IES20070064A2 - Method and apparatus for measuring the endpoint of a plasma etch process - Google Patents

Abstract

A method for detecting the endpoint of a plasma etch process being performed on a semiconductor wafer is provided. The light being generated from the plasma is detected. The detected light is filtered to extract modulated light. The detected modulated light is then processed to determine when the endpoint of the etch process has been reached. An indicator is generated when the endpoint has been determined. <Figure 7>

Description

The present invention relates to plasma etch processes. More‘~p; invention relates to a method and an apparatus for determining the process endpoint of a plasma etch process on a semiconductor wafer of a particular wafer batch by monitoring the modulation intensity of emitted radiation from the plasma.

Background of the Invention One of the main processes involved in semiconductor manufacturing is the etching of the semiconductor. A typical etch process requires plasma discharge to remove a patterned layer of exposed material on the wafer surface.

There are a number of etching processes which are in use by the semiconductor industry. Two commonly used reactors for the etching process are the Capacitive Coupled Plasma (CCP) tool, and the Transformer Coupled Plasma (TCP) tool.

The principles of the etching process may be explained with reference to Figures 1 to 3 of the drawings. Figure 1 shows a cross sectional view of typical CCP processing tool. A vacuum chamber 10 incorporates a bottom electrode 2, on which the wafer or substrate 3 is placed, and a top electrode 7. A gas inlet 8 and an exhaust line 9 are also provided. The chamber also includes a bottom electrode radio frequency (RF) power supply 1.

Figure 2 shows a cross sectional view of a typical TCP processing tool. This processing tool incorporates substantially the same components as the CCP processing tool, but does not include a top electrode. It also includes a second RF power supply 12, an antenna 13 and a dielectric window 6. It is customary to place a matching network between the RF power supplies 1 and 12 and the powered electrode/antenna 070064 (not shown). The purpose of the network is to match the power supply impedance, which is typically 50Ω, to the electrodes/antenna impedance.

Typical operation of such tools is explained in Figure 3, which shows a CCP tool. It involves placing a wafer or substrate 3 on the bottom electrode 2 and igniting the plasma, by the radio-frequency power supply 1 applying a constant amount of energy to the electrode 2 and/or antenna. A constant gas flow of a selection of feedstock gases 11 indicated by arrow x should also be provided which should be pumped at a constant throughput into the chamber.

The etch process results in the removal of material from the wafer 3 by sputtering, chemical etch or reactive ion etch. The removed material is then volatised into the plasma discharge 5. These volatile materials are called etch by-products 4, and, together with the feedstock gases 11, contribute to the chemistry of the plasma discharge 5. The etch by products 4 and the gases 11 are pumped away through the exhaust or pumping port 9, as indicated by arrow y. A TCP tool process works in a similar fashion.

It will be appreciated that it is highly desirable to be able to detect when the etch process has finished, in order to reduce material costs and to avoid damage to the electronic devices under construction.

A number of parameters of the etching process change when the etching process is complete. For example, underneath the top layer of the wafer, another layer of a different chemical composition is provided. If this layer is exposed to the same plasma as the first layer, a change in the chemistry of the discharge will result. The change in chemistry is due to the change in the composition of etch by-products coming from the wafer or substrate surface, as a new layer of material is uncovered and begins to be volatised. This chemical change may affect the power, matching network settings, pressure and plasma optical emission of the etching process. ι 0700 64 The etch processing endpoint may therefore be defined as the time period in which there is a change in any, some, or all of the parameters of the etching process which corresponds to the end of the etching of a layer (such as an unmasked top layer), exposing an underneath layer.

To detect the process endpoint, sensors have been used to monitor the time evolution of one or more of these parameters. These parameters may include not only the physical and chemical processes in the discharge and the surface of the processing wafer described above, but also the plasma tool operating conditions. Other parameters which have been found to change during the etching process include radiofrequency power, gas pressure and flow for various gases, and plasma light intensity at various wavelengths (i.e. Optical Emission Spectroscopy (OES)).

