Patent Publication Number: US-6669539-B1

Title: System for in-situ monitoring of removal rate/thickness of top layer during planarization

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to planarization in a chemical mechanical polishing process, and more particularly to in-situ monitoring of removal rate and thickness of a top layer during planarization. 
     2. Description of the Related Art 
     The semiconductor industry is continually striving to improve the performance of semiconductor devices, while still attempting to reduce the cost of these same devices. These objectives have been successfully addressed by the ability of the semiconductor industry to practice micro-miniaturization, or to fabricate semiconductor devices with sub-micron features. Several fabrication disciplines, such as photolithography, as well as dry etching, have allowed micro-miniaturization to be realized. The use of more sophisticated exposure cameras, as well as the use of more sensitive photoresist films, have allowed the attainment of sub-micron images in photoresist films, to be routinely achieved. In addition, the development of more advanced dry etching tools and processes, have allowed the sub-micron images, in masking photoresist films, to be successfully transferred to underlying materials used for the fabrication of semiconductor devices. 
     Integrated circuits are chemically and physically integrated onto a substrate, such as a silicon substrate, by patterning conductive regions in the substrate and by patterning conductive and insulation layers over the substrate. The various conductive and insulation layer create uneven surfaces on a semiconductor structure. Interlevel dielectric (ILD) layers are formed between conductive layers (e.g., metal or polysilicon) in a semiconductor device or between conductive lines formed from the same conductive layer (in the same level). Contact holes are formed through the ILD layers to make electrical contact with conductive layers and device regions there below. A typical ILD stack of oxides is shown with reference to FIG.  1 . 
     FIG. 1 is a diagram showing a prior art ILD based structure  100 . The prior art ILD based structure  100  includes a first oxide layer  102  upon which a metal line  104  has been formed. Over these is formed a first film of conformal oxide  106 . The first conformal oxide film  106  typically is formed using a plasma enhanced chemical vapor deposition (PECVD) process in order to deposit the film  106  such that it conforms to the topography on the surface of the wafer. 
     A second oxide film  108 , which is also highly conformal, is deposited over the first conformal film  106  to fill any gaps between the metal lines  104 . A cap-oxide layer  110 , which is thicker than the other oxide layers, is deposited over the second oxide film  108 . During a chemical mechanical polishing (CMP) process, most of the cap-oxide layer is removed or polished away. In particular, the process control is required to monitor the thickness of the cap-layer  110  and stop the CMP process at a predefined thickness. 
     In view of the foregoing, there is a need for systems and methods for efficiently polishing oxide layers during ILD CMP processes. The methods should provide fast and efficient removal of the cap-oxide layer to a predetermined thickness. 
     SUMMARY OF THE INVENTION 
     Broadly speaking, the present invention fills these needs by providing a two step polishing process having fast and slow removal rates, respectively. To increase accuracy during the first portion of the polishing process, embodiments of the present invention provide in-situ monitoring of the removal rate and thickness of a top wafer layer during planarization. In one embodiment, a method is disclosed for removing a top wafer layer during a CMP process. Time series data is collected based on a reflected wavelength from a top layer of a wafer. A frequency of peak intensities in the time series data is used to determine a removal rate of the top layer, and the removal rate is used to calculate a current thickness of the top layer. The CMP process is discontinued when the current thickness of the top layer is equal to or less than a target thickness, and a separate polishing process is performed to remove an additional portion of the top layer. The frequency can be determined by applying a Fourier Transform to the time series data. The Fourier Transform of the time series data can be analyzed to determine a peak magnitude in the frequency, which corresponds to the frequency of peak intensities in the time series data. The removal rate for top layer can be calculated based on the peak magnitude in the frequency, which can be used to calculate the current thickness of the top layer. 
     In another embodiment, a system is disclosed for removing a top wafer layer during a CMP process. The system includes a light source for illuminating a top layer of a wafer, and an optical detector for collecting time series data based on a reflected wavelength from the top layer. Further included in the system is logic that determines a removal rate of the top layer based on a frequency of peak intensities in the time series data, and logic that calculates a current thickness of top layer based on the removal rate. A process controller is also included that discontinues the CMP process when the current thickness of the top layer is equal to or less than a target thickness. Optionally, the system can include an endpoint detection subsystem that performs a separate polishing process to remove an additional portion of the top layer. As above, the logic can apply a Fourier Transform to the time series data to determine the frequency, which can be analyzed by addition logic to calculate a removal rate for top layer based on a peak magnitude in the frequency. 
    
