Systems and methods for correction of impact of wafer tilt on misregistration measurements

A method for correcting misregistration measurements of a semiconductor wafer for errors therein arising from tilt of the wafer including measuring, for at least one location on a wafer, a difference between a Tool Induced Shift (TIS) of a metrology device in a first illumination arrangement with respect to the wafer wherein a surface of the wafer is generally orthogonally illuminated by an illumination source of the metrology device and a TIS of the metrology device in a second illumination arrangement with respect to the wafer, wherein the surface is obliquely illuminated by the illumination source, and correcting a misregistration measurement measured by the metrology device at the at least one location for errors therein arising from tilt of the wafer at the location by subtracting from the misregistration measurement a weighted value of the difference between the TIS in the first and second illumination arrangements.

FIELD OF THE INVENTION

The present invention relates generally to metrology and more particularly to misregistration measurements on semiconductor wafers.

BACKGROUND OF THE INVENTION

Various systems and methods for the measurement of misregistration in the manufacture of semiconductor wafers are known in the art.

SUMMARY OF THE INVENTION

The present invention seeks to provide novel systems and methods for the correction of misregistration measurements performed on semiconductor wafers for the impact thereon of localized wafer tilt.

There is thus provided in accordance with a preferred embodiment of the present invention a method for correcting misregistration measurements of a semiconductor wafer for errors therein arising from tilt of the wafer including measuring, for at least one location on a wafer, a difference between a Tool Induced Shift (TIS) of a metrology device in a first illumination arrangement with respect to the wafer wherein a surface of the wafer is generally orthogonally illuminated by an illumination source of the metrology device and a TIS of the metrology device in a second illumination arrangement with respect to the wafer, wherein the surface is obliquely illuminated by the illumination source, and correcting a misregistration measurement measured by the metrology device at the at least one location for errors therein arising from tilt of the wafer at the location by subtracting from the misregistration measurement a weighted value of the difference between the TIS in the first and second illumination arrangements.

Preferably, the difference between the TIS in the first and second illumination arrangements includes a signature profile varying as a function of a parameter of the metrology device.

Preferably, the metrology device illuminates the wafer by a multiplicity of wavelengths and the difference in TIS of the metrology device is measured as a function of the multiplicity of wavelengths.

Preferably, the measuring the difference in TIS is performed at a plurality of locations Nson the wafer and the difference in TIS of the metrology device is measured as a function of the multiplicity of wavelengths for each of the plurality of locations.

Preferably, the weighted value of the difference between the TIS in the first and second illumination arrangements is calculated by multiplying the difference between the TIS in the first and second illumination arrangements by a weighting coefficient.

Preferably, the weighting coefficient is calculated based on only variable parts of the misregistration measurement and the difference between the TIS in the first and second illumination arrangements.

Preferably, the weighting coefficient is calculated in accordance with

α⁡(Ns)=∑λmeasured(Ns,λ)·⁢(λ)∑λ(λ)
wherein α(Ns) is the weighting coefficient,measure (Ns, λ) is the variable part of the misregistration measurement and(λ) is the variable part of the difference between the TIS in the first and second illumination arrangements.

Preferably, the variable part of the misregistration measurement in calculated in accordance with:
measured(Ns,λ)=MISmeasured(Ns,λ)−MISmeasured(Ns)
wherein MISmeasured(Ns, λ) is the misregistration measurement andMISmeasured(Ns)is the misregistration measurement averaged over the multiplicity of illumination wavelengths.

Preferably, the subtracting from the misregistration measurement a weighted value of the difference between the TIS in the first and second illumination arrangements is performed in accordance with:
MIScorrected(Ns)=MISmeasured(Ns,λ)−α(Ns)·ΔTIS(λ)

wherein MIScorrected(Ns) is a corrected value of the misregistration measurement and ΔTIS (λ) is the difference between the TIS in the first and second illumination arrangements.

In accordance with one preferred embodiment of the method of the present invention, the method also includes optimizing an operating parameter of the metrology device based on the difference in TIS of the metrology device.