Figure 4 details a graph of an ideal representation of the variation in a process parameter over time during the etch process. It consists of the following five parts: 1. The initial transient (IT) area, when the discharge is turned on. 2. The main etch (ME) area, when the unmasked material on the wafer is continuously etched. 3. The endpoint (EP) area, which is the transition from the main etch to the overetch. The endpoint begins when the material being etched starts to be cleared from the wafer. 4. The over-etch (OE) area, which is when most or all of the material has been removed from the wafer and the discharge continues etching the following layers. In many cases it is critical to avoid over-etch.

. The final transient (FT) area, which occurs when the discharge is turned off.

It will be appreciated that for an ideal signal of a parameter of the etching process, the main etch is a continuous process, with the endpoint being identified by a sudden change in the level of the signal. The over-etch of an ideal signal is a uniform process. In an ideal signal, the endpoint is therefore typically seen as a sharp fall in the intensity of the signal. This corresponds to a depletion of the etch-by-products that caused the signal. However, it could also be a rise in the signal, for example possibly 00 64 due to an increase in other species in the plasma that were initially depleted by the etch-by-products.

As the chemistry of the process is affected by the material being etched on the wafer, one would expect that when the layer is completely removed there would be a simultaneous change in the chemistry of the discharge. However, during a real etching process, it will be appreciated that the wafer may not be etched uniformly over all its area, and this does not follow the ideal representation of Figure 4. Accordingly, the etched layer may be removed in some areas of the wafer before others. Therefore, in a real signal of a process parameter, the endpoint is not a sharp fall or rise, but a transition from the main etch to the over-etch in a certain amount of time. This is illustrated in Figure 5, where the real etch signal has a fall endpoint over a period of time At. It should also be noted that the parameters may also have a complex time structure associated with various changes through the process, not all of which are associated with the endpoint, e.g. multi-step etch process. Therefore, the determination of the endpoint must be carefully analysed with the corresponding signal change observed by the tool monitoring sensors.

In some cases, one of the parameters of the etching process is sufficient for use as a process monitor signal for monitoring the endpoint of the plasma process, as it is able to detect a change clearly enough. However, a real signal may also contain a fair amount of noise, and in some cases a drift. A poor signal to noise ratio and/or a strong drift may result in poor sensitivity to endpoint detection algorithms. These are the main problems in low open area situations where only a small fraction of the wafer is etched (1 to 0.5 % of the total area). Where this is the case, a number of parameters can be used as process monitor signals. These process monitor signals can then be combined to condense the process evolution into a single monitor signal using multivariate analysis techniques (MVA). MV A techniques are well known in the art, and therefore will not be elaborated further here.

Traditionally, endpoint detection of plasma etch processes has been carried out with the use of optical sensors. Electrical sensors may also be used for endpoint detection. 0 0 6* However, as new processes have been developed in the semiconductor manufacturing industry, there has been a drive to achieve a reduction in geometry of the semiconductors. Accordingly, there has been a corresponding need for the development of advanced sensors for process control and process endpoint detection.

In the last few years therefore, optical systems have been further developed to include broadband Optical Emission Spectroscopy (OES) systems, which use multiwavelength measurements and various algorithms to more accurately determine the occurrence of an endpoint in a process.

A typical optical sensor consists of an array of fast photo-sensitive devices, such as photo-diodes or photo-multipliers. These detect the light emission from the plasma and record them as electrical signals for use as process monitor signals. The sensor may be exposed to light emission from the plasma through view ports in the tool chamber, by placing the sensor against the window, or by using optical fibre light guides between the view port and the sensor. The use of lenses and/or optical filters between the view port and the sensor is optional and may depend on the specific plasma process. Optical filters allow for the detection of light for particular optical wavelength bands. In order to improve the sensor’s sensitivity to the process, the optical fibres and the sensor may be preferred in some situations.

As previously discussed, these methods of endpoint detection may measure the timeaveraged intensity of one or more spectral lines from the plasma emission. The spectral emission measured is dominated by emissions with long decay times within the bulk plasma, which results in a non-modulated or DC signal. Most systems use a charge coupled device to measure the intensity with an integration time of the order of 10-100ms. Various univariate and multivariate statistical algorithms can then be implemented to enhance the signal to noise ratio of the endpoint transition. However, these techniques can be unsatisfactory for accurate endpoint detection of plasma etch processes, in particular due to the ever decreasing size of components on semiconductor chips. 00 64 US patent number 6,830,939 entitled ‘System and method for determining endpoint in etch processes using partial least squares discriminant analysis in the time domain of optical emission spectra’, shows that chemometric algorithms are increasingly being applied for use in endpoint detection systems.

