The present invention relates to any of various methods for planarizing a surface of a workpiece such as a semiconductor wafer during manufacturing semiconductor devices on the wafer. A representative such method is xe2x80x9cchemical mechanical polishingxe2x80x9d (xe2x80x9cCMPxe2x80x9d) as used to planarize semiconductor wafers between certain manufacturing steps. More specifically, the invention pertains to methods and apparatus for detecting a xe2x80x9cprocess endpointxe2x80x9d (i.e., time to stop planarizing).
Semiconductor devices (e.g., integrated circuits, displays, and the like) are becoming increasingly dense and highly integrated. With this trend, certain processes such as processes directed to formation of multiple wiring layers, formation of insulating films between wiring layers, formation of inter-layer connecting plugs and the like, and formation of electrodes are becoming increasingly more critical. For example, controlling the thickness and depth profiles of inter-layer insulating films or metal layers is now very important to ensure attainment of target integration levels in devices with high reliability. Achieving such control requires that layer thicknesses in steps such as layer-formation steps and etching steps be monitored.
Responsive to the need to achieve ever-increasing device density along with ever-decreasing feature sizes, the microlithography industry has developed microlithography apparatus that utilize extremely short wavelengths of light, such as deep-UV light, but at large numerical apertures. Projection optics used in such apparatus have extremely short focal ranges. The focal ranges are now so short that the uneven surface that results from stacking multiple layers atop one another during manufacture of an integrated circuit is no longer in sharp focus from peaks to valleys of the surface. Consequently, it has become increasingly important to planarize the surface of the wafer (at least within exposure areas) accurately between certain layer-formation steps. It is also important to perform a planarization step after embedding an inlay of a metal electrode layer to form inter-layer connecting plugs and the like.
Planarization typically involves the removal of material from the surface of the wafer. Whereas several candidate techniques have now been developed for performing planarization, as summarized below, a key problem has been how to accurately detect, during planarization, when to stop planarizing so as to ensure the desired amount of material has been removed without removing excessive material.
Among the several conventional planarization methods, a polishing process termed xe2x80x9cchemical mechanical polishingxe2x80x9d or xe2x80x9cchemical mechanical planarizationxe2x80x9d (abbreviated xe2x80x9cCMPxe2x80x9d) has received considerable favor. This is because, inter alia, CMP is effective for planarizing wafers having a large surface area and is effective for planarizing microscopic bumps and other surficial irregularities from wafer surfaces. CMP achieves such results from a combination of mechanical abrasion (using an abrasive in liquid suspension) and chemical action (using a mild surface-eroding chemical in the liquid suspension). More specifically, during CMP, the wafer surface is urged against a polishing pad, saturated with a polishing slurry, as the wafer and polishing pad undergo motion relative to each other. The polishing slurry is a suspension of polishing granules (silica, alumina, cerium oxide, or the like, depending on the material on the surface of the wafer) in an acidic or basic (depending on the material on the wafer surface) carrier liquid. With CMP, the entire surface of the wafer can be polished uniformly by making sure that the applied polishing pressure, amount of slurry used, and velocity of relative motions are uniform over the entire wafer surface.
Unfortunately, achieving consistent results with CMP is much more difficult than with other semiconductor processing steps such as layer-forming and etching. Therefore, there is a great need for improved methods for monitoring the thickness of the layer(s) on a wafer being polished by CMP, especially such methods that can provide quick, accurate, and efficient feedback to the CMP apparatus.
According to one conventional approach for monitoring polishing, changes in friction between the wafer and the polishing pad are monitored as corresponding changes in the torque being applied by a motor used to effect rotation of the wafer or polishing pad. For example, a change in torque is encountered when polishing has progressed to an underlying layer made of a material having a substantially different coefficient of friction than the layer being polished away. Unfortunately, monitoring polishing by monitoring torque is notoriously inaccurate and unreliable.