    
     A further method for removing a top wafer layer during a CMP process is disclosed in another embodiment of the present invention. As above, time series data is collected based on a reflected wavelength from a top layer of a wafer. A Fourier Transform is applied to the time series data, and a frequency of peak intensities in the Fourier Transform of the time series data is analyzed to determine a peak magnitude in the frequency. A first removal rate of the top layer is determined based on the peak magnitude in the frequency, and a current thickness of top layer is calculated based on the first removal rate. The CMP process is discontinued when the current thickness of the top layer is equal to or less than a target thickness, and a separate polishing process is performed to remove an additional portion of the top layer. In one aspect, the separate polishing process can be based on a soft endpoint detection process having second removal rate that is lower than the first removal rate. Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with further advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a diagram showing a prior art ILD based structure; 
     FIG. 2 is a flowchart showing a method for in-situ monitoring of removal rate and thickness of a top layer during ILD planarization, in accordance with an embodiment of the present invention; 
     FIG. 3A is a diagram showing an ILD based structure polished in accordance with an embodiment of the present invention; 
     FIG. 3B is a flowchart showing an exemplary method for performing a soft planarization process based on endpoint detection, in accordance with an embodiment of the present invention; 
     FIG. 3C is a graph showing a sample trace having an upswing at the start of the soft planarization process, in accordance with an embodiment of the present invention; 
     FIG. 3D is a graph showing a sample trace having a downswing at the start of the soft planarization process, in accordance with an embodiment of the present invention; 
     FIG. 4 is flowchart showing a method for in-situ monitoring of removal rate and thickness of a top layer during a high removal rate ILD planarization process, in accordance with an embodiment of the present invention; 
     FIG. 5A shows a CMP system in which a pad is designed to rotate around rollers, in accordance with an embodiment of the present invention; 
     FIG. 5B is an illustration showing an endpoint detection system, in accordance with an embodiment of the present invention; 
     FIG. 6 is an intensity graph  600  showing the intensity of a single wavelength λ as a function of time, in accordance with an embodiment of the present invention; 
     FIG. 7 is flowchart showing a method for preprocessing and applying a Fourier Transform to the time series data, in accordance with an embodiment of the present invention; 
     FIG. 8 is a frequency graph showing the intensity as a function of cycle frequency, in accordance with an embodiment of the present invention; and 
     FIG. 9 is an estimated thickness graph, in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An invention is disclosed for in-situ monitoring of removal rate/thickness of a top layer during an ILD planarization process. To this end, embodiments of the present invention determine the removal rate of the top layer and perform a two step process for ILD planarization, which includes a fast removal rate process and a soft process with a lower removal rate. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without some or all of these specific details. In other instances, well known process steps have not been described in detail in order not to unnecessarily obscure the present invention. 
     FIG. 1 was described in terms of the prior art. FIG. 2 is a flowchart showing a method  200  for in-situ monitoring of removal rate and thickness of a top layer during ILD planarization, in accordance with an embodiment of the present invention. In an initial operation  202 , preprocess operations are performed. Preprocess operations can include determining an initial thickness of the top layer, defining a target thickness, defining a final thickness, and other preprocess operations that will be apparent to those skilled in the art after a careful reading of the present disclosure. 
     In operation  204 , a fast planarization process is performed based on a calculated cap-oxide removal rate. Embodiments of the present invention utilize a two-step process to polish the surface of a wafer. In the first step, operation  204 , a fast process having a high removal rate is used to polish the cap-oxide layer to a predefined target thickness within a prescribed tolerance, as described next with reference to FIG.  3 A. 
     FIG. 3A is a diagram showing an ILD based structure  300  polished in accordance with an embodiment of the present invention. The ILD based structure  300  includes a first oxide layer  102  upon which a metal line  104  has been formed. Over these is formed a first film of conformal oxide  106 . The first conformal oxide film  106  typically is formed using a PECVD process in order to deposit the film  106  such that it conforms to the topography on the surface of the wafer. 
