Method for etching or depositing a thin film

Abstract

A method for plasma etching or plasma depositing a target material layer in or on a sample including an additional material layer, comprising, during the plasma etching or plasma deposition process of the target material layer, a step of directing light of a first wavelength onto the sample, a step of measuring the intensity of the light of the first wavelength reflected by the sample, a step of directing light of a second wavelength onto the sample, and a step of measuring the intensity of the light of the second wavelength reflected by the sample, and a step of determining an end point of the plasma etching or plasma deposition process of the target material layer based on the intensity of the light reflected by the sample, and a step of terminating the plasma etching or plasma deposition process of the target material layer based on the determined end point.

Description

Technology Field

[0001] The present invention relates to a method for etching or depositing a thin film, and in particular to a method for etching a thin film layer within a defined distance of the interface between dissimilar materials using plasma etching and in situ dual-wavelength reflectance measurement. Background Technology

[0002] When performing a dry etching process that requires partial etching, it is advantageous to monitor the etching to ensure that the desired degree of etching is achieved. This applies even to techniques such as atomic layer etching (ALE), which enables a high level of control over the etching process, because variability in how such a process is performed can have a negative impact on the process. For example, if the gas used to dope the surface of the material to be etched is not sufficiently pumped out of the etching chamber, residual gas may alter the etching process or cause a change in the initial thickness of the layer to be etched, resulting in partial etching.

[0003] Endpoint detection technology has been widely used to detect when the etching process reaches the material interface, but technology to accurately and briefly etch at the interface to leave a layer of defined residual thickness on the interface has rarely been developed.

[0004] Examples of previously used endpoint detection techniques include monitoring changes in sample reflectance during the etching process. Since the layer to be etched generally has different optical properties than the underlying material, the reflectance of the sample changes during etching. If a frequency is selected at which the layer to be etched or the underlying material exhibits a peak or trough of reflectance, the change in the sample's reflectance increases or decreases during etching, and reaches a plateau when the layer to be etched is completely removed. Thus, it is theoretically possible to monitor the etching process and determine the endpoint of the etching process.

[0005] However, this technique is not suitable for precisely and briefly etching at the interface between the layer to be etched and the underlying material to leave a predetermined residual thickness of the layer to be etched. In addition, there are practical limitations to monitoring the reflectance of a single wavelength, namely, it is difficult to accurately determine when the etching process has reached the interface with the underlying material, which can lead to over-etching or under-etching.

[0006] For example, a similar problem is faced when attempting to deposit a material layer of a specific thickness on a substrate during atomic layer deposition. While the deposition rate can be estimated, it would be more desirable to be able to monitor the thickness of the deposited material layer during the deposition process.

[0007] Therefore, a more precise technique is required to etch or deposit a material layer inside or on a sample containing at least one additional material layer. means of solving the problem

[0008] According to a first aspect of the present invention, a method is provided for plasma etching a target material layer within a sample or plasma depositing a target material layer on a sample, wherein the sample comprises an additional material layer. The method comprises the following steps: during a plasma etching or plasma deposition process of a target material layer: directing light of a first wavelength to the sample; measuring the intensity of the light of the first wavelength reflected by the sample; directing light of a second wavelength to the sample; and measuring the intensity of the light of the second wavelength reflected by the sample; determining an endpoint of the plasma etching or plasma deposition process of the target material layer based on the intensity of the light reflected by the sample; and terminating the plasma etching or plasma deposition process of the target material layer based on the determined endpoint.

[0009] The terms “target layer of material” and “target layer” are used throughout the specification to refer to a layer of material to be etched or deposited. A sample on which the target layer of material is etched or the target layer of material is deposited comprises an additional layer, which means that it comprises at least one additional layer in addition to the target layer. The “further layer” may be a layer immediately below the target layer, or there may be one or more intermediate layers. The additional layer will typically be made of a material different from that of the target layer.

[0010] Accordingly, throughout the specification, the expression “target material layer” should be regarded as synonymous with “layer to be etched” in the case of an etching process and “layer to be deposited” in the case of a deposition process. The target layer is often an upper layer of the material, but additional layers may be provided on top of this layer that is etched first; for example, in the case of an AlGaN layer provided on top of a GaN layer, a “cap” layer of GaN may be provided on top of the AlGaN layer. However, for the sake of ease of understanding, throughout the specification, the expression “upper layer” always refers to the target layer.

