DE102005054552B4 - Apparatus and method for testing semiconductor substrates for cracks - Google Patents

Abstract

Device (1) for testing semiconductor substrates for cracks (8), comprising a holder (2) for holding one or more semiconductor substrates, a test unit and a movement unit (3) for moving the semiconductor substrate and the test unit relative to each other, the test unit being at least a heat source (4) for local heating of the semiconductor substrate and at least one pyrometer with a detector (5) for local measurement of the temperature of the semiconductor substrate and a subtraction device for calculating the difference of two local temperature measurements, as detector (5) a thermopile is provided and at least one detector (5) is designed without imaging optics.

Description

The invention relates to a device for testing semiconductor substrates, in particular semiconductor wafers, for cracks. It further relates to a method for inspecting semiconductor substrates.

A semiconductor wafer, for example made of silicon, is provided after production with a plurality of layers of electrically conductive or insulating material, which are patterned by etching processes. During the manufacturing process, the wafer is repeatedly planarized.

Its small thickness makes a semiconductor wafer susceptible to damage such as cracks. Because such cracks can lead to failure of a component, the semiconductor wafer must be checked for damage between the individual manufacturing steps. For reasons of cost, it is expedient to use a process that is as sensitive as possible in order to be able to detect damage as early as possible in the production process and to be able to sort out the affected wafer.

From the publication US 6,906,794 B2 For example, a device for inspecting semiconductor wafers is known, which detects damage by means of an optical method. For this purpose, the device has a light source for irradiating the semiconductor wafer and an imaging unit with a camera for imaging the region to be inspected on the semiconductor wafer. The semiconductor wafer is movable relative to the light source to the camera and also arranged tiltably.

However, the semiconductor wafer inspection apparatus does not allow automatic inspection. Rather, it is operated by an operator and the actual examination and assessment of the recorded images is performed by the operator and is supported by the imaging process and image processing software only.

The inspection of semiconductor wafers is thus more accurate than an inspection with the naked eye, but very time-consuming and thus costly. In addition, damage such as very fine cracks with optical methods are difficult to detect.

The pamphlets US 5,894,345 A . US 3 451 254 A . DE 197 23 080 A1 . US 4,468,136 A . US 6,906,794 B2 and US 2002/0 011 852 A1 each discloses an apparatus for detecting a fault in a substrate

The publication US 5 147 498 A discloses an apparatus for controlling a temperature in processing a substrate. The US Pat. No. 6,552,561 B2 discloses an apparatus for controlling the temperature of an object to be tested.

The object of the invention is therefore to provide a device with which a simple and fast, but at the same time very sensitive testing of semiconductor wafers for cracks is possible.

In addition, it is another object of the present invention to provide a method for inspecting semiconductor wafers for cracks.

According to the invention, these objects are achieved with the subject matters of the independent patent claims.

Advantageous developments of the invention are the subject of the dependent claims.

A device according to the invention for testing semiconductor substrates for cracks comprises a holder for holding one or more semiconductor substrates, a test unit and a movement unit for moving the semiconductor substrate and the test unit relative to one another, the test unit having at least one heat source for locally heating the semiconductor substrate, at least one pyrometer a detector for locally measuring the temperature of the semiconductor substrate and a subtraction device for calculating the difference of two local temperature measurements. As a detector, a thermopile is provided, and at least one detector is designed without imaging optics.

A crack here and below is understood to mean a crack-shaped damage to the semiconductor substrate which affects not only its surface but the entire thickness of the semiconductor substrate or at least substantial parts thereof and which measurably influences the mechanical and thermal conduction properties of the semiconductor substrate.

The invention is based on the consideration that the inspection of semiconductor substrates, in particular of semiconductor wafers, should be as automatic as possible and nevertheless allow a precise examination of the semiconductor. For this purpose, the property is exploited that an undamaged semiconductor wafer conducts heat relatively well, while over a damage such as a crack, the heat conduction almost comes to a standstill.

Advantageously, one or more thermopiles are provided as a detector. A thermopile is characterized by easy handling and a relatively low susceptibility to disturbances. A thermopile usually comprises a plurality of consecutively switched thermocouples whose function is based on the Seebeck effect: Thereafter arises at the boundary layers of two different metals, which are interconnected, a thermoelectric voltage. This is temperature-dependent and allows by its measurement a conclusion on the temperature.