     A second oxide film  108 , which is also highly conformal, is deposited over the first conformal film  106  to fill any gaps between the metal lines  104 . A cap-oxide layer  110 , which is thicker than the other oxide layers, is deposited over the second oxide film  108 . During a CMP process, most of the cap-oxide layer is removed or polished away. In particular, the process control is required to monitor the thickness of the cap-layer  110  and stop the CMP process at a predefined thickness. To increase production, embodiments of the present invention separate the process into two steps. The first step polishes the cap-oxide layer  110  down to a predetermined target thickness T 304  within a defined tolerance band, by polishing away a top portion  302   a  of the cap-oxide layer  110 . 
     Referring back to FIG. 2, a soft planarization process is performed based on endpoint detection, in operation  206 . As mentioned above, embodiments of the present invention utilize a two-step process to polish the surface of a wafer. In the second step, operation  206 , a slow process having a low removal rate is used to polish the cap-oxide layer to a predefined final thickness. 
     FIG. 3B is a flowchart showing an exemplary method  350  for performing a soft planarization process based on endpoint detection, in accordance with an embodiment of the present invention. In one embodiment, the expected variations of incoming thickness at the second operation  206  in FIG. 2 is mean ±λ/4n, where λ is the probing wavelength used in the second step and n is the refractive index of the top layer. Hence, the first operation  204  in FIG. 2 generally should meet these criteria. 
     In operation  352 , sample data is acquired using a probing wavelength that provides a trace characterized by cosine wave shifted by π radians. Generally, the sample data is acquired by capturing reflectance data from a light source directed at the surface of the wafer. A decision is then made as to whether the slope of the trace is greater than zero, in operation  354 . If the slope of the trace is greater than zero the method  350  continues to operation  356 . Otherwise the method  350  branches to operation  358 . 
     In operation  356 , a decision is made as to whether a predefined slope threshold has been reached. When the slope of the trace is greater than zero, the trace has an upswing at the start of the soft planarization process. In this case, embodiments of the present invention stop the soft planarization process at an appropriate point on the down swing of the trace, which follows immediately after the upswing of the slope. FIG. 3C is a graph  370  showing a sample trace  372   a  having an upswing at the start of the soft planarization process, in accordance with an embodiment of the present invention. As shown in FIG. 3C, the slope  374   a  of the sample trace has an upward swing. As mentioned above, embodiments of the present invention stop the soft planarization process at an appropriate point on the down swing of the trace  372   a , which follows immediately after the upswing of the slope  374   a , for example, at point  376   a . Hence, if the predefined slope threshold has been reached the method  350  terminates in operation  360 . Otherwise, the method  350  continues with another data sample acquisition operation  352 . 
     Referring back to operation  358  of FIG. 3B, a decision is made as to whether an upswing flag is set. Generally, embodiments of the present invention utilize an upswing flag to record whether an upswing in the slope has occurred. If the upswing flag is set, an upswing has already been detected and the method  350  continues with operation  356 . Otherwise, an upswing has not been detected and the method  350  continues to operation  360 . 
     A decision is made as to whether the current slope of the trace is greater than zero, in operation  360 . When operation  360  is first reached, a downward or horizontal slope has been detected in the trace. In this case, embodiments of the present invention wait until an upswing in the slope occurs. FIG. 3D is a graph  380  showing a sample trace  372   b  having a downswing at the start of the soft planarization process, in accordance with an embodiment of the present invention. As shown in FIG. 3D, the slope  374   b  of the sample trace has a downward swing. As explained below, embodiments of the present invention stop the soft planarization process at an appropriate point on the downswing of the trace  372   b , which follows immediately after the upswing of the slope  374   b , for example, at point  376   b . Hence, if the current slope of the trace is not greater than zero, the method  350  continues with another sample acquisition operation  352 . However, if the current slope of the trace is greater than zero, the method  350  continues to operation  362 . 
     Referring back to FIG. 3B, the upswing flag is set in operation  362 . As mentioned above, embodiments of the present invention utilize an upswing flag to record whether an upswing in the slope has occurred. As with operation  360 , when operation  362  is first reached, a downward or horizontal slope has been detected in the trace. In this case, embodiments of the present invention wait until an upswing in the trace slope occurs and then terminate the soft planarization process when a downswing in the trace slop is detected after the upswing. Hence, after setting the upswing flag, in operation  362 , the method  350  continues to operation  356 , where the slop is compared to the slop threshold. 