[0011] A method performed according to an embodiment of the first aspect of the present invention enables accurate monitoring of the thickness of a target material layer within a sample during a plasma etching or deposition process, thereby enabling accurate determination of the endpoint of the plasma etching or plasma deposition process. For clarity, "endpoint of the process of plasma etching or plasma deposition" refers to the point at which the plasma etching or deposition process ends. In the case of a plasma etching process, which is a preferred application of the first aspect of the present invention, this may be when the target material layer is completely removed from at least one region of the sample, when the target material layer is partially etched from at least one region of the sample, or a combination of both. Likewise, a plasma deposition process may end when the target material layer is deposited over the entire sample or when the target material layer is deposited in at least one region of the sample.

[0012] When applied to a plasma etching process, this method enables plasma etching with a precisely controlled residual thickness. Specifically, this method allows for thickness control with an accuracy of ±0.5 nm and a reproducibility of ±0.2 nm. Both of these yield excellent results when manufacturing devices with closely grouped electrical characteristics, such as enhanced mode (usually off) high electron mobility transistors (HEMTs). In particular, it is important to minimize damage to the residual layer when manufacturing such devices, which is possible in the embodiment of the first aspect of the present invention.

[0013] These advantages are partially realized by monitoring the intensity of light reflected by the sample at two wavelengths. This makes it possible to overcome problems that may arise when monitoring only one wavelength, such as window clouding, wafer tilt, or variability in signal levels between runs caused by stray reflections.

[0014] The above method is particularly useful when applied to the ALE process because ALE, which is actually performed in a limited surface area, is highly uniform and can compensate for the general non-uniformity of plasma etching. The uniformity of the residual layer thickness then approaches the non-uniformity inherent in the original epitaxially grown thin film, and combined with precise monitoring of thickness variations of the target material layer within the sample, it enables highly uniform etching with a precisely controlled residual thickness. ALE is also advantageous because it operates close to the sputter threshold energy for the layer being etched, making it suitable for minimizing damage to the residual layer.

[0015] However, it should be noted that embodiments of the first aspect of the present invention may also use conventional plasma etching with a low bias or a combination of conventional plasma etching and ALE. For example, the method may control the transition between continuous plasma etching and ALE using time-dependent etching or measurement, and may control the final residual thickness while minimizing damage to the corresponding layer using an endpoint method. The combined method has a higher throughput than using atomic layer etching.

[0016] Likewise, the advantage of the first aspect of the present invention is also achieved when applied to a plasma deposition process. In this case, the target material layer of the sample, in this case the deposited material layer, can be deposited with a precisely controlled thickness if monitored very precisely.

[0017] As described in more detail below, the method of the first aspect of the present invention utilizes the phenomenon in which the optical properties of a sample change as the thickness of the target layer changes, such that when the target layer is thicker, the optical properties of the target layer are governed, and when the target layer is thinner or absent, the optical properties of the underlying material are governed. Reflectance is the optical property of most interest, as the method includes a method for measuring the intensity of light reflected from the surface of the sample.

[0018] Therefore, to maximize this effect, the first wavelength will generally be selected at the peak or lowest point of the reflectance of the target layer. For the use of a preferred method, such as when the target layer is AlGaN, the first wavelength is in the range of 280 nm to 340 nm, and more preferably in the range of 320 nm to 340 nm.

[0019] Likewise, the second wavelength will be at the peak or trough of the reflectance of an additional material layer, which may typically be a material layer located immediately below the target layer as described above. For the use of a preferred method, such as when the underlying material is made of GaN, the second wavelength is in the range of 340 nm to 390 nm, and more preferably in the range of 360 nm to 370 nm.

[0020] That is, it may be proven advantageous to provide a reference signal that allows comparison of the intensity of light reflected at different wavelengths by selecting a first or second wavelength, respectively, in the reflection spectrum region of the target layer or underlying material, which has no peak or lowest point.