There is also provided in accordance with another preferred embodiment of the present invention a system for correcting misregistration measurements on a semiconductor wafer for errors therein arising from tilt of the wafer including an illumination source forming part of a metrology device and operative to illuminate a wafer in at least a first illumination arrangement wherein a surface of the wafer is generally orthogonally illuminated by the illumination source and a second illumination arrangement wherein the surface is obliquely illuminated by the illumination source, a TIS calculator operative to find a difference between a TIS of the metrology device in the first and second illumination arrangements and a wafer tilt corrector operative to correct a misregistration measurement by the metrology device at a location on the wafer for errors therein arising due to tilt of the wafer at the location, based on subtracting from the misregistration measurement a weighted value of the difference between the TIS of the metrology device in the first and second illumination arrangements.

Preferably, the difference between the TIS in the first and second illumination arrangements includes a signature profile varying as a function of a parameter of the metrology device.

Preferably, the illumination source is operative to illuminate the wafer by a multiplicity of wavelengths and the TIS calculator is operative to find the difference in TIS of the metrology device as a function of the multiplicity of wavelengths.

Preferably, the TIS calculator is operative to find the difference in TIS at a plurality of locations Ns, on the wafer, as a function of the multiplicity of wavelengths for each of the plurality of locations.

Preferably, the wafer tilt corrector is operative to calculate the weighted value of the difference between the TIS in the first and second illumination arrangements by multiplying the difference between the TIS in the first and second illumination arrangements by a weighting coefficient.

Preferably, the wafer tilt corrector is operative to calculate the weighting coefficient based on only variable parts of the misregistration measurement and the difference between the TIS in the first and second illumination arrangements.

Preferably, the wafer tilt corrector is operative to calculate the weighting coefficient in accordance with:

α⁡(Ns)=∑λmeasured(Ns,λ)·⁢(λ)∑λ(λ)
wherein α(Ns) is the weighting coefficient,measured(Ns, λ) is the variable part of the misregistration measurement and(λ) is the variable part of the difference between TIS in the first and second illumination arrangements.

Preferably, the wafer tilt corrector is operative to calculate the variable part of the misregistration measurement in accordance with:
measured(Ns,λ)=MISmeasured(Ns,λ)−MISmeasured(Ns)
wherein MISmeasured(Ns, λ) is the misregistration measurement andMISmeasured(Ns)is the misregistration measurement averaged over the multiplicity of illumination wavelengths.

Preferably, the wafer tilt corrector is operative to subtract from the misregistration measurement a weighted value of the difference between the TIS in the first and second illumination arrangements in accordance with:
MIScorrected(Ns)=MISmeasured(Ns,λ)−α(Ns)·ΔTIS(λ)
wherein MIScorrected(Ns) is a corrected value of the misregistration measurement and ΔTIS (λ) is the difference between the TIS in the first and second illumination arrangements.

In accordance with a preferred embodiment of the system of the present invention, an operating parameter of the metrology device is optimized based on the difference in TIS of the metrology device.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference is now made toFIG.1, which is a simplified schematic partially pictorial, partially block diagram illustration of a system for the correction of misregistration measurements for the impact thereon of wafer tilt, constructed and operative in accordance with a preferred embodiment of the present invention.

As seen inFIG.1, there is a provided a metrology system100including a metrology tool102for measuring misregistration in the manufacture of a semiconductor device, such as a semiconductor wafer104. Metrology tool102preferably includes an illumination source, here schematically represented as an illumination source110, operative to illuminate wafer104in order to allow imaging of target structures formed on layers of wafer104so as to measure misregistration between layers of wafer104. It is appreciated that, in addition to illumination source110, metrology tool102typically includes various additional components as are well known in the art, omitted here for clarity, in order to facilitate imaging of wafer104thereby and the measurement of misregistration between layers of wafer104.

In accordance with a particularly preferred embodiment of the present invention, metrology tool102may be embodied as a multi-wavelength tool, such as an Archer700tool, commercially available from KLA, of California, USA, operative to perform multi-wavelength measurements on wafer104. Metrology tool102is preferably operative to output at least one misregistration measurement representing misregistration between layers of wafer104at at least one measurement location thereon. In the case that metrology tool102is a multi-wavelength instrument, the misregistration measurement MISmeasuredoutput thereby may be expressed as MISmeasured(Ns, λ), wherein NSrepresents the particular location on wafer104with respect to which the measurement is performed and λ represents the multiplicity of illumination wavelengths provided by illumination source110.