     Turning to FIG. 3A, the slow planarization process is used to polish the remaining cap-oxide layer  110  down to a final thickness T 306 , by polishing away a second portion  302   b  of the cap-oxide layer  110 . If the thickness T 304  varies greatly at the beginning of operation  206 , and in particular, which does not meet the tolerance requirement mentioned above, cycle aliasing can occur. As a result, false endpoints can be detected. However, using the embodiments of the present invention, the thickness at the beginning of operation  206  is known, namely the target thickness T 304 . Thus, embodiments of the present invention avoid cycle aliasing and therefore can provide better control during the soft planarization process. In this manner, embodiments of the present invention polish the cap-oxide layer  110  allowing a final portion  302   c  of the cap-oxide layer  110  having a final thickness of T 306  to remain. However, it should be noted that embodiments of the present invention can be utilized to allow any thickness of the cap-oxide layer  110  to remain, including removing all the cap-oxide layer  110 , resulting a no final portion  302   c  remaining after polishing. 
     Referring to FIG. 2, post process operations are performed in operation  208 . Post process operations can include further ILD processing, further wafer etch, and other post process operations that will be apparent to those skilled in the art after a careful reading of the present description. Embodiments of the present invention can divide operations  204  and  206  between polishing stations, which can allow increased overall throughput. To remove the cap-oxide layer using a high removal rate, embodiments of the present invention monitor, in real-time, the removal rate and compute the thickness of the cap-oxide layer, as described in greater detail next with reference to FIG.  4 . 
     FIG. 4 is flowchart showing a method  400  for in-situ monitoring of removal rate and thickness of a top layer during a high removal rate ILD planarization process, in accordance with an embodiment of the present invention. In an initial operation  402 , preprocess operations are performed. Preprocess operations can include determining an initial thickness of the top layer, defining a target thickness, and other preprocess operations that will be apparent to those skilled in the art after a careful reading of the present disclosure. 
     In operation  404 , a data buffer is created for time series data at a particular wavelength λ. Embodiments of the present invention utilize a reflectometery apparatus, for example a broad band reflectometery apparatus to capture time series data. Specifically, a fiber bundle periodically carries a pulse, or flash, of white light from a lamp source and delivers the flash to the surface of a wafer through an opening of the polishing belt using a triggering mechanism. Reflected light from the wafer is then collected and passed through a further fiber bundle to a spectrometer, which disperses the reflected light into various wavelength components. The intensity at each wavelength is then digitized and delivered to an on-board computer for further processing. 
     For example, FIG. 5A shows a CMP system in which a pad  550  is designed to rotate around rollers  551 , in accordance with an embodiment of the present invention. A platen  554  is positioned under the pad  550  to provide a surface onto which a wafer will be applied using a carrier  552 . Time series data is obtained using an optical detector  560  in which light is applied through the platen  554 , through the pad  550  and onto the surface of the wafer  500  being polished, as shown FIG.  5 B. In order to apply light to the wafer, a pad slot  550   a  is formed into the pad  550 . In some embodiments, the pad  550  may include a number of pad slots  550   a  strategically placed in different locations of the pad  550 . Typically, the pad slots  550   a  are designed small enough to minimize the impact on the polishing operation. In addition to the pad slot  550   a , a platen slot  554   a  is defined in the platen  554 . The platen slot  554   a  is designed to allow the broad band optical beam to be passed through the platen  554 , through the pad  550 , and onto the desired surface of the wafer  500  during polishing. 
     FIG. 6 is an intensity graph  600  showing the intensity of a single wavelength λ as a function of time, in accordance with an embodiment of the present invention. As illustrated in FIG. 6, the intensity at wavelength λ varies over time as a result of changing optical interference caused by layer thickness changes. Specifically, at particular thicknesses constructive optical interference occurs creating peaks  602  in the intensity graph  600  of wavelength λ, and at other thicknesses destructive optical interference occurs creating valleys in the intensity graph  600  of wavelength λ. Hence, to a first order approximation, assuming no contribution from a patterned surface, the time variation of the reflected wave at wavelength λ as the thickness of the top layer decreases do to polishing is described by the following equation: 
     
