Abstract:
A method for detecting the transition between different materials in semiconductor structures during alternating etching and covering steps for anisotropic depthwise etching of defined patterns performed using a plasma. Provision is made for ascertaining, by way of an intensity measurement of at least one specific substance contained in the plasma, the beginning of each etching step by the fact that a characteristic threshold is reached, this also being achievable by way of an external synchronization signal which indicates the beginning and end of each etching step; for then, when the threshold value is reached, starting a delay time which is longer than the course of a first concentration maximum; for a second concentration maximum then to be ascertained after the delay time has elapsed; and for the second concentration maxima of the etching steps to be monitored as to whether they exceed or fall below the predefined value, in order to detect a material transition.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for detecting the transition between different materials in semiconductor structures during alternating etching steps and covering or deposition steps for anisotropic depthwise etching of defined patterns performed using a plasma. 
     BACKGROUND INFORMATION 
     German Patent No. 42 41 045 describes a plasma etching process for silicon in which a Teflon-like protective film is applied onto the side walls of etched structures, and in a subsequent inherently isotropic etching step the silicon is etched down with fluorine radicals. By removal of the overlying side wall protective film, subsequent transport downward, and redeposition during the individual etching steps, the newly produced side wall sections are always protected from the next etching attack, resulting in a smooth side wall. If layers of different materials are present in a semiconductor structure, etching must often be performed through one layer down to the layer lying below it; i.e. the etching operation is performed until a new layer begins, and then stopped. For example, if the upper layer is made of silicon and the layer lying below it of silicon dioxide, it is usual to use an optical spectroscopy method to stop the etching process after the silicon layer has been etched through and directly after the silicon dioxide layer has been reached. The optical method involves examining the strength of the plasma emission in terms of a specific substance, by way of its characteristic emission wavelength(s). During the etching of silicon as defined in German Patent No. 42 41 045, a relatively large quantity of fluorine radicals is consumed in the etching reaction, i.e. the concentration of fluorine radicals in the plasma is relatively low. At the same time gaseous reaction products such as SiF 2 , SiF 3 , SiFP 4 , etc., which in turn exhibit a characteristic emission, are produced. When etching reaches the dielectric intermediate layer, i.e. the silicon dioxide layer, the result is more or less an etching stoppage, since the etching operation proceeds much more slowly in silicon dioxide than in silicon. Because of this etching stoppage, the fluorine consumption decreases, and the quantity of free fluorine in the plasma rises correspondingly. This is detected in the optical measurement by a corresponding light emission at characteristic wavelengths. The slowing of the etching process when the silicon dioxide layer is reached moreover causes the SiF x  concentration to decline, so that its light emission at characteristic wavelengths decreases, and emission of oxygen from the silicon dioxide takes place at its characteristic wavelengths. These effects can also be sensed with the optical measurement method. Using the aforementioned optical method, it is thus possible to terminate the etching process or continue it with modified parameters. This is necessary because uncontrolled overetching results in an undesired etching profile, for example in incipient etching of the vertical side walls, so that undercutting of the mask occurs with pattern loss. The originally achieved accuracy thus no longer exists, and pockets can form at the dielectric interface (between silicon and silicon dioxide). 
     SUMMARY OF THE INVENTION 
     The method according to the present invention provides, for detection of the transition between different materials in semiconductor structures with alternating etching and covering steps as defined in German Patent No. 42 41 045, a method with which precise, reproducible results can be achieved. (The terms “concentration” and “concentration maximum” will be used hereinafter; these may be replaced by the terms “intensity” and “intensity maximum,” respectively, since the concentration of a substance contained in the plasma and the intensity by way of an intensity measurement of the characteristic wavelengths are proportionally related to one another.) For this purpose, it is provided that by way of an intensity measurement of at least one specific substance contained in the plasma, via its emission strength and thus its concentration, the beginning of each etching step be ascertained by the fact that a characteristic threshold is reached; that then, when the threshold value is reached, a delay time be started which is longer than the course of a first uncharacteristic concentration maximum; that a second concentration maximum then be ascertained after this delay time has elapsed; and that the second concentration maxima of the etching steps be monitored as to whether they exceed or fall below a predefined value, in order to detect the material transition. Instead of the fact of exceeding or falling below a threshold value, it is also optionally possible to ascertain the end or the beginning of the etching step by way of a synchronization signal, the generation of which will not be discussed in further detail. The present invention utilizes proposes to utilizes as an essential variable a second concentration maximum, occurring at each etching step, of a specific substance contained in the plasma. Detection of this second maximum is accomplished by the fact that by waiting for the delay time, an uncharacteristic signal peak located in each case before the maximum to be analyzed, namely a first concentration maximum, is excluded from consideration. The delay time is selected so that it is longer than the course of the first concentration maximum, so that this first concentration maximum is, e.g., blanked out. Analysis of the second characteristic concentration maximum makes available a characteristic and thus informative value for detecting the transition between different materials at each etching step. This value is preferably ascertained continuously, i.e. at each etching step. Alternatively, however, it is also possible for this value to be ascertained not at each etching step but, for example, at every other etching step. The individual second concentration maxima change in magnitude over the course of the plurality of etching steps; a dependence on the etched material thus exists. During the etching operations, as one material is left and etching of a second material begins, the informative second concentration values change, i.e. the fact of exceeding or falling below a predefined value allows the conclusion that the underlying second material has been reached. The method according to the present invention moreover permits a conclusion in terms of trend, since the change in the second concentration maximum in the course of the individual etching steps proceeds essentially continuously, thus resulting in a rise or fall in the maxima; it is evident from this that the material transition is about to be reached. It is thus possible, by way of a suitable continuous analysis, to draw conclusions as to trends. 