Optical methods for monitoring polishing offer prospects for high accuracy. According to one conventional method, a small wafer xe2x80x9cblankxe2x80x9d region (i.e., a location on a wafer where the surficial layer is essentially planar and desirably not patterned) is subjected to the same CMP process as the remainder of the wafer and used as a measurement sample. Measuring the thickness of the surficial layer as polishing progresses is performed by monitoring changes in the blank region.
Unfortunately, the xe2x80x9cblankxe2x80x9d-measurement method has several serious disadvantages. First, the method requires considerable time to execute and to provide feedback to the actual polishing process. Wafers are normally imprinted with as many devices as possible placed side by side. A non-imprinted portion, for use as a blank, of the wafer must be located among the devices on the wafer. The area of such a blank is normally very small and, on some wafers, has an unspecified location. Because the size of the blank region is normally very small, the range of measurements that can be performed at the location is also very small. Available apparatus for accurately measuring layer thickness within such a small area are simply not available. It is also very difficult to perform measurements in such a small area at sufficiently high speed because the required mechanism for picking up, recognizing, and processing the image of the blank region is very complex.
Also, positioning the wafer for measurement at the blank location poses many problems. Because the blank location is normally very small (and sometimes not even specified), accurate alignment mechanisms are required to ensure that the measurements are consistently performed at the blank location.
Another substantial contributor to the excessive amount of time required to perform the xe2x80x9cblankxe2x80x9d technique is the need to interrupt polishing, clean the wafer, and transport the wafer to a remote but stable location for measurement.
In another conventional optical method for monitoring polishing, the thickness of the layer being polished is monitored by optical interference. In such a technique, an optical path is provided through the polishing pad to the wafer surface being polished, and a laser light beam is directed through the polishing pad to the wafer surface during polishing. Alternatively, light (e.g., infrared light) is transmitted through the wafer holder and through the wafer from the rear surface of the wafer to the surface being polished. Temporal changes in the intensity of light reflected from the surface being polished are monitored as polishing progresses, and a polishing endpoint is believed to be reached when the intensity of the reflected laser light ceases to change with further polishing. Use of such a method for measuring the thickness of a layer at a xe2x80x9cblankxe2x80x9d location is normally effective in achieving a satisfactory accuracy. However, requisite accuracy is not obtained whenever the method is used to detect a polishing endpoint for a patterned layer (which must be performed when attempting to monitor polishing in real time as polishing progresses). This problem is even more pronounced when the wafer surface is patterned with logic circuits or a combination of logic and memory circuits.
Moreover, because the wafer undergoes motion during polishing, signals from a specific (blank) location on the wafer are not normally obtainable, either directly or extracted from signals from other portions of the wafer surface. This method is also susceptible to obfuscating effects of signal noise, making determinations of polishing endpoint even more difficult to obtain. Consequently, polishing endpoint detection by this technique as conventionally applied cannot be reliably realized, especially in real time.
According to yet another conventional optical approach, the layer thickness is monitored by illuminating onto the wafer surface a light beam having a relatively large spot diameter. Optical measurements of light reflecting from the wafer surface are made and averaged. Unfortunately, if the wafer is imprinted with device patterns, the waveform of reflected light is usually extremely complex since the signal depends not only upon the layer thickness but also on the device patterns. Therefore, the thickness of a layer being polished cannot be measured reliably or accurately using this method.
Other methods that have received some attention include detecting changes in vibration or sounds, or detecting changes in slurry composition that purportedly occur whenever a polishing endpoint is being approached. None of these techniques has proved to be effective or reliable.
As can be ascertained from the foregoing summary, none of the conventional techniques for monitoring the thickness of a layer during polishing is satisfactory for current requirements. Each technique as conventionally applied suffers from one or more deficiencies such as, inter alia, excessive time required for performing the measurements and providing feedback to the polishing process, inaccuracy of determinations of layer thickness, lack of repeatability, and/or inapplicability to certain types of layers or to certain steps during manufacture of a semiconductor device.
