak rate, 
     FIG. 3 b  a magnified section of the curve shown in FIG. 3 a,    
     FIG. 4 a:  the principle structure of a conventional micromechanical acceleration sensor, 
     FIG. 4 b:  the principle of operation of the conventional acceleration sensor shown in FIG. 4 a,  and 
     FIG. 4 c:  the equivalent circuit diagram for the conventional acceleration sensor shown in FIG. 4 a.   
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 4 a  shows the principle structure of a micromechanically manufactured acceleration sensor  1 . A first fixed comb electrode  4  with fingers  5 , as a first capacitor C 1 , and a second fixed comb electrode  6  with fingers  7 , as a second capacitor C 2 , are arranged on an n-conducting substrate  2  (base wafer) and an insulating SiO 2  layer  3 . An electrode  9  with fingers  10 , which is moveable in the direction of an arrow  8 , is located as a seismic mass between the first fixed comb electrode  4  and the second fixed comb electrode  6 . The moveable electrode  9  is attached to two fixed holding blocks  12  by means of two arms  11 . A stop  13  prevents the fingers  10  from touching the fingers  5  or  7 . Finally, a cover in the form of a hermetically sealing lid  21 , made of silicon or similar materials, ensures that the capacitive acceleration sensor  1  is protected against external influences. 
     Should an acceleration force act on the moveable electrode  9 , as a seismic mass, said electrode is deflected in its sensitive direction, indicated by the arrow  8 . In so doing, one of the two capacitances C 1  or C 2  between the moveable electrode  9  and the fixed comb electrode  4  or  6  is increased, while the other of the two capacitances C 1  or C 2  is simultaneously reduced. 
     This principle of operation is shown in FIG. 4 b . The fingers  5  of the first fixed comb electrode  4  form the capacitor C 1 , whereas the capacitor C 2  is formed by the fingers  7  of the second fixed comb electrode  6 . The capacitor C 1  is measured between a connection W 1  (wall  1 ) of the first fixed comb electrode  4  and a center tap c (center) of the moveable electrode  9 , the capacitor C 2  is measured between a connection W 2  (wall  2 ) of the second fixed comb electrode  6  and the center tap c of the moveable electrode  9 . 
     When acceleration forces cause the moveable electrode  9  to move in the direction of the arrow  8 , the capacitances C 1  and C 2  change in the manner already described. The fingers  5 ,  7  and  10 , which are electrically isolated from one another, form a differential capacitor with a capacitance ranging from a few hundred fF up to approximately 600 fF. 
     The equivalent circuit diagram shown in FIG. 4 c , derived from the acceleration sensor  1 , consists of the variable capacitors C 1  and C 2 , in which each capacitor C 1  and C 2  is composed of at least two parallel connected capacitors. Should the capacitance C 1  increase by a specific value as a result of acceleration forces, then the capacitance C 2  falls by this value, and vice versa. 
     The Course of the Electrical Testing Method 
     The covers  21  of the finished processed wafers are either sawn down to the base wafer  2  as sensor elements or the sensors  1  are sawn completely free on the wafer saw as DIEs. The sawn wafer is then put in a vessel and exposed to a vacuum of 5·10 −1  mbar for thirty minutes. After flooding with a suitable test fluid, for example with a FLUOROCARBON for example FC  40  FC  72 , EC  75 , FC  77  produced by the Minnesota Manufacturing and Mining company, the test piece is exposed to the vacuum for a further thirty minutes. During this, TIME the wafer must be completely surrounded by the test fluid. 
     After bringing the vacuum up to normal pressure, the wafer remains in the same vessel and is immersed in the same testing medium. A defined overpressure of 8 bar is then generated in the vessel, and the test piece is exposed to these conditions for one hour. 
     After pumping out the test fluid and opening the vessel, the test piece (wafer) is removed. The wafer is held at an inclined angle for two minutes to allow it to drain before the wafer is removed from the vessel. After the wafer has been lightly blown off with a nitrogen jet, the wafer is placed in a nitrogen box to dry at room temperature. Any test fluid which had not been completely removed from the surface of the wafer evaporates within a period of fifteen minutes without leaving any residue. The wafer must now be electrically tested (wafer test) within twelve hours. 
     Wafer Test 
     As already mentioned, an untight sensor cavity is detected by the increase in the sensor capacitance as measured by an LCR meter capable of measuring inductance (L), capacitance (C) and resistance (R) under the defined test conditions f=400 kHz; U=0.5 V rms  wherein U is a voltage for C and C pd . 
     Depending upon the test fluid used, with a dielectric constant, in the case of coarser leak rates (ca. 10 −2  mbar·dm 1 ·s −1 ) one can expect the test fluid to penetrate and almost completely wet the sensor finger structure. The capacitance of the sensor fingers  5 ,  7 ,  10  in the rest position thereby increases by a factor corresnonding to the dielectric constant of the test fluid. In this case, the increase in capacitance is substantially different from that of tight reference sensors and may be used as an unambiguous test criterion within the wafer test. 
     FIG. 1 shows the typical curve of the capacitance against the voltage in the case of a tight acceleration sensor  1 , its so-called CV characteristic (C stands for the capacitance, V stands for the voltage). Here, a first curve  14  shows the capacitance of the first variable capacitor C 1  measured between the measuring point W 1  and the center tap c, and a second curve  15  shows the capacitance of the second variable capacitor C 2  measured between the measuring point W 2  and the center tap c. Both measurements show a curve of the sensor characteristic which runs symmetrically on the voltage axis for both variable capacitors C 1  and C 2 . 
