TRENCH ETCH ENDPOINT DETECTION BY LASER-INDUCED FLUORESCENCE

Abstract: Laser induced fluorescence is utilized to detect the reactive ion etch-through of a given layer in a wafer by detecting a large change in the concentration of a wafer selected minor species from the wafer in the etching plasma. This selected minor species must be present in a significantly different concentration in the etched given layer compared to any of the other layers in the wafer in order to provide a proper endpoint detection. In one embodiment, when the large change in the selected minor species concentration is detected, then the RF electrodes for the reactor are de-energized.

Description: The present invention relates generally to reactive ion etching endpoint detection, and more particularly, to endpoint detection via laser induced fluorescence. Reactive ion etching or plasma etching is now typically used to delineate fine line patterns and trenches in thin films of either insulators, semiconductors, or metals by means of the removal of portions of these films in a plasma discharge. The wafer to be etched is placed in a plasma chamber into which an inert gas is directed at a reduced pressure. In the presence of a plasma discharge generated by the application of RF energy, reactive species are generated from the inert gas by processes such as dissociative or impact ionization. Portions of the thin film are removed by chemical reaction between the reactive species and the film, as well as by bombardment of the film by ions present in the plasma. The gaseous reaction products formed by the chemical reactions and the ion bombardment of the film are continuously removed from the chamber using a vacuum pump. One of the problems encountered in the use of reactive ion etching is the insufficient reproducibility of the etching rate. This reproducibility problem is caused by difficulties in completely controlling the purity of the plasma gas and in completely controlling the surface temperature of the wafer to be etched. Because of this variation in the etching rate, reactive ion etching in many cases requires monitoring to detect the completion of the etching process. In this regard, it is important to detect the end of the etching process in order to terminate the etch before over-etching occurs in the sublayer below the layer being etched. Such over etching is detrimental not only because it attacks the substrate or sublayer below the layer being etched, but also because it causes under-cutting of the etch mask, thereby altering the dimensions of the desired pattern in the etched layer. In a typical endpoint detection scheme, a majority chemical species from the layer being etched enters the etching plasma and is observed by monitoring a relevant spectral line for that majority species as the etching process consumes the layer being etched. The time to terminate the process is inferred from changes in the intensity of this monitored majority species spectral line. The monitoring of the majority species coming from the etched layer works well in many situations where the etched layer and the sublayer therebelow are composed of different materials. However, when the composition of the etched layer and its sublayer therebelow are similar or the same, then monitoring of the majority species from the etched layer will not provide a determination of the etch endpoint. Such a process is disclosed in EP-A-0 070 523 in which a gas etching is monitored and adjusted via an induced fluorescence. The gaseous phase used in the process to be monitored is probed by excitation with a suitable energy source. The emission from the gas phase induced through this excitation is then monitored and yields an accurate measure of concentration of the active species present. In turn the conditions of the fabrication process are adjusted based on these discerned concentrations. A similar problem is encountered when techniques are utilized to compensate for etch loading non-uniformities. For example, an aluminum film on a wafer is many times etched by disposing the wafer on a high purity aluminum target to thereby prevent a sudden large excess of etching ions near the end of the etch process which would cause an attendant undercutting of the aluminum film. However, the use of this aluminum target prevent the determination of the etch endpoint by monitoring the majority aluminum species. The invention as claimed is intended to remedy the above-described drawbacks. Specifically, it solves the problem of detecting the etching endpoint through a film when the majority species in the etched film and in the sublayer therebelow are the same. Additionally, the present invention solves the problem of detecting the etching endpoint through a film when the majority species in that etched film is the same as a specially designed target disposed adjacent to the wafer being etched. Briefly, the present invention comprises a method for automatically controlling the etching of an integrated circuit wafer in the chamber of a reactive ion etching reactor when a given layer