Notwithstanding the various processes described above it is desirable to provide an accurate endpoint detection process.

Summary of the Invention The present invention provides a method for detecting the endpoint of a plasma etch process being performed on a semiconductor wafer, the method comprising the steps of: detecting light being generated from the plasma; filtering the detected light to extract modulated light; processing the detected modulated light to determine when the endpoint of the etch process has been reached; and generating an indicator when the endpoint has been determined.

The semiconductor wafer typically comprises a plurality of layers, with the etch process involving the removal of portions of a layer. By detecting the modulated light emission, an accurate determination of the etch process endpoint can be achieved, as the modulation of the light will change at the endpoint, for example on transition to the next layer.

The detecting may further comprise the step of filtering the light to detect selected wavelength bands.

The processing may further comprise performing an endpoint detection algorithm on the detected modulated light.

Desirably, the endpoint detection algorithm comprises the steps of: 070064 converting the detected light into a digital signal; transforming the digital signal into a frequency domain signal; determining whether a signal level transition of one or more pre-selected frequencies matches a stored signal level transition value which corresponds to when the endpoint in the etch process is reached.

The step of determining whether a signal level transition of one or more pre-selected frequencies matches a stored signal level transition value may comprise the steps of: extracting the one or more pre-selected frequencies from the frequency domain signal for use as process monitor signals; generating a plot of the intensity of the process monitor signals over the elapsed time of the etch process; and determining whether a signal level transition in the plot matches a stored signal level transition value.

The transforming of the digital signal may comprise performing a fast fourier transform on the digital signal.

The indicator may be a control signal to stop the etch process.

The indicator may be a visual or aural indicator that the etch process is complete.

Desirably, the stored signal level transition value and the process monitor signals are determined during a test wafer analysis of wafers of the same batch as the wafer.

The test wafer analysis of the batch may comprise the steps of: detecting modulated light being generated from the plasma of a test wafer being etched over the duration of the etch process; converting the detected modulated light signals into digital signals; transforming the digital signals into frequency domain signals; determining the main frequencies of the frequency domain signals; 070064 selecting those main frequencies which exhibit a signal level transition when the endpoint of the etch process is reached as the process monitor signals; and storing the value of this signal level transition for use as the stored signal level transition value.

The step of selecting those main frequencies which exhibit a signal level transition when the endpoint of the etch process is reached as the process monitor signals may comprise the step of generating a plot of the intensity of the main frequencies over the duration of the time of the etch process; and selecting those main frequencies which exhibit in the plot a signal level transition when the endpoint of the etch process is reached as the process monitor signals.

The present invention also provides a method to determine the process monitor signals and a signal level transition value for use in a method of detecting the endpoint of a plasma etch process to be performed on a semiconductor wafer from a particular wafer batch, the method comprising the steps of: placing a test wafer of the wafer batch in a plasma etching tool and initiating the etch process; detecting modulated light being generated from the plasma of the test wafer over the duration of the etch process; converting the detected modulated light signals into digital signals; transforming the digital signals into frequency domain signals; determining the main frequencies of the frequency domain signals; generating a plot of the intensity of the main frequencies over the duration of the time of the etch process; selecting those main frequencies which exhibit in the plot a signal level transition when the endpoint of the etch process is reached as the process monitor signals; and selecting the value of this signal level transition as the signal level transition value to be stored.

The method may further comprise the step of: generating electron microscopy images of the test wafer; 070064 and wherein the step of selecting may further comprise selecting those main frequencies which exhibit in the plot a signal level transition when the test wafer images show that the endpoint of the etch process is reached as the process monitor signals.

The determining of the main frequencies may comprise the step of determining those frequency domain signals having the higher signal intensity values.

There is also provided a computer program comprising program instructions for causing a computer program to carry out the above methods which may be embodied on a record medium, carrier signal or read-only memory.