In view of the above-summarize shortcomings of the prior art, an object of the invention is to provide, inter alia, simple and convenient detection methods by which the thickness of one or more layers on the surface of a workpiece (e.g., wafer) can be determined. Such determinations are especially useful in the determination of a process endpoint.
According to a first aspect of the invention, methods are provided for determining a thickness of a superficial thin-film layer on a substrate as the thin-film is being subjected to a process resulting in a change in thickness of the thin-film layer. According to a representative embodiment of such a method, a probe light is illuminated onto a region of a surface of the thin-film layer to produce a signal light propagating from the thin-film layer. The signal light is detected. From the detected signal light, a spectral characteristic of the signal light is measured to produce a spectral-characteristic signal. A value of a parameter of the spectral-characteristic signal is calculated, wherein the value is a function of the thickness of the thin-film layer. From the calculated value of the parameter, the thickness of the thin-film layer is determined. Such a process can also include the step of determining, from the calculated value of the parameter, an endpoint at which to terminate the process and thus cease changing the thickness of the thin-film layer. An exemplary and especially useful spectral characteristic is spectral reflectance. (R(xcex)).
Although such a method can be applied to any of various workpieces bearing one or more surficial thin layers, the method is especially applicable to semiconductor wafers having one or more surficial thin-film layers including, for example, one or more electrode layers, metal layers, and insulator layers.
Any of various parameters of the spectral characteristic can be used for determining the thickness of a layer. For example, the parameter can be a local maximum of the spectral-characteristic signal (wherein a xe2x80x9clocal maximumxe2x80x9d is a peak appearing in a plot of the spectral characteristic), a local minimum of the spectral-characteristic signal (wherein a xe2x80x9clocal minimumxe2x80x9d is a valley appearing in a plot of the spectral characteristic), a difference of a selected local minimum from a selected local maximum, or a quotient of a selected local minimum to a selected local maximum. Other possible parameters include a largest local maximum of the spectral-characteristic signal, a smallest local minimum of the spectral-characteristic signal, a difference of the largest local minimum from the largest local maximum, and a quotient of the smallest local minimum to the largest local maximum. Yet other parameters include a spectral dispersion of the spectral-characteristic signal (wherein a xe2x80x9cspectral dispersionxe2x80x9d is a variance of the spectral characteristic) and a component of a Fourier transform of the spectral-characteristic signal.
The signal light that is detected can be probe light reflected from the surface of the workpiece or transmitted through the workpiece.
As summarized above, various conventional methods for optically measuring the thickness of a thin film are known. In conventional methods employing interference phenomena, the requisite high accuracy is achieved only when the methods are used to measure the thickness of a blank film (i.e., a uniformly flat film). Methods according to the present invention (such as the method summarized above), in contrast, can be utilized for measuring not only the thickness of a blank film but also the thickness of, e.g., a layer on a wafer having a device pattern (substrate pattern) that is two-dimensionally non-uniform. In such an application instance, the obtained signals differ substantially from the signals obtainable from a blank film.
In methods according to the invention, such as those summarized above, signal light that is reflected from or transmitted through the workpiece are processed to obtain a measure of a parameter that desirably rapidly changes when the process is at or near a process endpoint.
According to another aspect of the invention, in a process for progressively reducing a thickness of a thin-film layer on a surface of a substrate, methods are provided for detecting a process endpoint representing a minimum desired thickness of the thin-film layer. According to a representative embodiment of such a method, a probe light is directed onto a region of a surface of the thin-film layer to produce a signal light propagating from the thin-film layer. The signal light is detected and a spectral characteristic of the signal light is measured from the detected signal light so as to produce a spectral-characteristic signal. A cross-correlation function is calculated. The cross-correlation function is of the spectral-characteristic signal with a predetermined reference spectral-characteristic signal. The cross-correlation function exhibits a change with a corresponding change in the thickness of the thin-film layer. From the cross-correlation function, the process endpoint is determined.