A thermopile measures the temperature of a body using the radiation emitted by it. Thermal radiation is absorbed at one terminal of the thermopile blackened for better absorbency, resulting in heating over the other pre-radiation protected terminal and inducing the thermoelectric voltage.

Thermopile can multiply the thermoelectric voltage by connecting several thermocouples in series and thus form a sensitive heat detector, which allows a non-contact and very fast temperature measurement.

The heat source is advantageously arranged on the same side of the semiconductor substrate as the detector. Thus, the re-radiated rather than the much weaker transmitted radiation is analyzed.

The test unit can be stationary and the semiconductor substrate can be arranged to be movable relative to the test unit. When a semiconductor wafer is provided as the semiconductor substrate, the wafer is advantageously rotated about an axis through its center perpendicular to its surface to allow inspection with the stationary inspection unit.

The heat source, which may be for example a light source, a heated gas or a sound source, has a distance d to the detector. between the heat source and the detector is thus an area with an extent of the order d. If the semiconductor material in this area is undamaged, then it has relatively homogeneous heat conduction properties. Thus, if the wafer rotates around an axis through its center, and if the detector and the heat source are arranged in a stationary manner, the detector measures a temperature which is largely constant over time.

However, if the heat source or detector sweeps across a crack, ie if there is a crack within the area between the heat source and the detector, the heat conduction properties of the substrate change abruptly, which is manifested by a temperature jump.

Another possibility of detecting a crack results when the test unit comprises a further pyrometer with a detector which is arranged at a distance d 'from the detector of the first pyrometer.

In a simultaneous measurement with both detectors, a crack can be detected by the fact that the difference between the temperatures measured by them is greater than would be expected by the different distance from the heat source alone. At a distance d 'of at most 10 mm, a particularly good resolution can be achieved in the localization of cracks.

Advantageously, at least one detector is designed without an imaging optics. Such a detector has, on the one hand, a relatively high temperature sensitivity and allows the testing of a semiconductor substrate even with a rather slight heating. On the other hand, he can be at a short distance from. Semiconductor substrate can be arranged, thereby achieving a particularly good spatial resolution.

The semiconductor substrate typically has a thickness of at most 1 mm, its transmissivity in the wavelength range between 1 .mu.m and 10 .mu.m is at least 50%.

The device according to the invention has the advantage that it allows a particularly sensitive and rapid automatic inspection by the utilization of the heat conduction properties of the semiconductor substrate and by the use of thermopiles. Even very fine cracks, which can not be detected by optical inspection or only with great effort, affect the heat conduction of the semiconductor substrate strong enough to be detected by the device according to the invention. The non-contact measurement also avoids influencing the local wafer temperature by the measurement itself as well as damage to the wafer surface.

According to the present invention, a method for testing semiconductor substrates from cracks comprises the following steps: First, the semiconductor substrate is locally heated at a point y. Subsequently or at the same time, the local temperatures T (x) and T (x ') are pyrometrically measured at two different points x and x' from the semiconductor substrate.

From the temperatures T (x) and T (x '), the difference D = T (x) - T (x') is formed and compared with a predetermined threshold value T _S. If D exceeds this threshold value T _S , then this is a sign that there is a crack between the positions x and x '. This can be done using a Signal indicating a crack between points x and x 'if D> T _s . The temperature T is measured with one or more thermopiles, and at least one detector is designed without imaging optics.

For local heating of the semiconductor substrate, a heat source, for example a light source or a heated gas is used. The temperature is measured by a pyrometric method using thermopiles as detectors.

The semiconductor substrate and the test unit, which comprises the heat source and the thermopile, are moved relative to one another, so that the entire surface of the semiconductor substrate or at least the marginal area, which is particularly at risk due to cracks, is swept over during the test procedure.

In particular, when the semiconductor substrate is a nearly circular semiconductor wafer, it is advantageously rotated about an axis through its center perpendicular to its surface at the angular velocity ω. In this way, the semiconductor wafer can be particularly easily inspected by the fixed test unit.

The temperatures T (x) and T (x ') at locations x and x' can either be measured at two consecutive times with a single detector sweeping through positions x and x 'one after the other. At a point in time t, the point x is then located in a measuring window of the detector and at the point in time t ', the point x', so that T (x) at time t and T (x ') at time t' are measured.

However, they can also be measured simultaneously with two different detectors, the first of which is at a certain point in time above the point x, while the other is simultaneously above the point x '.

To calculate the difference D, instead of the measured temperature T _measurement (x), the temperature T (x) = T _measurement (x) - ΔT (x) corrected by the local signal background ΔT (x) is advantageously used.