The present invention also provides an apparatus for detecting the endpoint of a plasma etch process to be performed on a semiconductor wafer, comprising: a plasma etching tool; means for detecting light to be generated from the plasma during an etch process; means for filtering the detected light to extract modulated light; means for processing the detected modulated light to determine when the endpoint of the etch process has been reached; and means for generating an indicator when the endpoint has been determined.

The means for detecting may further comprise a means for filtering the light to detect selected wavelength bands.

I'he means for processing may comprise: a means for converting the modulated light into a digital signal; a means for transforming the digital signal into a frequency domain signal; and a means for determining whether a signal level transition of one or more preselected frequencies matches a stored signal level transition value which corresponds to when the endpoint in the etch process is reached. 070064 The means for determining whether a signal level transition of one or more preselected frequencies matches a stored signal level transition value may comprise: a means for extracting the one or more pre-selected frequencies from the frequency domain signal for use as process monitor signals; a means for generating a plot of the intensity of the process monitor signals over the elapsed time of the etch process; and a means for determining whether a signal level transition in the plot matches a stored signal level transition value.

The means for detecting may be a photo-sensitive device.

The means for transforming may comprise a microcontroller.

The means for transforming may comprise a Field Programmable Gate Array.

The means for extracting the one or more pre-selected frequencies from the frequency domain signal for use as process monitor signals, generating a plot of the intensity of the process monitor signals over the elapsed time of the etch process and determining whether a signal level transition in the plot matches a stored signal level transition value which corresponds to when the endpoint in the etch process is reached may comprise a computer.

The present invention also provides an apparatus for determining the process monitor signals and the signal level transition value to be stored for use in detecting the endpoint of a plasma etch process to be performed on a semiconductor wafer from a particular wafer batch, comprising: a plasma etching tool; a means for detecting modulated light to be generated from the plasma of a test wafer of the wafer batch over the duration of an etch process; a means for converting the detected modulated light signals into digital signals; a means for transforming the digital signals into frequency domain signals; a means for determining the main frequencies of the frequency domain signals; 0700 64 a means for selecting those main frequencies which exhibit a signal level transition when the endpoint of the etch process is reached as the process monitor signals; and a means for selecting the value of this signal level transition as the signal level transition value.

The means for selecting those main frequencies which exhibit a signal level transition when the endpoint of the etch process is reached as the process monitor signals may comprise a means of generating a plot of the intensity of the main frequencies over the duration of the time of the etch process; and a means of selecting those main frequencies which exhibit in the plot a signal level transition when the endpoint of the etch process is reached as the process monitor signals.

The present invention also provides a method for detecting the endpoint of a plasma etch process being performed on a semiconductor wafer, the etch process generating a plasma sheath proximate the wafer, the method comprising the step of determining an endpoint using substantially only light emitted from the plasma sheath.

The light emitted from the plasma sheath and the remainder of the plasma may be detected together, but the endpoint is determined using substantially only light emitted from the plasma sheath.

The detected light may include both modulated light and non-modulated light.

Brief Description of the Drawings The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which:Figure 1 is a cross sectional view of typical CCP processing tool; Figure 2 is a cross sectional view of a typical TCP processing tool; Figure 3 is a cross sectional view of the CCP processing tool of Figure 1 detailing the etch by-products; 070064 Figure 4 is a ideal graph of the variation in a process parameter over time during the etch process; Figure 5 is a real graph of the variation in a process parameter over time during the etch process; Figure 6 is a diagram of one embodiment of the components involved in the implementation of the present invention; Figure 7 details the process flow of one embodiment of the present invention; Figure 8 details further steps of the process flow of Figure 7; Figure 9 details the process flow of the first steps in determining the optimum process monitor signals for a particular wafer batch; Figure 10 shows an example voltage waveform generated from the detection of modulated light; Figure 11 shows the FFT waveform generated from applying the FFT to the waveform of Figure 10; Figure 12 details the process flow of further steps in determining the optimum process monitor signals for a particular wafer batch; and Figure 13 shows an example of a time process signal from one of many frequencies in the FFT’ recorded in a plasma tool.