According to yet another aspect of the invention, apparatus are provided for determining a process endpoint of a process for reducing a thickness of a thin-film layer on a substrate. A representative embodiment of such an apparatus comprises a source of a probe light, a probe-light optical system, a detector, a signal-light optical system, and a signal processor. The probe-light optical system directs the probe light to a location on a surface of the thin-film layer so as to produce a signal light propagating from the location. The detector detects the signal light, and the signal-light optical system directs the signal light from the location to the detector. The signal processor, which is connected to the detector, measures a spectral characteristic of the signal light from the detected signal light, calculates a parameter of the spectral characteristic that is a function of the thickness of the thin-film layer, and determines the thickness of the thin-film layer from the calculated parameter.
According to yet another aspect of the invention, apparatus are provided for planarizing a surface of a workpiece. A representative embodiment of such an apparatus comprises a polishing pad and a polishing head. The polishing head is configured to support the workpiece and contact the workpiece against the polishing pad. A mechanism is provided to move the polishing pad and the polishing head relative to each other as the workpiece contacts the polishing pad for polishing the workpiece. This apparatus also includes a device for determining a process endpoint as summarized in the preceding paragraph, for example.
According to yet another aspect of the invention, processes are provided for detecting, for example while reducing a thickness of a thin-film layer on a surface of a workpiece, the thickness of the thin-film layer. According to a representative embodiment of such a process, a probe light is directed to a location on the thin-film layer so as to produce a signal light propagating from the location. A signal waveform is produced from the signal light, and a value of a parameter of the signal waveform is calculated. From such a value, a thickness of the thin-film layer is detected. The parameter can be, for example, a difference between a largest local maximum of the signal waveform and a smallest local minimum of the signal waveform. Other exemplary parameters are the smallest local minimum of the signal waveform, a quotient of the smallest local minimum of the signal waveform to the largest local maximum of the signal waveform, and the mean of the signal waveform. An example mean is of spectral reflectance: xcexa3(R(xcex))/n, wherein n is the sample size.
The method summarized in the preceding paragraph solves the problem of optically measuring a process endpoint with a satisfactory degree of accuracy in the case in which a device pattern exists on the wafer surface. It has been discovered that signal light obtained by illuminating a probe light onto the wafer surface is a superposition of a pattern interference component and a film-thickness interference component. It has also been discovered that the magnitude of the pattern-interference component changes according to changes in the device pattern on the wafer. In other words, since the magnitude of the pattern-interference component changes in accordance with changes in the device pattern on the wafer, the signal light contains a degree of uncertainty that corresponds to the device pattern (i.e., the type of device corresponding to the pattern). For example, there is a significant difference in the degree of uncertainty between a DRAM that is a part of a memory, part of a logic device, and part of a device having both a logic component and a memory component. This uncertainty arises because the pattern-interference components of these devices differ significantly from each other due to differences in the degree of integration, even if the devices are of the same type.
Another type of uncertainty by which the magnitude of the pattern-interference component differs is the location on the pattern at which measurement is performed, even among devices of the same type and having the same degree of integration. In a memory device such as a DRAM, the degree of uncertainty of this type is small since the pattern on the memory device can be regarded as a uniform continuation of its periodic structure. However, the pattern on a logic device or a device having a logic component and a memory component is not uniform. Hence, in such a device, the degree of this type of uncertainty is high. It has been discovered that such types of uncertainties are major causes of large errors encountered in the detection of process endpoints and the like in the prior art. According to the method summarized above, the causes of such uncertainty are alleviated by specifying a position for measurements of the device pattern based on a suitable parameter obtained from the signal-light waveform.