of wafer is etched through to a second layer therebelow, wherein the given layer and the second layer have different concentrations of a minor species, with the layer concentration thereof being less than or equal to 10% of the wafer layer by weight. The method comprises the steps of etching the wafer in the etching reactor with an etching ion plasma suitable for etching the given layer through to the second layer and for causing the minor species to enter the plasma, wherein when the given layer is etched through to the second layer, the concentration of the minor species changes; directing a laser beam through the plasma in the chamber, with the laser beam having a first frequency suitable for pumping the minor species in the plasma to an excited energy state thereof, with the pumped minor species in the excited energy state then emitting a second frequency of radiation as the pumped minor species decays to a lower energy state; detecting a second frequency of radiation in the chamber; generating a control signal when a change in an intensity parameter of the second frequency of radiation is detected, thereby indicating that the given layer has been etched through to the second layer; and changing the plasma generation in the chamber in accordance with the control signal. In one embodiment of the present invention, the given layer being etched is an Al film with a Cu minor species therein, and a second layer is disposed therebelow which does not contain Cu as a minor species. In this embodiment, a plasma containing Cl is utilized and the laser beam is directed with a first frequency which is suitable for pumping CuCl. In a further embodiment of the present invention, the given layer to be etched is SiO2 without Al doping, and the second layer therebelow includes a plurality of Al alloy studs therein with Al comprising the minor species to be monitored. In this embodiment, a plasma containing F is utilized and a laser beam with a first frequency suitable for pumping an Al minor species of AlF is utilized. In yet a further embodiment of the present invention, the given layer to be etched is chosen from a group of Si with As as the minor species, and Si with P as the minor species. When the given layer is Si with As as the minor species, a plasma is utilized which causes the As minor species to form a species in the plasma from the group consisting of AsCl and AsF. In yet a further embodiment of the present invention, polycrystalline silicon which is heavily doped with a minor species of B is utilized and a second layer is disposed therebelow containing a B minor species therein with a concentration that is at least 50% less than in the given layer. Brief Description of the Drawings Fig. 1 is a schematic block diagram of one embodiment of the present invention. Fig. 2 is a schematic cross-section of an Al wafer layer which may be etched utilizing the present invention. Fig. 3 is a schematic cross-sectional diagram of a set of polysilicon layers which may be etched utilizing the present invention. Fig. 4 is a schematic cross-sectional diagram of a chip being etched down to Al studs. Fig. 5 is a schematic cross-sectional diagram of a silicon wafer being etched through a doped subcollector. The present invention sets forth a method and a means for modifying the etching in a reactive ion etching device in accordance with the change in concentration of a minor species as a given layer in the wafer is etched through to a second layer therebelow. It has been discovered that minor species of less than 0,1% of a wafer layer by weight can be detected by the laser-induced fluorescence technique utilizing a pulsed laser beam. It has been discovered that such minor species can be detected with unusually high resolution using the LIF technique, despite the required pulsed nature of the LIF laser beam. In this regard, the present invention broadly covers the monitoring by laser induced fluorescence of a minor species in one or more layers of a wafer being etched in an etch reactor and the use of a control signal generated in accordance with this minor species monitoring to control the etching in the etch reactor. The invention will be described in the context of a number of different examples. It should be noted that these examples are set forth, not by way of limitation, but to illustrate a number of different aspects of the present invention. Referring now to Fig.1, there is shown one embodiment of a system 10 for the preferential etching of monocrystalline silicon or other semiconductor material (in regions not masked by silicon dioxide or silicon nitride) and which may be utilized to implement the present invention. The system 10 includes a plasma etching reactor 12, which may be implemented by any of a variety of currently available plasma or reactive ion etching devices. One such apparatus is the parallel-plate reactive ion etcher described in the article by J. Lechaton and J.L. Mauer in the Proc. of the Symposium on Plasma Etching and Deposition Electrochem Soc., Vol. 81-1, pp. 75-85, 1981. A standard planar-type