[0021] In a preferred embodiment of the first aspect of the present invention, the step of determining the endpoint of a plasma etching or deposition process generally includes the step of determining the reflectance of a sample at a first and second wavelength by obtaining (taking) the ratio of the intensity of light of the wavelength reflected by the sample to the intensity of light of the wavelength directed toward the sample for each wavelength.

[0022] The reflectance of the above sample is easily measured during the etching or deposition process, so it is a characteristic particularly suitable as a basis for determining when to stop the plasma etching or deposition process or when to switch from conventional plasma etching or deposition to atomic layer etching or deposition.

[0023] In a more preferred embodiment, the step of determining the endpoint of the plasma etching or deposition process includes the step of obtaining the ratio of the sample reflectance of the first wavelength to the sample reflectance at the second wavelength.

[0024] By obtaining the ratio of the two reflectance measurements, it is possible to overcome the difficulties that may arise when monitoring only one wavelength, such as the variability in signal levels between runs caused by window clouding, wafer tilt, or stray reflection.

[0025] Often, determining when to end the plasma etching or deposition process will be based on using the ratio of two reflectance measurements to determine the thickness of the target layer and ending the plasma etching or deposition process when the target layer reaches the desired thickness. However, the form of the ratio trend may not be a simple linear (or nearly linear) relationship throughout the plasma etching or deposition process, in which case the slope of the ratio trend from a part of the plasma etching or deposition process can be used to calculate the etching or deposition rate, and the time to end the plasma etching or deposition process calculated based on this etching or deposition rate can be used to leave the desired residual thickness.

[0026] Deviation from a simple linear relationship can occur due to one or more of quantum size effects, optical band gap shifts caused by proximity of the underlying material, temperature, or strain effects. As such, these effects can be incorporated into a thin film thickness model to determine the endpoint of a plasma etching or deposition process in order to improve the precision of the residual thickness of the target layer.

[0027] Another example of a nonlinear change in the optical properties of a sample is the change in reflectance during ALE. As will be explained in more detail below, ALE involves a cycle in which the sample surface is doped, for example, with a monolayer of chlorine atoms, and then a single atomic layer of the sample is etched. During the doping step, the reflectance is generally stable, and during the etching step, the reflectance changes. The change in the sample's reflectance can be used to determine the etching rate per cycle, and then the etching can be stopped after an appropriate number of ALE cycles.

[0028] Using the ratio between reflectance measurements to determine the endpoint of a plasma etching or deposition process allows for consistency in the expected ratio at various points in the process, so the value or slope of this ratio can be used to infer the progress of the plasma etching or deposition process. However, determining when to terminate the plasma etching or deposition process is often based on observing when the ratio reaches a steady state or when the slope of the ratio reverses. In such cases, it is not necessary to obtain the ratio of reflectance measurements, and determining the endpoint of the plasma etching or deposition process may involve obtaining the ratio of the two measured intensities. This is often a simpler and more direct method, while still allowing for many of the advantages of the preferred embodiment in which the reflectance measurement ratio is used to determine the endpoint of plasma etching.

[0029] The process described above can be applied to an additional layer provided on top of a target layer to determine when to start plasma etching the target layer of the material, and in the case of a GaN cap layer provided on top of an AlGaN layer, to determine when to etch the AlGaN layer. In such cases, a turning point or other recognizable feature of light intensity or reflectance measurement can be used to determine the start of target layer etching using a dual-wavelength reflectometer. Since the first and second wavelengths are ideally selected for each layer stack type or configuration, it is desirable for the device used to perform the method of the first aspect to include a broadband light source or an adjustable laser to provide maximum flexibility.

[0030] Generally, an external light source is used to generate light of first and second wavelengths, in which case it is desirable that neither one or both of the first and second wavelengths correspond to the characteristic wavelengths of the emission spectrum of the plasma used in the plasma etching or deposition process. This simplifies the process of distinguishing between the light emitted from the plasma used in the plasma etching or deposition process and the light reflected from the sample.

[0031] However, one or both of the first wavelength light and the second wavelength light directed toward the sample may be provided by the plasma used in the plasma etching or deposition process.