Detailed Description of the Invention The present invention provides a method for monitoring a plasma reactor with a sensor which is sensitive to the modulation intensity of radiation emitted from the plasma during the etch process. This is in turn sensitive to, and can be correlated with, the transition of the plasma interface to a surface with a wholly or partially different surface chemistry.

In order to understand the principles behind the present invention, the chemical reactions which occur during the etch process should be appreciated. During the etching of a substrate such as a silicon wafer, a transition will occur in the concentration of by-products from the etch process as the endpoint is reached. This by-products concentration change will result in a transition in the optical emission from the plasma. 070064 One of the main sources for excitation of atoms or molecules in the discharge is electron impact excitation. These excitations are directly proportional to the electron density. The excitation of atoms and molecules is time uniform in the plasma bulk where the electron density is time uniform. On the other hand, the electron density in the plasma sheaths, i.e. the region between the plasma and the electrode/wafer, as indicated by 4 in Figures 1 to 3, is highly modulated at the driving radio-frequency.

The excited species emit light via spontaneous emission with a characteristic decay rate. The excited species can also emit radiation through stimulated emission from the radio-frequency cycle. In general, the plasma emission is directly proportional to the number density of species in an excited state. If the density of the species in excited states is modulated, it is expected that the light emission will be modulated in a similar fashion. This gives rise to a non-modulated or DC emission component together with an additional component, which is modulated at the driving radio-frequency. The modulated light is that light which exhibits a periodic temporal intensity variation at a particular frequency.

Etch by-products resident near the wafer surface are more likely to be excited by the electrons, as the local by-product density is higher in the plasma sheath region. Since the electrons are strongly modulated in the plasma sheath regions, the light from these regions will be highly modulated and the modulation will be correlated with the driving radio-frequency.

It has been found that the modulated light emission is more sensitive to endpoint, as it is independent of memory effects from species with long de-excitation times, such as gases desorbing from the walls and tool drifts, and, because it corresponds to light emitted significantly by excited etch-by products. Therefore, it is ideal for use in the detection of the etch process endpoint.

In a single frequency etching tool, it is expected that the modulated light will correspond to the driving RF frequency and harmonics. But in dual frequency 070064 systems, it is probable to find light modulated at the mixed up products of the two driving RF frequencies, as well as at the RF frequencies themselves and their harmonics.

The optical sensor of the present invention makes use of the above described plasma light modulation effect to detect, if present, this plasma light modulation, and use the detected plasma light modulation to determine the etch process endpoint. As the modulated light is substantially in the plasma sheath, the invention therefore involves determining the endpoint by using substantially only light emitted from the plasma sheath.

Figure 6 shows a diagram of one embodiment of the components involved in the implementation of the present invention. A plurality of sensors 14 provide for the detection of plasma light from the plasma 15 located in the etching tool (etching tool not shown). The sensors 14 can take the form of photo-diodes or photo multiplier tubes. In order to successfully detect the plasma light modulation, the sensors should have fast response times. A plurality of optical filters 16 may be used in conjunction with the sensors 14, each filter adapted to detect a particular optical wavelength band, the filters located between the sensors and the plasma. This has the effect of removing unwanted wavelength bands. The filters therefore allow the real time monitoring of specific optical lines, enabling the classification of plasma chemistry at the sheath. Λ signal conditioning block 17 receives the output data from the sensors 14. At the signal conditioning block 17, the detected light signals from the sensors 14 are conditioned and digitised. In one embodiment of the invention, the conditioning is carried out by a transimpedance amplifier and a programmable voltage amplifier. The transimpedance amplifier converts the signals from the sensors to voltage signals, while the voltage amplifier amplifies these voltage signals. The amplified voltage signals are digitised by an analog to digital converter (ADC). In a preferred embodiment of the invention, the ADC operates at frequencies up to 70 MHz. A processor 18 provides for the processing of the digital signals into the format required in order to enable the endpoint to be determined by the computer (PC) 19. The 070064 processor may be any suitable processing device, such as a micro-controller or a Field Programmable Gate Array (FPGA). The computer 19 provides for the further processing of the processor output signal to determine when the endpoint in an etching process is reached and to generate an indicator when it is reached.