     According to another embodiment of the present invention, provision is made for a peak value of the second concentration maximum to be ascertained. The greatest value in each case is thus acquired, and utilized as an analysis criterion. Alternatively, it is also possible for several values of the second concentration maximum lying in the region of the peak to be utilized for the analysis. 
     Such is the case, for example, if the second concentration maximum is ascertained using a sample-and-hold method. A corresponding sample-and-hold apparatus can be used for this purpose. 
     It is furthermore advantageous if the semiconductor structure to be etched using fluorine radicals has at least one region made of silicon and at least one other region made of silicon dioxide, the silicon being the material first processed from the etching side and the silicon dioxide the material to be reached by the etching process as it proceeds, the rise in the second fluorine concentration to more than the predefined value resulting in detection of the material transition from silicon to silicon dioxide. The second fluorine concentration maxima thus constitute the aforementioned second concentration maxima. Alternatively, in the case of the etching of silicon down to an underlying silicon dioxide layer, provision can also be made for the decrease in a SiF x  concentration maximum contained in the plasma to less than the predefined value to be ascertained and analyzed in order to detect the material transition from silicon to silicon dioxide. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a graph which reproduces a fluorine concentration as a function of time for a plurality of etching steps. 
     FIG. 2 shows a graph of an individual etching step illustrated in FIG.  1 . 
     FIG. 3 shows a graph of two concentration maxima of the etching steps illustrated in FIG.  1 . 
     FIG. 4 shows a block diagram for performing an analysis of the measurement results. 
     FIG. 5 shows a detailed circuit diagram of an embodiment according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a diagram on whose ordinate the fluorine concentration S (signal strength), and on whose abscissa the time t, are plotted. The curve on this diagram indicates a plurality of etching steps Z, i.e. at each etching step Z the fluorine concentration rises to a maximum value and then, when the etching step is complete, decreases to a minimum value. The etching operation being performed is depthwise plasma etching of a semiconductor structure, for example for a sensor; when viewed from the etching side, silicon (Si) is the material which is etched down to an underlying layer of silicon dioxide (SiO 2 ). This is accomplished by way of a plurality of etching steps evident from FIG. 1, the side wall of the etched structure being protected with a Teflon-like protective film during each etching step. The plasma contains fluorine radicals; because of the discontinuous process, the concentration of free fluorine fluctuates greatly in periodic fashion, as is evident from the shape of the curve in FIG.  1 . At the beginning of each etching step Z, the fluorine concentration S reaches a first concentration maximum  1  which is followed, after a specific time period has elapsed, by a second concentration maximum  2 . This is clearly shown in FIG. 2, which shows, in an enlarged illustration, an etching step Z in terms of the change over time in fluorine concentration S. The first concentration maximum  1 , which occurs at time t 1 , is clearly evident. The second concentration maximum  2  occurs at a later time t 2 . A comparison between the first concentration maximum  1  and second concentration maximum  2  of etching steps Z in FIG. 1 shows that the first concentration maximum  1  is a value that is uncharacteristic in terms of the continuing etching process, i.e. a process artifact; in other words, the variable does not represent a characteristic variable in terms of reaching the silicon dioxide layer. The situation with the second concentration maximum  2  is different. A consideration of the individual etching steps over time (FIG. 1) shows that as the etching periods continue, the second concentration maximum  2  rises. This rise results from the increase in free fluorine in the plasma, since when the silicon dioxide layer is reached, the fluorine consumption for etching decreases and thus the concentration of fluorine in the plasma increases. Determination of the free fluorine in the plasma is preferably accomplished using an optical method, for example the light emission at wavelengths characteristic of fluorine is measured. To ensure that the uncharacteristic first concentration maxima are not taken into consideration in the analysis, FIG. 2 shown that a delay time τ is provided, beginning with a threshold value W of the fluorine concentration at a time t w , and ending at a time t e . 