     FIG. 2 shows three curves of the capacitance against the voltage for an untight acceleration sensor  1  with a high leak rate (ca. 10 −2  mbar·dm 3 ·s −1 ). A first curve  16  shows the course of the variable capacitance C 1  or C 2  before the acceleration sensor  1  has been subjected to the seal test described. The curve  16  shows a curve of the sensor characteristic which is symmetrical on the voltage axis. 
     A second curve  17  shows the course of the variable capacitance C 1  or C 2  immediately after the acceleration sensor  1  has been subjected to the seal test described. The curve  17  shows a course of the sensor characteristic that is clearly asymmetrical on the voltage axis, in which the capacitance increases steeply on one side at +3.3 V and on the other side at −2.8 V. The capacitance of the variable capacitors C 1  or C 2  has approximately doubled as a consequence of the penetration of the test fluid. 
     A third curve  18  shows the course of the variable capacitance C 1  or C 2  48 hours after the acceleration sensor  1  has been subjected to the seal test described. The curve  18  again shows a course of the sensor characteristic that is clearly asymmetrical on the voltage axis, in which the capacitance increases steeply on one side at +4.0 V and on the other side at −3.6 V. The course of the curve  18  shows that an untight acceleration sensor  1  can still be distinguished from a tight one even  48  hours after the described seal test has been carried out. 
     An extended electrical test method, as described in the following, is required for finer leak rates in the range of ca. 10 −4  mbar·dm 3 ·s −1 . 
     The Course of the Extended Electrical Testing Method 
     The working principle of the extended electrical test method is that tiny particles of the penetrating test fluid remain adhered to the sensor fingers  5 ,  7  and  10  (FIGS. 4 a, b ). These Flourinert Dielektrika are polarized in the electrical field by electrical influence. Penetrating particles of the test fluid have an electrically measurable effect on the sensor characteristic. 
     An additional direct voltage (bias) is superimposed on the measuring alternating voltage during the measurement of the capacitance. While doing so, the moveable sensor structure  9  with its fingers (moving fingers)  10  is moved along into the area of the stop  13  (over force stopper) by the electrical force field generated. 
     In the case of the measured sensors  1 , the bias voltage required lies at 4.5 volts and tests the sensor response in both directions of polarity (+4.5 V and −4.5 V). The large electrical force field generated by the high bias voltage exerts a strong attractive force on the moving fingers  10 , which leads to a minimal plate gap. 
     As already described and shown in FIG. 1, a CV characterization on tight reference sensors shows a symmetrical and typically parabolic curve for the capacitance up into the stop area (over force stopper). In the case of untight sensors  1 , this symmetrical curve of the sensor capacitance remains disturbed for a long time. 
     This is caused by polarized particles of the test fluid penetrating into the cavity and some of them adhering to the sensor fingers  5 ,  7  and  10 . In so doing, surface forces effect a local increase in the attractive and repulsive reactions between the sensor fingers  5 ,  7  and  10 . As a result of the high field strength concentration occurring in this area, the finger structure  5 ,  7  and  10  is elastically deformed and the gap further reduced, which in the extreme case may lead to the affected fingers  5 ,  7  and  10  touching. 
     This sensor response is detectable in the CV characteristic by a spontaneous, premature and frequently asymmetric capacitance change, as can be seen in FIGS. 3 a  and  b . FIG. 3 a  shows a first curve  19 , which illustrates the typical course of the capacitance change of a tight sensor  1  after the described, extended test method. In the case of a voltage ranging from ca. +4.0 V to −4.0 V, the capacitance of the sensor increases steeply from ca. 7·10 −13  F until the capacitance finishes in each case at ca. 1·10 −12  F in the stop area. 
     FIG. 3 a  also shows a second curve  20 , which illustrates the typical course of the capacitance change of an untight sensor  1  with a leak rate in the range of ca. 10 −4  mbar·dm 3 ·s −1  after the described, extended test method. As in the case of a tight sensor  1 , the capacitance changes sharply in the positive voltage range at ca. +4.0 V from, once again, ca. 7·10 −13  F up to ca. 1·10 −12  F (stop area). In contrast to that, the capacitance already changes, equally sharply, at −3.5 V, which leads to the asymmetrical course of the curve  20 , as can be clearly seen in FIG. 3 a.    
     The steep increase in the capacitance in the negative voltage range is shown again in FIG. 3 b  in magnified form. This makes it clear that the course of the steep increase in the capacitance from ca. 7·10 −13  F up to ca. 1·10 −12  F in the stop area is almost the same in both curves  19 ,  20 , nevertheless, the steep increase in capacitance of the tight sensor  1 , shown in curve  19 , takes place at −4.0 V, whereas the steep increase in capacitance of the untight sensor  1 , shown in curve  20 , already takes place at −3.5 V. 
     With the aid of the extended test method, it is therefore possible to unambiguously determine whether a micromechanically manufactured sensor  1 , which has a leak rate of ca. 10 −4  mbar·dm 3 ·s −1 , is tight or untight. It can be integrated into the wafer test as an unambiguous, electrically measurable test criterion within the scope of the parameter test. 
     This extended test method is also suitable for detecting other weak points in the sensor  1  (sticking, mouse bite, trench derivatives, foreign material etc.) by technical measurement of hermetically sealed sensors.