Figure 7 details the process flow of one embodiment of the present invention. In step 1, light is generated from the plasma of a wafer of a particular batch which is to be etched in an etching tool. The optical sensors continuously detect the modulated light emitted from the plasma sheath and the non-modulated light from the remainder of the plasma (step 2). The light may be additionally filtered to only detect light of particular optical wavelength bands. In step 3, the detected plasma light modulation signals are processed in real time by an endpoint detection algorithm, to determine when the endpoint of the etch process has been reached and generate an indicator when the endpoint has been determined.

The process flow can be broken down into a number of further steps, which are described in more detail below in relation to Figure 8. The etch process is started in step 1. In step 2a, the modulated plasma light of different optical wavelength bands is detected by the optical sensors. The non-modulated light may also be detected. The light is converted to a voltage signal by the transimpedance amplifier, and then subsequently amplified by the voltage amplifier (step 2b). The amplified voltage signal is then digitised by the ADC to provide a digital signal (step 2c). A Fast Fourier transform filter in the processor transforms the digital signal into the frequency domain by calculating a FFT of the digital signal (step 2d).

Steps 2a to 2d are repeated approximately two thousand times, and the resulting set of FFTs averaged to generate a sample FFT (step 2e). It should be noted that the entire averaging process only takes about 250ms. This sample FFT is recorded by the computer (step 3). 070064 In step 4, the data values of the one or more frequencies of the sample FFT which have been pre-selected to act as process monitor signals are extracted in order to determine if the endpoint of the etch process has been reached. This has the effect of filtering the detected light to extract modulated light from the light, which may include both modulated and non-modulated light, so that the modulated light may be used for the endpoint detection(the selection of the process monitor signals is earned out during test wafer analysis, details of which will be described later). The process then moves to step 5.

In step 5, if a single frequency has been selected as a process monitor signal, a plot of its corresponding intensity as a function of time is generated in real time based on the data values for that frequency extracted from the sample FFT values which have already been generated over the elapsed time of the etch process. Where there is more than one frequency selected as a process monitor signal, the time evolution of the intensity of the various frequency components may be combined as a single plot.

In step 6, the plot is analysed to determine whether the endpoint condition of the etch process has been satisfied. In one embodiment of the invention, this is achieved by determining whether a signal level transition in the plot matches a stored signal level transition value which corresponds to when the endpoint in the etch process has been reached for the selected process monitor signals of the wafer batch. This stored signal level transition value was determined during test wafer analysis and then preprogrammed into the computer, and will be described in detail later. If a match is found, the process moves to step 7. If a match is not found, the process flow returns to step 2, provided that the etch process has not already been completed.

In step 7, an indicator is generated by the computer that the endpoint in the etch process has been detected. In one embodiment of the invention, the indicator generated by the computer is a visual or aural indicator. In another embodiment of the invention, the indicator is a control signal for the etching tool to stop the etch process. 070064 It will be understood that that the processor could perform a number of alternative (asks once the endpoint has been detected, depending on a user’s requirements for the etch process.

Other numerical techniques could equally well be used instead of Fourier analysis to determine when the endpoint is reached.

It will be appreciated that other methods could also be used to determine the endpoint from the selected process monitor signals. For example, pattern recognition techniques could be used to compare the plot of the selected process monitor signals with a stored characteristic plot.

As explained in the background to the invention section, in order to be able to accurately detect when the endpoint is reached for a particular wafer, it is necessary to first select the most suitable process monitor signals. In the case of the present invention, this involves determining which of the frequencies of the modulated light are most suitable to act as process monitor signals. In reality, each wafer batch has its own unique characteristics. Accordingly, prior to being able to determine the endpoint of the etch process for wafers of a particular wafer batch, it is necessary to carry out advance preparation by performing an analysis of each individual wafer batch to both select the most appropriate frequencies which should be monitored in order to enable the endpoint to be detected for that particular batch, and also to establish the point at which the endpoint is reached for the selected frequencies. This is carried out through test wafer analysis of the batch.