The method summarized above can further comprise the step of providing a reference value of the parameter corresponding to a reference thickness of the thin-film layer. The actual measured thickness of thin-film layer can be compared with the reference value to obtain a comparison value. From the comparison value, a process endpoint can be calculated, wherein the process endpoint represents a point at which to cease reducing the thickness of the thin-film layer. The method can also further comprise the step of specifying on the thin-film layer a measurement position that includes the location. In such a method, the measurement of the value of the parameter is performed at the measurement position. A reference value of the parameter, corresponding to a reference thickness of the thin-film layer at the measurement position, can be provided. In such an instance, the actual measured thickness of the thin-film layer can be compared with the reference value to obtain a comparison value. From the comparison value, a process endpoint can be calculated at which to cease reducing the thickness of the thin-film layer.
According to a further modification of the method, an optical signal (e.g., a reflectance signal) is obtained from a desired measurement position on the surface of the workpiece. From the optical signal, a thickness of the thin-film layer is calculated. For example, thickness can be algebraically calculated from one or more local maxima in a plot of the optical signal, or by fitting a measured waveform to a pre-calculated waveform.
The calculated thickness is compared with a reference thickness at the measurement position so as to determine a process endpoint at which to cease reducing the thickness of the thin-film layer.
According to yet another aspect of the invention, methods are provided, for use in processes for reducing the thickness of a thin-film layer on an integrated circuit device formed on a surface of a semiconductor wafer, for detecting the thickness of the thin-film layer. According to a representative embodiment of such a method, a probe light is directed to a location on the thin-film layer so as to produce a signal light propagating from the location. The signal light is produced either by reflection of probe light from the thin-film layer or transmission of probe light through the thin-film layer. All orders of diffracted light are removed from the signal light except a zeroth order of diffracted light from which a signal waveform is produced. A value of a parameter of the signal waveform is calculated, from which value a thickness of the thin-film layer is calculated. The higher orders (i.e., greater than zeroth order) of diffracted light are removed by passing the signal light from the location through an aperture (which can be variable-sized) defined by an aperture plate, wherein the aperture plate blocks the higher orders of diffracted light. Alternatively, a two-dimensionally distributed measurement of a spot pattern of the signal light can be provided while blocking the higher orders of signal light. (A two-dimensional spot pattern can be obtained by, e.g., detecting signal light using a sensor comprising two-dimensionally distributed sensor elements, such as a CCD panel.)
With respect to the method summarized above, a conventional cause of difficulty encountered while attempting to detect the thickness of a film on the device pattern on a wafer with satisfactory accuracy is addressed. It has been discovered that reflected signal light obtained by illuminating a probe light onto the device pattern exhibits many diffraction spots caused by a diffraction of light reflecting from a device pattern having regular fine features (wherein diffraction spots are generated from a two-dimensionally distributed pattern, in comparison to diffraction fringes generated from a one-dimensionally distributed pattern). Each of the diffraction spots changes, in a different manner depending on the film thickness, for each order of diffraction. Thus, the obfuscating influence of higher diffraction orders is eliminated.
According to yet another aspect of the invention, apparatus are provided for determining a process endpoint of a process for reducing a thickness of a thin-film layer on a substrate. A representative embodiment of such an apparatus comprises a source of probe light, a probe-light optical system, a detector, a signal-light optical system, an aperture-defining plate, and a signal processor. The probe-light optical system directs the probe light to a location on a surface of the thin-film layer so as to produce a signal light propagating from the location. The detector detects the signal light that passes from the location through the signal-light optical system to the detector. The aperture plate is situated in the signal-light optical system, wherein the aperture (which can be variable) functions to remove all orders of diffracted light from the signal light except zeroth-order reflected light. The signal processor, to which the detector is connected, measures a spectral characteristic of the signal light from the detected signal light. The signal processor then calculates a parameter of the spectral characteristic that is a function of the thickness of the thin-film layer, and determines the thickness of the thin-film layer from the calculated parameter. This apparatus can be incorporated into an apparatus for planarizing the surface of a workpiece such as a wafer.