       
           R ( t )= r   A   +r   B   e   0   −i·2(d·r·t)n·2π/λ ,  (1) 
       
     
     where r A  is a constant bias and r B  is a scaling factor, both of which are determined as products of Fresnel&#39;s coefficients. In addition, r is the removal rate of the cap-oxide layer as the polishing proceeds, and d 0  is the initial thickness of the cap-oxide layer. As can be seen from the intensity graph  600 , the reflectance at a given wavelength λ is approximately sinusoidal in time of monotone frequency. However, it should be noted that complex patterned structures can result in signals with multiple sinusoidals. 
     The amount of oxide removed during a particular time period can be determined by examining the peaks  602  in the intensity graph  600 . The interval between the peaks in the intensity graph  600  represents a cycle. In particular, the amount of oxide removed during a single cycle, which is the time period between time t 1  and time t 2 , is given by the following equation: 
     
       
         Thickness_removed/cycle=λ/2 n  Å,  (2) 
       
     
     where λ is the probing wavelength and n is the refractive index. To determine the removal rate of the cap-oxide, the Thickness removed per cycle determined in equation (2) above can be divided by t 2 −t 1 , which is the time period of the cycle. To ensure adequate data is acquired to perform the removal rate calculation, calculations are delayed by a preset delay, during which thickness calculations are not performed. The preset delay time ensures that at least one cycle of the time series data is acquired before a reliable estimation of the removal rate is performed. 
     Although the two peak analysis technique discussed above can be used to determine the removal rate, it can be subject to errors caused by noise and other unwanted interference occurring during the data capturing process. Hence, embodiments of the present invention utilize a large number of peaks  602  to determine the removal rate of the cap-oxide. In particular, embodiments of the present invention utilize a Fourier Transform to facilitate calculation of the removal rate of the cap-oxide, as described next with reference to FIG.  4 . 
     Hence, the time series data is preprocessed and a Fourier Transform is applied to the time series data, in operation  406 . Embodiments of the present invention estimate the real-time frequency of the time series data and extract the removal rate from the estimated frequency. To achieve this, a discrete Fourier Transform is applied at each time step to the data segment available at that time. Essentially, the discrete Fourier Transform maps the time domain, illustrated in FIG. 6, to frequency space. 
     FIG. 7 is flowchart showing a method  700  for preprocessing and applying a Fourier Transform to the time series data, in accordance with an embodiment of the present invention. In an initial operation  702 , preprocess operations are performed. Preprocess operations can include determining an initial thickness of the cap-oxide layer, obtaining time series data, and other preprocess operations that will be apparent to those skilled in the art after a careful reading of the present disclosure. 
     In operation  704 , the time series segment is filtered. During the polishing process, light transmission through a dirty medium consisting of slurry and other optical path variations can cause sample to sample variations. To reduce these sample to sample variations, embodiments of the present invention filter the time series segment data. In one embodiment, a moving average filter is used to reduce noise occurring in the optical data. 