The temperature measurement is namely, since it is pyrometrically via the emitted radiation, in particular influenced in the infrared wavelength range relatively transparent semiconductor substrates or those with locally different surface properties by factors such as the emissivity of the surface material forming the signal background .DELTA.T (x).

There are various possibilities for determining the local signal background ΔT (x). Either it is determined by measuring the local temperature T (x) without prior heating of the semiconductor substrate.

Or it is determined by _measuring the temperature T _measurement (x) at time t with a detector and the local signal background ΔT (x) at time t + Δt with another detector.

While the first detector registers the sum of the effect of local heating, ie the actual temperature increase, on the one hand, and of surface effects on the other hand, if the second detector is located at a sufficient distance from the heat source, only the surface effects are measured.

For the correction of the measured temperature T _measurement (x), a kind of mapping of the surface properties is thus carried out, from which the locally different signal background ΔT (x) results.

The signal background can thus be measured with the same device as the signal itself; even a simultaneous measurement of the signal background is possible. This results in the advantage of the method a significant time savings and a relatively low expenditure on equipment. Even complex software such as an image processing program can be dispensed with. Only simply provided options for storing the measurement data of temperature T _measurement (x) and signal background .DELTA.T (x) and for calculating the corrected signal T (x) and the difference D and for the comparison of D with the threshold value T _S are necessary.

Embodiments of the invention are explained below with reference to the accompanying figures.

1 a schematic side view of a device according to the invention for testing semiconductor wafers;

2a - 2d a plan view of the device at different times t ₁ to t ₄ ;

3 the local temperature T (x) at the position x on the semiconductor substrate at the same times t ₁ to t ₄ ;

4 an alternative embodiment of the device according to the invention;

5 a further alternative embodiment of the device according to the invention, a Possibility to correct the measured temperature values to a local signal background.

The same parts are provided in all figures with the same reference numerals.

1 shows a device 1 for testing a semiconductor wafer 6 cracks and similar damage. The device 1 has a holder 2 on top of the semiconductor wafer 6 stops during the testing process. In addition, the device includes 1 a test unit with a heat source 4 and a detector 5 as well as a movement unit 3 for moving the semiconductor wafer and the test unit relative to each other.

The heat source 4 serves for local heating of the semiconductor wafer 6 , The local temperature T of the semiconductor wafer 6 is using a pyrometer, which is the detector 5 includes, measured.

In the embodiment according to 1 lies the semiconductor wafer 6 on the bracket 2 up and up with the help of the movement unit 3 rotated about an axis through its center perpendicular to its surface. The test unit with the heat source 4 and the detector 5 is fixedly installed so that the semiconductor wafer 6 rotates under the test unit.

Particularly vulnerable to damage such as cracks is the edge area 7 of the semiconductor wafer 6 , Therefore, especially this border area 7 be checked for cracks, which is why the test unit with the heat source 4 and the detector 5 preferably over the edge area 7 is installed. But it can also be arranged to be movable, so that it is flexible for the inspection of the entire semiconductor wafer 6 can be used.

The 2a to 2d show in a plan view of the device 1 with the semiconductor wafer 6 the inspection process. The semiconductor wafer 6 according to 2a rotates about an axis through its center M at an angular velocity ω counterclockwise. The test unit with the heat source 4 and the detector 5 is stationary, with the heat source 4 and the detector 5 have a distance d to each other.

The semiconductor wafer to be tested 6 has a crack 8th whose detection is described below.

At a first time t ₁ is the crack 8th even before the heat source 4 and the detector 5 the test unit. Through the heat source 4 , which may be a light source, a heated gas or a sound source, for example, becomes the semiconductor wafer 6 locally heated. A short time later, the area heated at time t ₁ reaches the semiconductor wafer 6 the detector 5 , The detector 5 has, for example, a thermopile, which generates a voltage signal from the local temperature and in this way allows a measurement of the local temperature T.

2 B shows the constellation of test unit and crack 8th to each other at a second time t ₂ . By the rotation of the semiconductor wafer 6 with the angular velocity ω, the crack has 8th now moved on and is located between the heat source 4 and the detector 5 , At this time t ₂ , the detector registers 5 another measured value of the local temperature T. This second measured value T (t ₂ ) may be as in 3 shown deviate from the previously recorded measured value T (t ₁ ).