reactor 12 with parallel plate electrodes is shown in the figure. The chamber of etching reactor 12 is typically pumped down to a vacuum equal to or less than 6.7x10⁻⁴ Pa (5x10⁻⁶ torr). This reactive etching chamber is then backfilled with a gas mixture appropriate to forming a desired plasma, for example, argon and chlorine with the chlorine being 3% to 7% of the argon-chlorine gas mixture. Other gases, such as SF₆/Cl/ He and CF₄ can also be used in this mixture. In the system 10 this example, a 13,56 MHz RF power supply 14 is connected to the reactor 12 to provide the RF power to the electrodes in the etch reactor chamber. An integrated circuit wafer 16 is shown positioned in the etch reactor. It is, of course, understood that a multi-wafer etch reactor may be utilised in place of the single wafer etch reactor shown in fig.1. One or mor gas inlets are provided (not shown) in order to insert the suitable gas mixture into the etch reactor chamber 12 in order to generate the desired plasma in the chamber. In order to obtain laser-induced flourescence (LIF) detection, a laser 18 is provided to direct a laser beam through a set of opposing windows 20 and 22 in the etch reactor chamber 12. A variety of portable and inexpensive lasers are available on the market which can be utilized to implement the present invention. The type of laser utilized will depend on the wavelength needed to pump the minor species to be monitored in accordance with the present invention. By way of example, and not by way of limitation, a Laser Science DCR1 nitrogen-pumped dye laser may be utilized as the laser 18. In the alternative, an Nd:YAG laser comprising a PDL1 dye laser pumped by a Quanta Ray DCR2 layer may be utilized with frequency doubling using a KD star P crystal. The windows 20 and 22 may be realized simply by standard quartz windows, which are capable of passing radiation in the range of 170 NM to 4500NM. As noted above, the spectral envelope of the light from the laser beam is chosen to overlap an absorption line of a selected minor species to be monitored. Directing the laser beam 18 with the appropriate spectral envelope through the plasma in the etch reactor chamber 12 causes the selected minor species in the plasma to be excited to a predetermined upper energy state. These excited minor species molecules then decay down to a lower energy state and emit a second frequency of radiation during this decay. This second frequency of radiation (fluorescence) is indicative of the presence of the minor species in the plasma. The relative intensity of this second frequency of radiation is indicative of the concentration of the minor species in the plasma. In order to detect this second frequency of radiation, a third window 24 of suitable material is provided in the etch reactor chamber 12. Typically, this window 24 will be located at some angle relative to the line between the two windows 20 and 22 in order to substantially reduce the detection of scattered light from the laser excitation source 18. The actual means for detecting this second frequency of radiation is not critical. By way of example, a set of lenses 26 and 28 in combination with a mirror 30 may be utilized to provide radiation obtained from this third window 24 to a detector such as a photomultiplier tube 34. By way of example, the photomultiplier tube made by RCA may be utilized. In order to remove other extraneous frequencies from the light obtained from the window 24, a monochromator or a filter 32 may be utilized. Generally, this monochromator or filter 32 is disposed between the lens optics 28 and the photomultiplier tube 34. By way of example, this filter 32 may be realized by a dielectric filter with a 5nm pass band. In order to improve the signal-to-noise ratio of the second frequency signal, a pulsed laser 18 is used in combination with a gated integrater 36 which is connected to receive the signal from the photomultiplier 34 and which is triggered by a control signal from the laser 18. A typical laser pulse duration is from 5 to 50 nanoseconds. The gated integrater 36 is synchronized to the beginning of the laser pulse to remove a significant amount of the background interference from the signal. The present system is designed to monitor a parameter of this second frequency of radiation in order to determine when a change has occurred in the etching plasma composition in the etch reactor chamber 12. Typically, this plasma composition change in the reactor will be the result of a given layer on the wafer being etched through down to a second layer therebelow. In general, the parameter that is monitored is the intensity of the second frequency of radiation that is detected by the photomultiplier 34. In order to detect a change in this intensity parameter of the second frequency of radiation, the gated signal from the integrater 36 is provided via a line 38 to a comparator 40. The comparator 40 compares the amplitude of the electrical signal from the integrater 36 to a reference provided on line 42. The comparator 40 generates a signal on line 