     The time series data is de-trended in operation  706 . Specifically, a quadratic curve is fitted to the time series data segment and subtracted from the signal to remove any linear or quadratic behavior in the data segment. De-trending stretches out the time series data curve by fitting a polynomial to the time series data curve and then subtracting out the polynomial. In this manner, the time series data curve begins essentially flat, thus allowing for easier detection of peaks during Fourier Transform. 
     In operation  708 , spectral smoothing is applied to the time series data. Spectral smoothing reduces spectral leakage introduced by discontinuities at the edges of the time series segment, which generally occur when the reflected time series data contains a non-integer number of cycles or oscillations. Zero padding is then applied to the time series data in operation  710 . Zero padding of the time series data helps to zoom the Fourier Transform onto a higher resolution grid. This procedure essentially does an interpolation of the Fourier Transform on to a finer grid. This, in turn, enables increased accuracy in peak detection. In one embodiment, Zero padding is performed by extending the number of discrete pixels of the reflected spectrum to a much larger grid. Any pixels in the extended grid not covered by the actual acquired data are can be filled with a value of zero. 
     In operation  712 , a Fourier Transform is applied to the Time series Segment. A mentioned above, embodiments of the present invention estimate the real-time frequency of the time series data and extract the removal rate for the cap-oxide from the estimated the real-time frequency. To estimate the frequency, a discrete Fourier Transform is applied to the data segment at each time step. The discrete Fourier Transform maps the time domain signal to the frequency space. FIG. 8 is a frequency graph  800  showing the intensity as a function of cycle frequency, in accordance with an embodiment of the present invention. The frequency graph  800  maps the intensity shown in FIG. 6 to the frequency space. Thus, the intensity of the time series data at wavelength λ is shown as a function of the cycle frequency. 
     Referring back to FIG. 4, a peak search of the Fourier magnitude in frequency space is performed in operation  408 . Turning to FIG. 8, embodiments of the present invention examine the frequency graph  800  to determine at what frequency the peak intensity magnitude  802  occurs. The peak intensity magnitude  802  indicates the frequency of the peaks  602  in the intensity graph  600  of FIG.  6 . As described in greater detail subsequently, the embodiments of the present invention utilize the peak intensity magnitude  802  to determine the removal rate of the cap-oxide in a robust manner. 
     Turning back to FIG. 4, a decision is then made as to whether the current process time is greater than the preset delay, in operation  410 . As mentioned above, removal rate calculations are delayed until a predetermined amount of time series data is obtained over a preset delay period. Once the preset delay has been reached, the time series data is preprocessed, a Fourier Transform is applied, and peak search is performed in the Fourier generated frequency space. Hence, in operation  410 , if the current process time is equal to the preset delay, the method  400  estimates the amount of cap-oxide removed during the preset delay period in operation  412 . Otherwise, the method  400  calculates the current thickness based on the current removal rate in operation  414 . 
     In operation  412 , the amount of cap-oxide removed during the preset delay period is calculated. As mentioned above, a preset delay is utilized to ensure at least one cycle of the time series data is acquired before an estimation of the removal rate is performed. Since the thickness of the cap-oxide layer is unknown at the preset delay time, embodiments of the present invention estimate the thickness of the cap-oxide layer by extrapolating backwards in time based on a removal rate computed at the present delay time using the following equation: 
     