[0032] The method of the first aspect may include a calibration step of measuring the intensity of light of first and second wavelengths reflected by the sample at the start of the plasma etching or deposition process. This allows determining a reference value for the light intensity or reflectance measurement, which can be used to calibrate the device used to perform the method. The calibration step may also be performed at other identifiable points in the plasma etching or deposition process. For example, the plasma etching process may involve a transition between etching layers of different materials, such as when the capping layer of the material is removed from the top of the sample in the example described above. Such a transition can be identified by measuring the intensity of light of first and second wavelengths reflected by the sample, meaning that the measurement obtained from this transition can be used to calibrate the method.

[0033] Not only is it suitable for calibrating the device used to perform the above method, but the measurement performed at the start of the plasma etching process can be used to determine the thickness of the target layer of the sample at the start of the plasma etching process, and then to determine how long the plasma etching process should be performed. If one or more additional layers are provided on top of the target layer of the material, these determinations can be made after the layers are removed.

[0034] An endpoint is generally used to terminate the plasma etching or plasma deposition process of a target material layer at a predetermined residual thickness, preferably 10 nm or less. In a preferred embodiment of the present invention in which the target material layer is etched to a predetermined residual thickness, prior to the process of plasma etching the target material layer, the method further comprises an initial step of etching the target material layer to a first residual thickness. In these embodiments, the predetermined residual thickness is a second residual thickness smaller than the first residual thickness. For example, the first residual thickness may be 25 nm and the second residual thickness may be 10 nm. The first etching step does not need to have an endpoint defined as precisely as the second etching step and therefore does not need to be performed according to the embodiment of the first aspect. This multi-step etching process is advantageous even when a relatively slow etching process, such as ALE, is used in the second etching step. ALE is very precise and can minimize damage to the target layer, but it is slower than conventional etching methods such as inductively coupled plasma (ICP) etching. Therefore, in the above multi-stage process, most of the etching can be performed using a faster conventional plasma etching process while achieving the final residual thickness using a more precise etching process such as ALE.

[0035] These embodiments are highly suitable for producing devices in which the target layer must be etched to a precisely defined residual thickness and the production time is reduced, thereby increasing the speed at which the device can be produced.

[0036] Plasma etching is generally performed at a pressure of about 1 mTorr to 100 mTorr, whereas plasma deposition is generally performed at a pressure of about 10 mTorr to 3000 mTorr.

[0037] According to a second aspect of the present invention, a plasma processing apparatus suitable for carrying out any one of the embodiments of the first aspect of the present invention is provided, the apparatus comprising: a plasma processing chamber; at least two light emitters; at least two light detectors; and at least one light port for connecting at least two light emitters and at least two light detectors inside the plasma processing chamber.

[0038] A typical processing apparatus includes a gas delivery system for controlling the flow of gaseous reactants into a plasma processing chamber; a plasma source capable of forming part of the plasma processing chamber; a support inside the plasma processing chamber for a sample to be processed; pressure control means for maintaining the pressure of the plasma processing chamber within a desired range; and a control system. The support is generally temperature-controlled and electrically insulated from the wall of the plasma processing chamber, and an RF power source may be connected to the support to generate an electric field. The pressure control means includes automatic pressure control and means for evacuating the plasma processing chamber to about 1 mTorr to 100 mTorr for etching and 10 mTorr to 3000 mTorr for deposition. Brief explanation of the drawing

[0039] Now, embodiments of the present invention will be described with reference to the drawings. Figure 1 shows the reflectance at the start of AlGaN / GaN etching provided according to an embodiment of the present invention for two different AlGaN compositions. FIG. 2 illustrates the reflectance spectrum of a sample before and after an etching process provided according to an embodiment of the present invention. FIG. 3 shows the change in the reflectance ratio of a sample at 320 nm / 365 nm during an etching process provided according to an embodiment of the present invention. FIG. 4 illustrates an example of a device that can be etched in a process provided according to an embodiment of the present invention. FIG. 5 illustrates the reflectance output from the reflectometer during the calibration process provided according to an embodiment of the present invention. FIG. 6 illustrates the change in reflectance of a sample during an ALE process provided according to an embodiment of the present invention. Figure 7 shows the change in reflectance of a sample at two different wavelengths during an etching process provided according to an embodiment of the present invention. Specific details for implementing the invention

[0040] The following description focuses on applying the present invention to an etching process. However, it should be noted that this description also applies to a deposition process.