44 to control the operation of the RF power supply 14 in accordance with this comparison. The operation of the foregoing apparatus will now be described for the etching of a given layer of a wafer down to a second layer set therebelow, wherein the given layer and the second layer have different concentrations of a predetermined minor species, with the larger of concentration thereof being less than or equal to l0% of either of the wafer layers by weight. The wafer to be etched is placed in the reactor chamber 12, and an appropriate gas mixture is provided to the chamber to facilitate plasma generation. Then, RF power is provided to the chamber in the well known manner by the RF power supply 14 to generate a desired etching plasma in the chamber for etching the given layer of the wafer 16. Majority and minor species atoms and molecules from the wafer layers being etched enter the plasma during the etching process. During this etching operation, a laser beam from the laser 18 with an appropriate wavelength envelope for exciting a selected minor species is directed through the plasma in the reactor chamber 12 via the windows 20 and 22. This laser beam excites the selected minor species in the plasma to an excited energy state. These excited minor species molecules then decay to a lower energy state and emit the second frequency of radiation during this decay. This second frequency of radiation is brought out from the etch reactor chamber 12 via the optics 26, 28, and 30 to the photomultiplier 34 and causes the photomultiplier to generate a signal indicative of a certain range of concentrations for the minor species in the plasma. When this given layer is etched through to the second layer therebelow, a relatively sharp change in the concentration of the minor species in the plasma in the reactor chamber 12 takes place because of the difference in concentration of the minor species in the given layer and in the second layer. This change in the minor species concentration in the plasma results in a change in the intensity of light detected at the photomultiplier 34, and thus a change in the electrical signal on line 38 applied to the comparator 40. The comparator 40 then operates to generate a control signal 44 to turn off the RF power supply 14, thereby stopping the generation of the plasma in the etch reactor 12. Various examples of the operation of the present invention will now be described in the context of specific wafer structure. Example 1 During the reactive ion etch processing of aluminum films, wafers are typically mounted on a high purity aluminum target to avoid loading non-uniformities during the tail end of the etch process. The high purity aluminum target prevents a sudden excess of etching species and thereby the undercutting associated with changes in the etch load. However, because the aluminum target contributes a large background of aluminum in the plasma concentration of the Al species cannot reliably indicate the etch endpoint for the Al film. However, standard Al films have approximately 2-5% copper therein as a dopant. When a standard Cl containing plasma, i.e., BC13 plasma, is utilized, then a Cu minor species of CuCl is generated and becomes part of the plasma during the etching of the Al film. Because of the very low concentration of the Cu in the Al alloy, a sensitive detection of the CuCl in the plasma was considered unlikely. However, when a nitrogen pumped dye laser at 100 Kwatt power was directed through the plasma in the chamber 12, and was tuned to the 515,2 nm line in the CuCl absorption band (which corresponds to the (1,0)A¹π-X¹ε transition), then very good resolution was obtained by monitoring the resultant fluorescence. In this particular experiment, the [CuCl] x emission band at 538,l nm which corresponds to the (1,2,)A-X transition was monitored. This transition wavelength was advantageous because it is well separated from the laser wavelength. It should be noted that the resolution of this CuCl line width was very good despite the fact that the laser pulsing in combination with gated detection was utilized in order to minimize background emission from the plasma. Accordingly, the present technique was utilized to determine the etch endpoint by monitoring the CuCl concentration in the plasma via the LIF detection of a particular laser transition line therefor. This technique of monitoring a transition from a minor species in the layer is advantageous in that it is insensitive to the interference generated by the aluminum target and thus provides a very accurate endpoint. Additionally, this minor species monitoring technique provides an endpoint detection which is usable in a multiple wafer tool where reflectance monitoring would only indicate an etch rate for a single spot on a single wafer. In contrast to reflectance monitoring, because the plasma composition is determined by the etching of all of the wafers present in the reactor, the CuCl transition detection technique disclosed herein may be utilized to determine when the last wafer is fully etched, i.e., when