The process of selecting the optimum process monitor signals is described below using an implementation performed through Fourier analysis. However, as previously advised, it should be appreciated that a number of other numerical techniques could equally well be used instead of Fourier analysis. 070064 The first few steps to determine the optimum process monitor signals are identical to those performed during the endpoint detection technique described above. However, for ease of understanding, they are briefly described below again.

Figure 9 details the process flow of determining the optimum process monitor signals for a particular wafer batch. In step 1, a test wafer of the batch is placed in the etching tool and the etching process begun. In step 2a, light from the plasma is detected by the sensors, and the light signal is converted to a voltage signal. This light may include both modulated and non-modulated components. The voltage signal is then amplified (step 2b). In step 2c, the voltage signal is digitised and input to the processor. The processor transforms the digitised voltage signal into the frequency domain using the Fast Fourier Transform to provide a FFT (step 2d).

Steps 2a to 2d are repeated approximately two thousand times, and the resulting set of FFTs averaged to generate a sample FFT (step 2e), which is recorded by the computer (step 21). It should be noted that the entire averaging process only takes about 250ms.

Steps 2a to 2f are repeated over time until the etch process is complete. At this stage, the processor will have recorded a set of sample FFTs covering the duration of the entire etch process of the test wafer. Once the process is complete, the generated sample FFT waveform is ready to be examined to determine the optimum frequencies for use as process monitor signals for that particular wafer batch.

The optimum frequencies of modulated light for use as process monitor signals to detect the endpoint in respect of all of the wafers of the batch are selected by first determining the main frequency components of the sampled FFTs, and then determining whether these main frequency components exhibit a change when the etch process endpoint is reached.

Figure 10 shows an example voltage waveform generated from the detection of modulated light. It will be appreciated that this waveform contains more than one frequency plus noise. Figure 11 shows the FFT waveform generated from applying the FFT to this voltage waveform. This is a plot of intensity versus frequency. It can be 070064 clearly seen that there are four peaks, each below 100 MHz. These peaks indicate the frequency signals that are contained in the waveform, with the height of the peaks indicating the relative intensity of their corresponding frequencies in the waveform. It will be appreciated therefore that the main frequency components correspond to the peaks in the sampled FFT waveform i.e. those frequency domain signals having higher signal intensity values.

Figure 12 details the process flow involved. In step 1, the main frequency components are determined, as described above in respect of Figure 11. In step 2, electron microscopy images of test wafers are examined, to determine which of the main frequencies can provide the most accurate detection of the endpoint. This can be determined by generating a time process signal for each of the main frequencies by plotting the intensity of the frequencies over the time of the etch process. The relevant frequencies are those process signals which exhibit a signal level transition when the test wafer images show that the endpoint has been reached. These one or more relevant frequencies are then selected for use as the process monitor signals.

The test wafer images may be obtained using any of the techniques known in the art. One such technique involves placing a first test wafer in the etching tool and running the etch process until a predetermined time period has elapsed. The test wafer is then removed from the etching tool and the state of its surface examined by slicing the wafer. A second test wafer is then placed in the etch tool and the etch process run until a second predetermined time period has elapsed, with the second time period being greater than the first time period. The second test wafer is then removed and its surface examined. This process is repeated on further test wafers until the predetermined time period exceeds the time taken for the endpoint to be reached for that particular wafer batch. The result is a set of test wafer images detailing the state of the surface of a plurality of test wafers over a number of predetermined time intervals of the etch process.

It should be noted that other surface analysis techniques could alternatively be used to determine the relevant frequencies, such as atomic force microscopy (AFM). ®700 6 4 It will of course be appreciated that if a frequency signal does not change at all over the etch process, then it is of no use for the endpoint detection. However, on the other hand, a signal may exhibit many changes throughout the process. Figure 13 shows an example of a time process signal from one of many frequencies in the FFT recorded in a plasma tool. The etch endpoint in this case has been found to correspond to the signal level transition between 85 and 100 seconds.

Accordingly, a process engineer’s knowledge is preferably used in conjunction with the test wafer analysis to determine which signal level transition actually corresponds to that which occurs when the endpoint is reached.