[0041] The term "sample" should be considered to represent one or more layer stacks of different materials prior to the etching (or deposition) process to be used to form the final device. The sample may undergo one or more processing steps prior to the etching (or deposition) process and one or more processing steps after the etching (or deposition) process.

[0042] When etching a sample composed of layers of various materials, it was observed that the optical properties of the sample change during etching. For example, the reflectance of the upper surface of the sample depends on the thickness of the upper layer of the sample; that is, as the upper layer of the sample is removed, the reflectance of the sample also changes. In addition, the optical properties change due to interactions at the interface between the upper layer and the lower material, particularly at the interface where the thickness of the upper layer is 10 nm or less.

[0043] This can be understood as a result of different layers of the sample having different optical properties; that is, at the start of the etching process, the optical properties of the upper layer of the sample are dominant, and once the etching process is completed, the optical properties of the lower layer dominate the optical properties of the entire sample. As described above, this is also a result of complex interactions occurring near the interface between the two material layers. For example, if the upper layer transmits light of a specific wavelength at least partially and the interface with at least one lower layer reflects light of that wavelength at least partially, the total intensity of light reflected by the sample will depend on the interference between reflections from the surface of the upper layer and reflections from one or more buried interfaces. Other dominant effects, particularly when the thickness of the upper layer is 10 nm or less, include quantum size effects, optical band gap shifts due to the proximity of the lower material, and temperature or strain effects.

[0044] Therefore, monitoring the optical properties of a sample during etching allows for understanding the progress of the etching and controlling the entire etching process. When monitoring the optical properties of a sample, the reflectance of the sample is particularly important, and one possible way to control the etching process is to monitor the location of characteristic peaks or troughs in the reflectance of the upper layer or the lower material of the sample. This allows the endpoint of the etching to be determined by observing when the peak or trough disappears when the peak properties of the upper layer are monitored, or when the peak properties of the lower material are monitored.

[0045] However, the peak or trough characteristics of the upper layer may disappear at the midpoint of the etching process or become double peaks or double troughs, making it difficult to accurately determine the endpoint of the etching. Similarly, the same phenomenon may occur when monitoring the peak or trough characteristics of the underlying material. Other difficulties that can negatively affect the accuracy of this method may also arise, such as variability in signal levels between runs due to window clouding, wafer tilt, or stray reflection.

[0046] Due to these problems, it can be difficult to determine when the top layer of the sample is completely removed, which is a particular problem when trying to control the etching to leave a certain thickness of the top layer.

[0047] To solve the above problem, the present invention monitors the optical properties of a sample at two different wavelengths.

[0048] Figure 1 shows the reflectance of an AlGaN layer provided on a bulk GaN substrate. This graph plots the reflectance (y-axis) of the top substrates of two different samples as a function of wavelength (x-axis); the upper line represents the reflectance characteristics of the 20% Al layer, and the lower line represents the reflectance characteristics of the 23% Al layer. These lines indicate peaks in reflectance within the wavelength range of 280 nm to 340 nm, respectively. The location of these peaks depends on the Al content of the layer; as the Al content increases, a peak appears in the 280 nm direction, while as the Al content decreases, a peak appears in the 340 nm direction.

[0049] For example, in the case of an AlGaN layer, changes in the reflectance of the sample at characteristic peaks in the wavelength range of 280 nm to 340 nm can be monitored and used to determine whether the AlGaN layer has been completely or partially etched. However, as mentioned above, it is difficult to accurately determine when to terminate the etching process if only this wavelength is monitored.

[0050] For example, an AlGaN layer provided on a bulk GaN substrate exhibits a peak reflectance in the wavelength range of 280 nm to 340 nm and a minimum reflectance at about 365 nm, whereas the reflectance of the GaN substrate is nearly constant throughout the range of 280 to 370 nm. By taking the ratio between the reflectance of the sample measured at the characteristic peak of the AlGaN layer and the reflectance of the corresponding sample measured at the characteristic minimum of the AlGaN layer, and identifying when said ratio passes a predetermined threshold, the endpoint of the etching can be determined more accurately and with enhanced sensitivity.