It is understood that local variations in the tilt of a surface120of wafer104may have an impact on the accuracy of the misregistration measurement MISmeasured(Ns, λ) output by metrology tool102, due to the influence of wafer tilt on the misregistration measurement, and may therefore introduce errors therein. Preliminary measurements performed by the present inventors have shown that the local wafer tilt at target locations may vary by about ±0.5 mrad. Such local wafer tilt may have a considerable impact on misregistration measurements, which impact may become increasingly significant with increasing wafer stack height.

In accordance with preferred embodiments of the present invention, the value of the local wafer tilt at a particular location on wafer104may be found and the contribution thereof to errors in the misregistration measurement MISmeasured(Ns, λ) may be quantified, so as to allow correction of the misregistration measurement for errors arising from local wafer tilt. This advantageously leads to the calculation of an improved misregistration measurement more accurately representing actual misregistration values for wafer104. In accordance with preferred embodiments of the present invention, both the measurement of the wafer tilt and the quantification of the error in the misregistration measurement arising therefrom are found based on the understanding by the present inventors that the effect of wafer tilt on the misregistration measurement MISmeasured(Ns, λ) is equivalent to the effect of the tilt of illumination source110with respect to surface120of wafer104illuminated thereby, due to the same telecentricity effect of both tilts.

The illumination source tilt contributes to the inherent limited measurement accuracy of metrology tool102, which inherent measurement accuracy may be quantified as the Tool Induced Shift (TIS) of metrology tool102, and does not change upon rotation of the wafer104by 180°. However, the wafer tilt contributes directly to the TIS calibrated misregistration measurement, and changes upon rotation of wafer102by 180°. Based on this understanding, measurement of changes in the TIS of metrology tool102under deliberately tilted illumination conditions, in comparison to non-tilted illumination conditions, may be used as a basis for finding corresponding changes in the misregistration measurement as a result of equivalent wafer tilt and hence both to quantify wafer tilt and correct the misregistration measurements for the impact of the wafer tilt thereon.

As seen inFIG.1, the effect of tilted illumination conditions on the TIS of metrology tool102may be found by a TIS differential calculator130. The preferred operation of TIS differential calculator130is best understood with additional reference toFIGS.2A-2C.

As seen inFIG.2A, the TIS of metrology tool102is preferably initially measured in a first non-tilted illumination arrangement, wherein illumination source110is preferably centered with respect to an illuminated location on surface120of wafer104, such that surface120is generally orthogonally illuminated along an illumination axis200by illumination source110. The TIS of metrology tool102may be found by any suitable method, various types of which are well known in the art. Preferably the TIS may be quantified by measuring the same feature on surface120at 0° and 1800 rotation of wafer104, the TIS being equal to half of the sum of the measurements in each wafer orientation.

As seen inFIG.2B, the TIS of metrology tool102is preferably additionally measured in a second, tilted illumination arrangement, wherein illumination source110is preferably shifted so as to be off-center with respect to the illuminated location on surface120of wafer104, such that surface120is generally obliquely illuminated with respect to illumination axis200by illumination source110. Here, by way of example, illumination source110is shown to be shifted by 10 μm, such that illumination provided thereby is incident upon surface120at an oblique angle of about 5 mrad.

It is understood that such measurements under first and second illumination conditions with respect to surface120of wafer104are preferably carried out prior to the performance of metrology measurements by metrology tool102on wafer104, optionally as part of a preliminary training procedure performed on metrology tool102prior to the operation thereof. Such measurements may be performed for multiple locations on wafer104and, in the case of a multi-wavelength imaging metrology tool, at multiple wavelengths of illumination.

The difference in measurement accuracy, preferably expressed as the difference in TIS, of metrology tool102under first centered illumination conditions, such as shown inFIG.2A, and under second off-center illumination conditions with respect to surface120of wafer104, such as shown inFIG.2B, for multiple measurement locations may be plotted as a function of illumination wavelength. An example of such a plot is shown inFIG.2C, in which the difference in TIS (ΔTIS) of metrology tool102between the first and second illumination conditions ofFIGS.2A and2Bis plotted as a function of illumination wavelength for 10 measurement locations, ΔTIS for each measurement location being represented by an individual line in the graph ofFIG.2C.