In step 3, when a single frequency signal is selected as a process monitor signal, the value of the signal level transition analysed to occur when the endpoint is reached for its time process signal should be recorded. Alternatively, if more than one frequencies arc selected as process monitor signals, then the signals can be combined using MultiVariate Analysis techniques (MVA) to output a single combined time process signal for analysis for when the endpoint occurs. Typical VMA techniques include Principal Component Analysis (PCA), Evolving Factor Analysis (EFA), Window Evolving Factor Analysis (WEFA), Hotelling’s T2, PCA-T2, Discriminant Analysis (DA) and Multi-wavelength Signal-to-Noise Ratio (YMSN). As in the case of a single process monitor signal, the value of the signal level transition analysed to occur when the endpoint is reached for the combined time process signal should be recorded.

The above described analysis on a single test wafer can be repeated if desired on several test wafers, in which case the endpoint is determined based on an averaged time process signal.

In the final step in the test wafer analysis process, the computer is programmed to monitor the selected one or more frequencies determined during the test wafer analysis as process monitor signals for analysis for endpoint detection. The computer should also be pre-programmed with the value of the signal level transition recorded 7 0 0 6 4 during the test wafer analysis to correspond to when the endpoint in the etch process is reached for the one or more selected frequencies (step 4).

Once the above described preparation has been completed, the endpoint in the etch process for any wafer from the analysed batch can be detected. This is achieved by placing any of the wafers from the batch into the etching tool, and following the steps of the invention as explained previously with reference to Figure 7.

It will be appreciated that the method and apparatus of the present invention can be used in Capacitive Coupled Plasma (CCP) tools, Transformer Coupled Plasma (TCP) tools and any other variation of these. It could also be used with any other plasma source driven by radio-frequency (RF) for the purpose of plasma etching/processing a substrate, surface or wafer.

This technique could also be used in combination with other sensors such as conventional optical emission, downstream plasma monitoring, RF current, voltage or power.

The embodiments in the invention described with reference to the drawings comprise a computer apparatus and/or processes performed in a computer apparatus. However, the invention also extends to computer programs, particularly computer programs stored on or in a carrier adapted to bring the invention into practice. The program may be in the form of source code, object code, or a code intermediate source and object code, such as in partially compiled form or in any other form suitable for use in the implementation of the method according to the invention. The earner may comprise a storage medium such as ROM, e.g. CD ROM, or magnetic recording medium, e.g. a floppy disk or hard disk. The carrier may be an electrical or optical signal which may be transmitted via an electrical or an optical cable or by radio or other means.

The invention is not limited to the embodiments hereinbefore described but may be varied in both construction and detail. The words “comprises/comprising” and the words “having/including” when used herein with reference to the present invention are °7°064 used to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

Claims (5)

Claims

1. A method for detecting the endpoint of a plasma etch process being performed on a semiconductor wafer, the method comprising the steps of: detecting light being generated from the plasma; filtering the detected light to extract modulated light; processing the detected modulated light to determine when the endpoint of the etch process has been reached; and generating an indicator when the endpoint has been determined.

2. The method as claimed in Claim 1, wherein the detecting further comprises the step of filtering the light to detect selected wavelength bands.

3. The method as claimed in Claim 1 or Claim 2, wherein the processing comprises performing an endpoint detection algorithm on the detected modulated light, wherein the endpoint detection algorithm comprises the steps of: converting the detected light into a digital signal; transforming the digital signal into a frequency domain signal; determining whether a signal level transition of one or more pre-selected frequencies matches a stored signal level transition value which corresponds to when the endpoint in the etch process is reached.

4. The method as claimed in Claim 3, wherein the step of determining whether a signal level transition of one or more pre-selected frequencies matches a stored signal level transition value comprises the steps of: extracting the one or more pre-selected frequencies from the frequency domain signal for use as process monitor signals; generating a plot of the intensity of the process monitor signals over the elapsed time of the etch process; and determining whether a signal level transition in the plot matches a stored signal level transition value. 070064

5. A method for detecting the endpoint of a plasma etch process being performed on a semiconductor wafer, the etch process generating a plasma sheath proximate the wafer, the method comprising the step of determining an endpoint using substantially only light emitted from the plasma sheath.