[0051] The characteristics of the AlGaN layer compared with the GaN substrate are illustrated in Fig. 2. The figure shows the reflection spectrum of a sample composed of an AlGaN layer applied to a GaN substrate at the beginning and end of the etching process. A trace with a peak at approximately 325 nm and a trough at approximately 365 nm shows the reflectance of the sample at the start of the etching process when the AlGaN layer is present, whereas the other trace shows the reflectance of the sample at the end of the etching process when the AlGaN layer is removed. As can be seen from this, there is a significant change in the reflectance of the sample at these two wavelengths during the etching process.

[0052] There are several reasons why the accuracy and sensitivity of endpoint detection are improved when monitoring two wavelengths, one of which is that errors caused by incorrect correction of absolute reflectance are eliminated by obtaining the ratio of the reflectances of the two wavelengths. In fact, if the incident light intensity at both wavelengths is constant throughout the etching process, this ratio can be simply determined by measuring the intensity of the light reflected from the samples at both wavelengths and obtaining the ratio of these two measurements.

[0053] Another reason for the improvement provided by this method is that it utilizes the fact that changes in the monitored signal are maximized during the measurement process.

[0054] When measurements are performed by monitoring changes in reflectance at a single characteristic wavelength of the upper or lower layer, even in the ideal case where there are no errors in the measurement process, the measured signal indicates only a decrease in reflectance at the characteristic peak of the upper layer of the sample (or, conversely, an increase in reflectance at the characteristic trough of the upper layer of the sample). Similarly, when a single characteristic wavelength of the lower layer is measured, the measured signal indicates only an increase in reflectance at the characteristic peak of the lower layer of the sample (or, conversely, a decrease in reflectance at the characteristic trough of the lower layer of the sample).

[0055] However, if the ratio between the reflectance (or actually the intensity of light) of two characteristic wavelengths is used instead, the variation in reflectance at the two wavelengths is taken into account. In the case of an AlGaN layer on a bulk GaN substrate, the reflectance measured in the range of 280 nm to 340 nm decreases, while the reflectance measured at approximately 365 nm increases, so the variation in both of these measurements will be taken into account. In the case of an AlGaN layer on a bulk GaN substrate, it is convenient to use two wavelengths characteristic of the reflection spectrum of the upper layer, but in other cases, it may be convenient to use two wavelengths characteristic of the reflection spectrum of the lower layer, or it may be convenient to use one wavelength characteristic of the reflection spectrum of the upper layer and one wavelength characteristic of the reflection spectrum of the lower layer.

[0056] Another advantage of monitoring two wavelengths is that the two wavelengths do not need to exactly match the characteristic peaks or troughs of the upper and lower layers of the sample, and even if the variation in reflectance at one or both wavelengths is reduced, the variation in the ratio between the two wavelengths can still be large. For example, the characteristic reflectance peak of an AlGaN layer depends on the aluminum content of the layer, but all wavelengths in the range of 280 nm to 340 nm produce sufficient variation in the ratio. Therefore, as long as the target threshold ratio is appropriately adjusted for the specific AlGaN layer being etched, a wavelength can be selected somewhere in the middle of the aforementioned range, such as about 320 nm, to allow for endpoint detection when etching the AlGaN layer with a specific range of aluminum content.

[0057] Fig. 3 shows 2nm GaN / 20nm Al with an etching rate of 2nm / min. 20 The change in reflectance ratio from 320 nm to 365 nm during the etching of GaN / 2 nm AlN to a GaN substrate is plotted, where Al 20The GaN has an Al content of 20%. As can be seen in the graph, the ratio briefly increases as the 2nm GaN layer is removed, and is expected to decrease from a value of approximately 1.15 at the start of etching to a value of approximately 1 when the AlGaN layer is completely removed. By adjusting the endpoint based on this ratio threshold, etching to the desired target thickness is possible.

[0058] Therefore, one method to determine the endpoint of etching is to perform a full etching of the sample to determine the relationship between the ratio values ​​of reflectance measurements at two wavelengths, and then use this relationship to determine the endpoint of subsequent partial etching.