As appreciated from consideration ofFIG.2C, ΔTIS is seen to vary significantly with illumination wavelength but not to vary significantly with illuminated location. The variation of ΔTIS as a function of wavelength, as shown inFIG.2C, thus may be considered to correspond to a differential signature profile or landscape representing the effect of deliberate, controlled, illumination tilt and hence the equivalent effect of wafer tilt on the measurement accuracy of metrology tool102. Based on this profile, the effect of local wafer tilt on misregistration measurements by metrology tool102may be corrected, as if further detailed hereinbelow.

It is understood that a profile of the type shown inFIG.2Cmay be output by TIS differential calculator130based on measurements under conditions shown inFIGS.2A and2B. It is appreciated that such a profile is both specific to wafer104and to a given layer of wafer104, and that such a profile thus is preferably obtained by TIS differential calculator130for each layer of wafer104upon which misregistration measurements by metrology tool102are to be performed. The output of such a profile by TIS differential calculator130is represented inFIG.1simply as ΔTIS. In one preferred embodiment of the present invention, TIS differential calculator130may be embodied as a computational module including computer code operative for finding a ΔTIS profile. It is appreciated that although TIS differential calculator130is shown inFIG.1to be embodied as a separate module, this is for the sake of clarity only, and the functionality of TIS differential calculator130may alternatively be included within metrology tool102.

The ΔTIS profile obtained by TIS differential calculator130is preferably provided to a wafer tilt corrector, here shown to be embodied as a wafer tilt correction calculator module140included in system100. Wafer tilt corrector140is preferably operative to correct at least one misregistration measurement measured by metrology device102at a given location on wafer104for errors therein arising due to tilt of wafer104at the given location. Preferably, the at least one misregistration measurement is corrected by wafer tilt corrector140based on subtracting from the misregistration measurement a weighted value of the difference between the TIS of metrology device102in first and second illumination arrangements, as quantified and output by TIS differential calculator130.

The misregistration measurement as corrected for wafer tilt effects at a particular location Ns, MISmeasured(Ns), may be expressed as:
MIScorrected(Ns)=MISmeasured(Ns,Δ)−α(Ns)·ΔTIS(λ)  (1)
wherein MISmeasured(Ns, λ) represents the at least one misregistration measurement, which is preferably provided as a misregistration landscape, being a function of the measurement location NSand the illumination measurement wavelength λ, ΔTIS (λ) represents the TIS differential signature profile, measured as described hereinabove with respect toFIGS.2A-2C, and α(Ns) represents a location-specific weighting coefficient, representing the specific localized wafer tilt at a given location. It is appreciated that the product of the weighting coefficient α(Ns) and the TIS differential signature profile ΔTIS (λ) represents the contribution of the local value of the wafer tilt of wafer104to the corresponding local measured misregistration value and thus must be subtracted from the measured misregistration landscape in order to provide a ‘clean’ misregistration landscape corrected for errors caused by local wafer tilt. The correction of the misregistration measurement based on the TIS differential signature profile ΔTIS (λ) is based on the equivalence between changes in TIS as a result of illumination tilt and as a result of wafer tilt, as detailed hereinabove.

It is appreciated that in order to find MIScorrected(Ns), which parameter is the desired output of wafer tilt correction calculator module140, the weighting coefficient α(Ns) must be ascertained. As a first step in ascertaining the weighting coefficient α(Ns), only the variable part of the measured misregistration at each location is considered, in accordance with:
measured(Ns,λ)=MISmeasured(Ns,λ)−MISmeasured(Ns)(2)
wherein MISmeasured(Ns) is the measured misregistration averaged overall measurement wavelengths.

The weighting coefficient α(Ns) may then be found by calculating the projection of the variable part of the measured misregistration at each location by the variable part of the ΔTIS in accordance with:

α⁡(Ns)=∑λmeasured(Ns,λ)·⁢(λ)∑λ(λ)(3)
where the variable part of the ΔTIS signature is defined for the same set of multiplicity wavelengths λ over which the misregistration is measured, by subtracting from the ΔTIS signature the value of the ΔTIS signature averaged over the multiplicity of wavelengths. It is appreciated that calculating the weighting coefficient at each location using only the variable part of the measured misregistration landscape and the variable part of the difference in TIS, rather than the full values of these parameters, allows separation of the component of these parameters arising from wafer tilt from the actual misregistration values at each location.