According to yet another aspect of the invention, methods are provided for measuring the thickness of at least one of an insulating layer and a metal electrode layer on a surface of a semiconductor device undergoing a process in which the layer (imprinted with a pattern) is being reduced in thickness (such as CMP). According to a representative embodiment of such a method, a probe light is illuminated onto at least a portion of a surface of the layer on the wafer so as to produce a signal light (e.g., an optical interference pattern) propagating from the layer. The signal light is detected, an intensity profile of the signal light is measured (desirably relative to wavelength), and a spatial coherence length of the signal light is determined. The spatial coherence length of the signal light is compared with a degree of fineness of the pattern illuminated by the probe light, and an optical model is determined based on the comparison. (The optical model is essentially a simulated distribution of features on the surface of the device, based on the particular waveform of signal light propagating from the actual surface of the device. The optical model is generated under the premise that it produces the same waveform of signal light as an actual corresponding device under the same incidence conditions of probe light. That is, signal light produced by the actual device and signal light produced by a surface according to the optical model would be substantially the same if the incidence conditions are the same.)
Based on the optical model, a theoretical intensity profile of the signal light is determined. The thickness of the layer and/or a process endpoint is determined by comparing the measured intensity profile of the signal light with the theoretical intensity profile of signal light. In such a method, the spatial coherence length of the probe light can be varied, desirably according to the degree of fineness of the pattern. The calculated theoretical intensity profile of signal light can be stored for later recall. The theoretical intensity profile can be calculated for a thickness of multiple films having an inter-film distance therebetween, wherein the comparison is made based on a similarity between the calculated theoretical intensity profile of signal light and the measured change in the signal-light intensity profile. Alternatively, a cross-correlation coefficient of the theoretical intensity profile of signal light and the measured intensity profile of signal light can be calculated. In the latter instance, the comparison can be made based on a similarity between a cross-correlation coefficient of a Fourier transform of the theoretical intensity profile of signal light and the measured intensity profile of signal light, and a position and magnitude of a Fourier component of the calculated theoretical intensity profile of signal light and a position and magnitude of a Fourier component of the measured intensity profile of signal light.
According to yet another aspect of the invention, apparatus are provided (in the context of an apparatus for planarizing a surface on a semiconductor wafer imprinted with a semiconductor device) for measuring a thickness of a layer on a surface of the semiconductor device imprinted on the wafer so as to provide a planarizing process endpoint. According to a representative embodiment, the apparatus comprises an illumination system, a measuring system, a numerical calculation system, and a detection system. The illumination system illuminates a probe light onto a portion of the surface of the layer on the wafer so as to produce a signal light propagating from the surface. The measuring system measures a change in an intensity of the signal light. The numerical calculation system is connected to the measuring system and calculates a theoretical intensity profile of signal light based on an optical model. The optical model is based on a comparison of a spatial coherence length of the probe light with a degree of fineness of a pattern for the semiconductor device illuminated with the probe light. The detection system detects at least one of a layer thickness and the process endpoint by comparing the measured intensity profile of signal light with the calculated theoretical intensity profile of signal light. A controller can be connected to the numerical calculation system and employed to control the spatial coherence length of the probe light. The controller can comprise a storage system (e.g., memory) that stores data concerning the calculated theoretical intensity profile of signal light. The detection system performs the comparison using a cross-correlation coefficient of the calculated theoretical intensity profile of the signal light and a measured intensity profile of the signal light. The detection system also performs a similarity comparison using a cross-correlation coefficient of a Fourier transform of the calculated theoretical intensity profile of the signal light and the measured intensity profile of the signal light, and/or a cross-correlation coefficient of a position and magnitude of a Fourier component of the calculated theoretical intensity profile of the signal light and a position and magnitude of a Fourier component of the measured intensity profile of the signal light.
The foregoing and other features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.