     It is apparent from FIG. 2 that the first fluorine concentration peak is located in the region of delay time τ. The magnitude of threshold value W is defined as a function of the prevailing process values. Threshold value W characterizes the respective beginning of the individual etching steps Z. It thus constitutes a “trigger” for the beginning of delay time τ. What is important in this context is that threshold value W and delay time τ are selected so that the first concentration maximum  1  always definitely lies within delay time τ and consequently is blanked out, i.e. is not utilized for the analysis. 
     The fact that the desired etching depth has been reached, i.e. that the silicon dioxide layer has been exposed, is detected using a predefined value A which is shown in FIG.  1  and whose value is determined based on the process parameters. When the respective second concentration maximum  2  exceeds this value A in the course of the etching steps, this indicates that the etching has reached the desired depth, i.e. that the silicon dioxide layer has been exposed. In FIG. 1, this occurs at time t f . 
     FIG. 3 illustrates this procedure. Fluorine concentration S is plotted on the ordinate of the diagram shown in FIG. 3, and time t on the abscissa. Curve K represents the change in second concentration maxima  2  over the entire etching process, i.e. over the plurality of etching steps Z. It is apparent that it rises over time (solid line); i.e. the magnitude of the second concentration maxima increases toward the end of the etching process, intersecting with the predefined value A at time t f . This means that the silicon dioxide layer has been exposed by the etching operation. The aforementioned procedure thus makes it possible to detect the transition between different materials. 
     In accordance with a further exemplary embodiment not explained below in further detail, FIGS. 1 and 3 also show a predefined value B and a downward trend in curve K (dashed line). If, in accordance with the process described above, what is ascertained during the etching process is not the fluorine concentration but the concentration of other gaseous reaction products, for example SiF 2 , SiF 3 , or SiF 4  (i.e. SiF x ), this concentration then decreases at the end of the etching process as shown by the dashed line in FIG.  3 . These respective concentration values are also second concentration values, i.e. second concentration maxima of the above-described SiF x concentration. When this concentration maximum falls below the predefined value B, this is an indication that etching can be terminated, since the silicon dioxide layer has been exposed in the course of the individual etching steps. 
     FIG. 4 shows an implementation in the form of a block diagram for ascertaining the second concentration maxima  2 . The block diagram in FIG. 4 shows a threshold value trigger element  3 , a delay element  4 , and a deactivatable peak value detector  5 , which are electrically interconnected. The input value used for threshold value trigger element  3  is the fluorine concentration S of the plasma, which is connected directly to input  3   a . Output signal  3   b  of threshold value trigger element  3  is made available to delay element  4  as input signal  4   a . Output signal  4   b  of delay element  4  is conveyed to peak value detector  5  as a first input signal  5   a , and activates or deactivates the latter. Peak value detector  5  receives a further second input signal  5   b  which represents the fluorine concentration S. The two concentration maxima K as shown in FIG. 3 are available at the output of peak value detector  5 . 
     The following functionality results: The threshold value trigger element compares the signal for fluorine concentration S present at its input  3   a  to threshold value W described described above. Threshold value W can be predefined in magnitude in terms of the process parameters. When the fluorine concentration S reaches a value which is greater than the predefined threshold W, threshold value trigger element  3  generates an output signal  3   b  which is delivered as input signal  4   a  to delay element  4 . Delay element  4  is thereby caused to start delay time τ. the magnitude of which has also be predefined as a function of process parameters during calibration prior to startup. The beginning of delay timeτcan also be triggered by a synchronization signal of the etching machine which indicates the beginning and end of each etching step, the beginning of each etching step being relevant for the start of delay time τ. While delay time τ is elapsing, peak value detector  5  is deactivated. Once delay time τ has elapsed, peak value detector  5  is activated at its input  5   b  by way of output signal  4   b  of delay element  4 . Once peak value detector  5  has been activated, it uses input signal  5   b  present at its input, namely the fluorine concentration S, to generate at its output  5   c  the peak value for fluorine concentration S. Lastly, subsequent components (not shown in FIG. 4) are used to make a comparison between the second concentration maxima  2  and the predefined value A or B, in order to reach a conclusion as to detection of the transition between different materials in semiconductor structures during the etching process. 