At time t ₃ , the crack has 8th as in 2c shown further moved and is located just before the detector 5 , The crack 8th represents a kind of barrier to the process of heat conduction. The heat source 4 Heat introduced into the semiconductor substrate may propagate significantly worse over the crack than through the undamaged semiconductor substrate. Therefore, that is from the detector 5 Temperature T (t ₃ ) measured at time t _{3 is} typically comparatively low.

Has the crack 8th however as in 2d shown below the detector 5 moved so that it is between the heat source 4 and the detector 5 no longer forms a barrier, the heat conduction can take place unhindered again. The at time t ₄ , the constellation in 2d is shown, measured temperature T (t ₄ ) is therefore comparatively high.

The measured at the times t _i temperatures T are as in 3 evaluated evaluated. Of the two successive points in time t _i and _{i + 1} t measured temperatures T, the difference D is formed. If, between these times t _i and t _{i + 1,} the heat conduction properties of the semiconductor substrate between the heat source 4 and the detector 5 have not changed significantly, the temperature difference D is relatively low.

However, has a crack between the times t _i and t _{i + 1} 8th into the area between the detector 5 and the heat source 4 pushed or in particular he has under the detector 5 moved through and thus leave this area again, the heat conduction properties of the semiconductor substrate have changed significantly in this area. This is reflected in a relatively large temperature jump D such as in 3 at D = T (t ₄ ) -T (t ₃ ).

To detect cracks, a threshold value T _{s is} set. The threshold value T _s is based on empirical values or on experiments with semiconductor wafers whose damage and heat conduction properties are precisely known. If the temperature difference D exceeds the threshold value T _s , there is a crack between the points in time 8th under the detector 5 moved through.

The difference D is not only a difference between temperature measurements at different times, but by the movement of the semiconductor wafer 6 at the same time a difference from temperatures at different points on the semiconductor wafer relative to the test unit 6 , It can therefore be written as D = T (x) - T (x ').

As an alternative to the temperature measurement at different times, the temperature T of the semiconductor wafer 6 also at the same time as in 4 can be measured with two detectors.

Another detector 9 is at a small distance d 'from the detector 5 arranged so that a point x on the semiconductor wafer first under the detector 5 and then under the other detector 9 moved through. However, the temperature measurement takes place simultaneously, while the detector 5 measures the temperature of the semiconductor wafer at point x, measures the other detector 9 the temperature at a point x 'at a distance from d' to x. The distance d 'should be at most 10 mm.

To evaluate the measured data, the difference D = T (x) - T (x ') is again formed. If this is above the predetermined threshold value T _s , there is a crack between the two detectors 5 and 9 ,

With the described method, cracks in the semiconductor substrate can be detected very accurately. The method has the advantage that, in contrast to optical inspection methods, it can also detect very fine cracks or oblique cracks, through which no light or, for an optical inspection, too little light would penetrate. Since the heat conduction properties of the semiconductor substrate are relatively strongly influenced by cracks and similar damage, the method described is very sensitive.

The local temperature T of the semiconductor wafer 6 is measured as described with one or more thermopiles. This measuring method is a pyrometric measuring method and is based on the fact that the heat radiation registered by the detector is independent of the temperature of the semiconductor wafer 6 depends. The temperature measurement thus takes place via the emitted infrared radiation and is thus contactless.

This has several advantages. On the one hand, the measurement can be carried out very quickly within milliseconds or microseconds, on the other hand there is neither a temperature influencing of the measurement object nor a mechanical damage of the sensitive wafer surface. Passing the semiconductor wafer 6 Under the test unit is thus easily possible.

However, the radiation emitted by the semiconductor wafer is influenced not only by the local temperature T but also by surface properties of the semiconductor wafer. In the case of semiconductor substrates which have a relatively high transmissivity in the infrared wavelength range, this can be done by the detector 5 also registered signal through the material, which is located on the detector 5 remote side of the semiconductor wafer 6 is affected.

For a particularly accurate inspection of a semiconductor wafer 6 Therefore, the measured local temperature T (x) can be corrected by the location-dependent signal background .DELTA.T (x). There are various possibilities for determining the signal background ΔT.

A first possibility is a pyrometric measurement of the temperature T (x) without prior heating of the semiconductor wafer 6 , In this way, when the heat source is switched off, a kind of mapping of the local signal background ΔT (x) can be carried out. However, this requires a separate process step and therefore a certain amount of time.