[0059] Another option is to compare the ratio of reflectance measurements at two wavelengths with the expected change model of the aforementioned ratio.

[0060] An example of a sample prior to the etching process is shown in Fig. 4. A SiC substrate was Ti / W sputtered onto the underside / backside of bulk GaN. An AlGaN layer approximately 20 nm thick is provided on the bulk GaN, and an additional GaN cap layer approximately 2 nm thick is provided on top of the AlGaN layer. A patterned photoresist PR is applied to the GaN cap area and serves as a mask for the etching process.

[0061] To calibrate the device used to perform the etching process, a calibration etch is performed on the sample as shown in FIG. 4 to detect the complete removal of the target layer and provide a reference result for subsequent partial etching. The etching is then continued into the GaN bulk. Reflectance measurements at 365 nm throughout the etching are shown in FIG. 5. A delayed rise in the ratio at the start of the process indicates the removal of the GaN cap layer, after which the reflectance measurement ratio rises substantially linearly until it reaches a plateau, also known as a knee. The knee indicates the complete etching of the AlGaN layer. In this example, the etching continued to the underlying GaN layer, confirming a plateau state at approximately 22 (arbitrary units of reflected light intensity (AU) are shown in FIG. 5). A similar calibration etch can be performed at a second wavelength.

[0062] The present invention is particularly suitable for monitoring various stages of an ALE cycle, and Figure 6 shows the reflectance measured at 340 nm compared to the plasma radiation count at 340 nm (from a spectrometer equipped with an intensity factor detector) during the ALE process. As can be seen in the figure, there is a distinct spike in the radiation from the plasma at 340 nm, which corresponds to a short pulse of the chlorine gas flow. At approximately 1390 counts, the stagnation of plasma radiation corresponds to the addition of RF bias to the table when the deformed surface layer is removed. The middle solid line, which increases stepwise from left to right in the graph, represents the reflectance of the sample at 340 nm, showing successive steps of reflectance corresponding to layer removal. The dotted line is a simulation of the layer thickness change, which can be determined by comparing the sample reflectance at 340 nm with the sample reflectance at 365 nm.

[0063] Another method for monitoring changes in the thickness of the top layer is to observe the oscillation of reflectance at the interface, as illustrated in Fig. 7. This figure shows the reflectance at 340 nm and 365 nm during another example of ALE of the sample shown in Fig. 4. Small oscillations throughout the etching process occur due to the ALE cycle. As the etching approaches the interface between the AlGaN layer and the GaN bulk, the amplitude of the oscillation at 365 nm becomes significantly larger, which is believed to be due to deformation of the AlGaN layer. If the desired residual thickness is within the range of this effect, the amplitude of these oscillations can be monitored to detect the approach of the interface. In this case, the reflectance at 365 nm acts as a reference for the amplitude of the oscillation, which will be expressed, for example, as the ratio between two reflectance measurements.

[0064] In the method provided according to the present invention, the reflectance ratio can be measured with a precision of approximately + / - 0.001, and the reflectance ratio can vary by about 20% during the complete etching of a 20 nm layer (varying from about 1.1 to 0.9 in FIG. 3a). This means that the remaining thickness of the target layer can be determined with a precision of + / - 0.1 nm through this technique.

[0065] To minimize the trade-off between etching speed and stop precision, it is advantageous to use time-integration smoothing when the etching speed is less than 0.1 nm in 3 seconds or less than 2 nm per minute, as it may take up to 3 seconds to generate a low noise reflectance ratio. Generally, when a residual thickness precision of 0.1 nm is required, the etching speed should preferably be less than 0.1 nm in the noise smoothing time, i.e., generally less than 0.1 nm in 3 seconds. Precision can be improved by adjusting the target endpoint for expected over-etching due to endpoint signal delay.

[0066] When etching a layer to a low residual thickness of 10 nm or less, it is advantageous to use a high-precision etching method such as ALE in conjunction with the aforementioned endpoint detection. However, a disadvantage of ALE is that its etching speed is slower compared to conventional etching processes, such as Inductively Coupled Plasma (ICP) etching. Therefore, it is advantageous to use a multi-stage etching process in which a conventional etching method is used in the first stage to etch the target layer to a first thickness, and then ALE is used in the second stage to etch the target layer to a desired residual thickness. This allows the etching process to be completed more quickly while providing high precision regarding the residual thickness after etching.