     In an implementation not explained in further detail, peak value detector  5  shown in FIG. 4 can be replaced by a sample-and-hold element; this sample-and-hold element, by multiple sampling of fluorine concentration S and storage of those fluorine concentration values using a comparator, makes available at its output the approximate second concentration maximum value ascertained in this fashion. As in the case of peak value detector  5 , the sample-and-hold element is activated and deactivated by input signal  5   a  arriving from delay element  4 . 
     FIG. 5 shows a more detailed circuit diagram of an embodiment for carrying out the method according to the present invention. The circuit comprises the following components: 
     An impedance converter  10  with a gain factor of one is provided, at whose input  11  the signal ascertained during the concentration measurement is present as the output signal of an optical emission spectrometer. Also provided is a comparator  12  with hysteresis. Further components to are two timers (monoflops)  13  and  14 , timer  13  being used to implement the delay time τ of a sampling stage  15 , and timer  14  being provided to transfer the information from sampling stage  15  to a hold stage  16 . 
     The following functionality results: The output signal of the optical emission spectrometer, which is present at input  11  of impedance converter  10 , is conveyed, with a gain factor of one for decoupling from the remaining circuit electronics, via a diode  17  and a protective resistor  18  to a storage capacitor  19  and thus to sampling stage  15 . The arrangement of diode  17  and storage capacitor  19  stores the respective maximum value of the output voltage of the optical emission spectrometer reached for each etching step. Connected in parallel with storage capacitor  19  is a discharge transistor  20 ; the latter short-circuits capacitor  19  during delay time τ, causing sampling to be deactivated. Memory capacitor  19  is thus set to zero during delay time τ, and therefore cannot be charged. Control of delay time τ is handled by comparator  12  with hysteresis: when the output voltage of the emission spectrometer exceeds the threshold value W preset with a “trigger level” potentiometer  21 , timer  13  for delay time τ is started (via a diode  22 , a resistor  23 , and a transistor  24 ) with the leading edge of the voltage signal at the collector of transistor  24 . Output 1Q of timer  13  thus goes to “high” potential, and via transistor  20  short-circuits storage capacitor  19  of sampling stage  15  during the period of delay time τ. After delay time τ has elapsed, storage capacitor  19  is enabled so that it can store the maximum value  2  of the output voltage of the optical emission spectrometer reached during the current etching cycle. As long as the etching cycle is in progress, i.e. the measured output voltage of the optical emission spectrometer is above threshold value W, the collector of transistor  24  is also at “high” potential. At the end of the etching cycle, the output voltage of the optical emission spectrometer, and thus the input voltage at input  11 , drop below threshold value W and the collector of transistor  24  drops to “low” potential. The trailing edge of the collector voltage of transistor  24  sets timer  14  in order to transfer to a storage capacitor  25  of hold stage  16  the maximum voltage reached during the previous etching cycle and stored in storage capacitor  19 . In this context, timer  14  generates a short output pulse at its output {overscore ( 2 Q)}, which briefly, via a transistor  26  and a switch IC  35 , connects storage capacitor  25  of hold stage  16  to the output of a buffer amplifier  27 . The maximum value of the voltage achieved in the previous etching cycle is thereby transferred from storage capacitor  19  of sampling stage  15  to storage capacitor  25  of hold stage  16 , and retained there. The two timers  13  and  14  are interlocked with one another, i.e. output {overscore ( 1 Q)} acts on clear input {overscore ( 2 CLR)} and {overscore ( 2 Q)} acts on clear input {overscore ( 1 CLR)}, so that only one timer can be set at any one time. This prevents sampled information from being deleted during the transfer of sampled information to hold stage  16 , and prevents any transfer to hold stage  16  while sampled information is being deleted. 
     Hold stage  16  thus stores the respective maximum value 2 of the signal intensity reached in the previous etching step, and is updated with the new maximum value 2 after each etching step. The input of comparator  12  with hysteresis can selectably be connected to the output of decoupling amplifier/impedance converter  10 , and thus, as described, the output voltage of the emission spectrometer can be utilized to detect etching or deposition steps, i.e. the beginning and end of each etching step; or the input of comparator  12  is connected, by placement of a jumper  28 , to the synchronization signal of the etching system via an external trigger input  29  of the circuit. The etching system then informs comparator  12 , via a voltage signal, when etching is being performed and when deposition is occurring. An operational amplifier  30  transfers the voltage value of hold stage  16  to its circuit output  31 , circuit output  31  being dimensioned such that a certain loading can occur. Buffer amplifier  27  makes possible the charge-transfer process from capacitor  19  to capacitor  25 , while preventing any voltage dip due to loading.