However, the measurement of the local signal background ΔT (x) can also take place at the same time as the actual test. 5 shows a plan view of a semiconductor wafer 6 with a test unit next to a heat source 4 and a detector 5 also another detector 9 for determining the signal background .DELTA.T. While the heat source 4 and the detector 5 have a distance from each other d, the further detector is located 9 at a distance d 'from the detector 5 arranged and behind this, leaving the detector 5 between the heat source 4 and the other detector 9 lies. So has the other detector 9 a greater distance from the heat source 4 as the detector 5 ,

The distance d 'is chosen such that the local temperature T at the position of the further detector 9 not or only slightly by the heat source 4 is affected. This corresponds to the other detector 9 recorded temperature signal the signal background .DELTA.T, which is used for correction by the detector 5 measured temperature value T is used.

LIST OF REFERENCE NUMBERS

1

contraption

2

bracket

3

moving unit

4

heat source

5

detector

6

Semiconductor wafer

7

border area

8th

Crack

9

another detector

M

Focus

ω

angular velocity

d

distance

d '

distance

Claims (17)

Contraption ( 1 ) for testing semiconductor substrates for cracks ( 8th ), which has a holder ( 2 ) for receiving one or more semiconductor substrates, a test unit and a movement unit ( 3 ) for moving the semiconductor substrate and the test unit relative to one another, wherein the test unit has at least one heat source ( 4 ) for local heating of the semiconductor substrate and at least one pyrometer with a detector ( 5 ) for local measurement of the temperature of the semiconductor substrate and a subtraction device for calculating the difference between two local temperature measurements, as a detector ( 5 ) a thermopile is provided, and at least one detector ( 5 ) is designed without imaging optics. Device according to claim 1, wherein the heat source ( 4 ) on the same side of the semiconductor substrate as the detector ( 5 ) is arranged. Apparatus according to claim 1 or 2, wherein the test unit is stationary and the semiconductor substrate is arranged to be movable relative to the test unit. Device according to one of claims 1 to 3, wherein the heat source ( 4 ) and the detector ( 5 ) have a distance d from each other. Device according to one of claims 1 to 4, wherein the test unit comprises a further pyrometer with a detector ( 9 ) disposed at a distance d 'from the first pyrometer. Apparatus according to claim 5, wherein the distance d 'is at most 10 mm. Device according to one of claims 1 to 6, wherein as a heat source ( 4 ) A light source is provided. Device according to one of claims 1 to 6, wherein as a heat source ( 4 ) is provided a heated gas. A method of inspecting semiconductor substrates for cracks, comprising the steps of: locally heating the semiconductor substrate at a point y by a heat source ( 4 ); - Pyrometric measurement of the local temperatures T (x) and T (x ') of the semiconductor substrate at two different points x and x' on the semiconductor substrate; - calculation of the difference D = T (x) - T (x '); Comparison of D with a predetermined threshold T _S ; - Output a signal that has a crack ( 8th ) between the points x and x 'if D> T _S ; wherein the temperature T is measured with one or more thermopiles and at least one detector ( 5 ) is designed without imaging optics. Method according to Claim 9, in which the temperatures T (x) and T (x ') coincide with two detectors ( 5 . 9 ), whereby a detector ( 5 ) the temperature T (x) at the point x and the further detector ( 9 ) measures the temperature T (x ') at point x'. Method according to claim 9 or 10, in which the semiconductor substrate is moved relative to the stationarily arranged test unit, so that at a point in time t the point x and at time t 'the point x' on the semiconductor substrate in a measuring window of the detector ( 5 ) and T (x) at time t and T (x ') at time t' are measured. Method according to one of claims 9 to 11, wherein the semiconductor substrate rotates about an axis through its center M perpendicular to its surface at an angular velocity ω. Method according to one of Claims 9 to 12, in which, to calculate the difference D instead of the measured temperature T (x), the temperature T _corrected by the local signal background ΔT (x) is _corrected (x) = T (x) - ΔT (x) is used. Method according to Claim 13, in which the local signal background ΔT (x) is determined by a measurement of the local temperature T (x) without prior heating of the semiconductor substrate. Method according to Claim 13, in which the local signal background ΔT (x) is determined by measuring the temperature T (x) at time t with a detector and the local signal background ΔT (x) at time t + Δt with another detector , Method according to one of claims 9 to 15, in which as a heat source ( 4 ) a light source is used. Method according to one of claims 9 to 15, in which as a heat source ( 4 ) a heated gas is used.

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