Claims

Claim 1 A method for determining an endpoint in a plasma etching or plasma deposition process for a target material layer in or on a sample including an additional material layer, comprising the following steps: during a process of plasma etching or plasma deposition of a target material layer: a step of directing light of a first wavelength onto the sample; a step of measuring the intensity of the light of the first wavelength reflected by the sample; a step of directing light of a second wavelength onto the sample; and a step of measuring the intensity of the light of the second wavelength reflected by the sample; a step of determining an endpoint in the plasma etching or plasma deposition process of the target material layer based on the intensity of the light reflected by the sample; and a step of terminating the plasma etching or plasma deposition process of the target material layer based on the determined endpoint; furthermore, the step of determining an endpoint in the plasma etching or plasma deposition process of the target material layer comprises a step of obtaining a ratio of two measured intensities. Claim 2 In claim 1, the method wherein the first wavelength is at the highest or lowest point of the reflectance of the target layer or additional material layer. Claim 3 In paragraph 2, the method wherein the first wavelength is in the range of 280 nm to 340 nm. Claim 4 A method according to any one of claims 1 to 3, wherein the second wavelength is at the highest or lowest point of the reflectance of the target layer or additional material layer. Claim 5 In paragraph 4, the method wherein the second wavelength is in the range of 340 nm to 390 nm. Claim 6 A method according to claim 1, wherein the step of determining the endpoint of the plasma etching or plasma deposition process of the target material layer includes the step of determining the reflectance of a sample at first and second wavelengths. Claim 7 In claim 6, the step of determining the reflectance of a sample at the first and second wavelengths comprises obtaining the ratio of the intensity of light of the wavelength reflected by the sample to the intensity of light of the wavelength directed toward the sample for each wavelength. Claim 8 A method according to claim 6 or 7, wherein the step of determining the endpoint of the plasma etching or plasma deposition process of the target material layer comprises the step of obtaining the ratio of the determined reflectance of the sample at the first wavelength to the determined reflectance of the sample at the second wavelength. Claim 9 A method according to claim 1, wherein the first wavelength or the second wavelength provides a reference signal capable of comparing the intensity of light reflected at a different wavelength. Claim 10 In claim 9, the method wherein the first wavelength or second wavelength corresponds to a region of the reflection spectrum of a target layer or an additional material layer having no peak or lowest point. Claim 11 A method according to claim 1, wherein one or both of the first wavelength and the second wavelength correspond to the lowest point of the emission spectrum of the plasma used in the plasma etching or plasma deposition process of a target material layer. Claim 12 A method according to claim 1, wherein one or both of the first wavelength light and the second wavelength light directed toward the sample are provided by a plasma used in a plasma etching or plasma deposition process of a target material layer. Claim 13 The method of claim 1, wherein the method comprises a calibration step of measuring the intensity of light of first and second wavelengths reflected by a sample at the start of a plasma etching or plasma deposition process of a target material layer. Claim 14 A method according to claim 1, wherein the step of determining the end point of the plasma etching process of the target material layer based on the intensity of light reflected by the sample comprises the step of determining the thickness of the target layer of the sample at the start of the plasma etching process. Claim 15 A method according to claim 1, wherein the endpoint is used to terminate the plasma etching or plasma deposition process of a target material layer at a predetermined remaining thickness. Claim 16 In item 15, the method wherein the predetermined remaining thickness is 10 nm or less. Claim 17 A method for etching a target material layer to a predetermined residual thickness according to claim 15 or 16, wherein prior to the process of etching the target material layer, the method comprises an initial step of etching the target material layer to a first residual thickness. Claim 18 A plasma processing apparatus suitable for carrying out the method of claim 1, wherein the apparatus comprises: a plasma processing chamber; at least two light emitters; at least two light detectors; and at least one light port for connecting the at least two light emitters and the at least two light detectors to the interior of the plasma processing chamber.

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