The weighting coefficient α(Ns) found in accordance with equations (2) and (3) may then be substituted back into equation (1) so as to yield the misregistration measurement corrected for wafer tilt in accordance with

It is appreciated that the calculations described hereinabove with reference to equations (1)-(4) are preferably performed by wafer tilt correction calculator module140and that the corrected misregistration value, cleaned for the inaccuracies introduced therein due to wafer tilt, is preferably output thereby. In one preferred embodiment of the present invention wafer tilt correction calculator module140may be embodied as a computational module including computer code operative to carry out calculations in accordance with equations (1)-(4). It is appreciated that although wafer tilt correction calculator module140is shown inFIG.1to be embodied as a separate module, this is for the sake of clarity only, and the functionality of wafer tilt correction calculator module140may alternatively be included within metrology tool102.

It is appreciated that the correction procedure carried out by system100may be implemented for correcting misregistration measurements performed on any type of symmetrical targets formed on wafer104and used as reference structures for measurement of misregistration in the manufacture of wafer104. In the case that the reference targets are not symmetrical, the asymmetry of the targets must be additionally accounted for in correcting the misregistration measurements, in order to distinguish between inaccuracies in the misregistration measurements arising from local wafer tilt and components of misregistration measurements attributable to target asymmetry.

It is further appreciated that in some preferred embodiments of the present invention, measurement of ΔTIS as a function of wavelength may be used to optimize settings of metrology tool102, for example by way of selecting a measurement wavelength having minimal sensitivity to illumination source tilt and hence to local wafer tilt. In such cases, the output of TIS differential calculator130may be fed back to metrology tool102and the settings thereof adjusted accordingly, as indicated by the double headed arrow showing optional bi-directional communication between metrology tool102and TIS differential calculator130inFIG.1.

It is further appreciated that although the operation of TIS differential calculator130is described hereinabove with reference to the measurement of the dependency of the changes in TIS as a result of illumination source tilt on metrology illumination wavelength, this is by way of example only. In alternative embodiments of the present invention, changes in TIS as a result of illumination source tilt may be characterized with respect to other parameters of the metrology tool, such as, by way of example only, the focus position of the metrology tool.

Reference is now made toFIG.3, which is a simplified flow chart illustrating steps in the correction of misregistration measurement for the impact thereon of wafer tilt, in accordance with a preferred embodiment of the present invention.

As seen inFIG.3, a method300for the correction of misregistration measurements for the impact of wafer tilt thereon may begin at a first step302, whereat a signature profile of the difference in TIS of a metrology tool under tilted and non-tilted illumination conditions is obtained. The signature profile may be obtained as a function of a variable characteristic of the metrology tool, for example as a function of wavelength. The signature profile may be calculated by a computerized TIS differential calculator module, such as module130ofFIG.1.

As seen at a second step304, misregistration of a semiconductor device is preferably measured by the metrology tool characterized at first step302, for at least one location on the semiconductor device. It is appreciated that although first step302may typically be performed prior to second step304, this is not necessarily the case and the order of steps302and304may be reversed.

As seen at a third step306, a weighted value of the signature profile of the difference in TIS is preferably found, which weighted value preferably corresponds to the value of the local wafer tilt at the measurement location on the semiconductor device under test. This correspondence is based on the understanding that the effect of wafer tilt on misregistration measurements is equivalent to the effect of illumination source tilt on misregistration measurements. Preferably, the weighted value is found in accordance with equations (2) and (3) detailed hereinabove.

As seen at a fourth step308, the weighted value found at third step306is preferably subtracted from the misregistration measurement, so as to correct the misregistration measurement for inaccuracies therein due to local wafer tilt. It is appreciated that the corrected misregistration measurement thus obtained is more accurately representative of actual misregistration between layers of the semiconductor device under test. Preferably, the subtraction is performed in accordance with equation (4) detailed hereinabove.

It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. The scope of the present invention includes both combinations and subcombinations of various features described hereinabove as well as modifications thereof, all of which are not in the prior art.