       
           d ( t   preset     —     delay )= d   0   −r ( t   preset     —     delay )· t   preset     —     delay ,  (3) 
       
     
     where d(t preset     —     delay ) is the thickness at the preset delay time, d 0  is the initial thickness of the cap-oxide layer, t preset     —     delay  is the preset delay time, and r(t preset     —     delay ) is the removal rate at the preset delay time, as illustrated next in FIG.  9 . 
     FIG. 9 is an estimated thickness graph  900 , in accordance with an embodiment of the present invention. The estimated thickness graph  900  shows the thickness of the cap-oxide layer as a function of time. In particular, the estimated thickness graph  900  shows the thickness of the cap-oxide layer between initial time to and the preset delay time t preset     —     delay . Embodiments of the present invention calculate the removal rate at time t preset     —     delay  using the Fourier Transform peak analysis. In particular, the frequency of the time series is determined by analyzing the magnitude peak of the Fourier Transform graph, as described previously with reference to FIG.  8 . For example, in FIG. 8, the peak intensity magnitude  802  indicates the frequency of the peaks  602  in the intensity graph  600  of FIG.  6 . Hence, in the example of FIG. 8, the peak magnitude frequency  802  of the peaks in time series data is 10 cycles per second. The removal rate at time t preset     —     delay  can then be determined using the following equation: 
     
       
           r ( t   preset     —     delay )=frequency·(λ/2 n ),  (4) 
       
     
     where r(t preset     —     delay ) is the removal rate at the preset delay time, frequency is the frequency of the peaks in the time series data, λ is the probing wavelength, and n is the refractive index. The thickness at time t preset     —     delay  can then be estimated by assuming that the removal rate during the time period from t 0  to t preset     —     delay  is r(t preset     —     delay ). Thus, the embodiments of the present invention estimate the thickness at time t preset     —     delay  by multiplying the removal rate at the preset delay time by the preset delay time and subtracting the product from the initial thickness, as shown in equation (3) above. It should be noted that initial thickness information can be obtained using an inline metrology tool. Referring back to FIG. 4, the method  400  continues to collect time series data in operation  404  after calculating the amount of cap-oxide removed during the preset delay period in operation  412 . 
     In operation  414 , the current thickness is calculated based on the current removal rate. Once the thickness at time t preset     —     delay  has been calculated, the method  400  branches to operation  414 , where the current thickness is calculated based on the current removal rate and the previous estimate of thickness iteratively. In particular, the top layer is initialized to d(t preset     —     delay ), as follows: 
     
       
         Current_thickness initial   =d ( t   preset     —     delay ),  (5) 
       
     
     where d(t preset     —     delay ) is the thickness at the preset delay time. Then, at any later point in time the current thickness can be determined as follows: 
     
       
         Current_thickness= d   previous     —     estimate −( r   previous     —     estimate ·sampling interval),  (6) 
       
     
     where d previous     —     estimate  is the previous thickness estimate, and r is the current estimate of the removal rate. A decision is then made as to whether the current thickness is equal to the target thickness, in operation  416 . As mentioned previously with respect to FIG. 2, embodiments of the present invention utilize a fast polishing process based on the cap-oxide removal rate before transferring the wafer to a soft process based on endpoint detection. As illustrated in FIG. 3, the first step polishes the cap-oxide layer  110  down to a predetermined target thickness T 304 , by polishing away a top portion  302   a  of the cap-oxide layer  110 . Referring back to FIG. 4, if the current thickness is equal to the target thickness, the method  400  is completed in operation  418 . Otherwise, the method  400  continues to collect time series data in operation  404 . 
     Post process operations are performed in operation  418 . Post process operations can include performing a soft polishing process based on endpoint detection, further wafer processing, and other post process operations that will be apparent to those skilled in the art after a careful reading of the present description. In this manner, the embodiments of the present invention can provide efficient polishing of oxide layers during ILD CMP planarization. 
     Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalents of the appended claims.