no more [CuCl] x is generated. As noted previously, the signal on line 38 is an electrical signal indicative of the concentration of the CuCl in the plasma. When the amplitude of this signal falls below a certain predetermined level, then the given layer of the wafer or wafers present in the reactor chamber 12 are considered to be etched through. Accordingly, when the signal on line 38 drops to this predetermined level, it is desired to generate a control signal on line 44 to turn off the RF power supply 14. The comparator 40 operates to generate this control signal on line 44 after comparing the amplitude of the signal on line 38 to an appropriate reference signal on line 42. By way of example, for the detection of the etch endpoint for an Al layer with 4% Cu, the reference signal on line 42 should be less than or equal to 5% of the initial signal value on line 38. Accordingly, when the level of the signal on line 38 is determined by the comparator to be less than the 5% signal reference on line 42, then the comparator 40 generates the control signal on line 44 to turn off the RF power supply 14. Fig. 2 provides a schematic cross-sectional representation of a given layer 50 of Al with 4% Cu and with a second layer 52 therebelow of SiO2. Various species of Al and Cu are shown entering the plasma during the etching of the given layer 50. Example 2 Referring now to Fig. 3, there is shown a schematic cross-sectional diagram of a given layer 60 of polycrystalline silicon with a heavy doping of boron of on the order of 10²⁰atoms/cm3 being etched through to a second layer 62 therebelow of polycrystalline silicon with a low boron doping of on the order of 10¹⁵ - 10¹⁶ atoms/cm3. Various boron species are shown entering the plasma. It can be seen that in this etching situation, the minor species of boron is present in both layers 60 and 62. However, the concentration of the boron minor species in the layer 62 is reduced by at least 50% below the concentration of the boron in the given layer 60. Accordingly, in order to determine the etching endpoint through the given layer 60, the reference value on line 42 of the comparator 40 is provided with a value which is decreased by at least 50% from the initial value of the signal on the line 38 when etching of the given layer is first begun. When the amplitude of the signal on line 38 drops below this 50% reference signal value on line 42, then the comparator 40 generates the control signal on line 44 to turn off the RF power supply 14. In the Example, a 100 Kwatt power laser beam with a wavelength of 272 nm is used to pump the ---> transition of BCl. The second frequency of radiation was obtained by monitoring the 284,75 nm wavelength for the ---> transition. Alternatively, a laser beam with a 195.75nm wavelength could be used to pump the ---> transition of BF. The second frequency of radiation is obtained by monitoring the 201.1nm wavelength for the ---> transition. Example 3 Example 3 relates to the endpoint detection for a vertical wiring process utilizing metallic studs. In this regard, it is important to stop the etching as soon as all of the metallic studs are exposed to prevent the capacitance and insulation problems that result from overetching into the surrounding layer. Fig. 4 is a schematic cross-sectional diagram showing a given layer 70 of SiO2 to be etched through down to metallized studs 72 of, for example, aluminum. The studs 72 are typically set in a layer 74 of, for example, SiO2. In this particular instance, the aluminum studs 72 constitute a very small portion of the entire layer 74 and thus constitute the minor species to be monitored. In operation, when the plasma etches through the given layer 70 down to the studs 72, various Al species enter the plasma. The amount of Al minor species in the plasma can be monitored to determine when the etching has reached the first studs, and to determine when all of the studs have been exposed. In the particular Example of Fig. 4, an F containing plasma is utilized to convert the surface 76 of the Al alloy studs 72 to AlF. This AlF surface 76 is sputtered off by the energetic ions in the plasma. The presence of the AlF minor species may be monitored by tuning the laser 18 to the 227,47 nm wavelength for AlF (transition ---> ) and then detecting the 231,64nm wavelength for the ---> transition flourescence. In order to detect when the plasma etching has reached the first Al studs 72, the reference value on line 42 to the comparator 40 may be set to a level which is on the order of l0% greater than the amplitude of the signal on line 38 when it is etching through the SiO2 given layer 70. When the amplitude of the signal on line 38 increases above the reference level on line 42, then the comparator 40 generates a signal on line 44. This signal on line 44 may be utilized to energize a timer 80, shown in dashed line representation in Fig. 1. The timer 80 may be used to count for a predetermined period of time equal to the time it takes to expose all of the studs after the first stud has been exposed. When the timer 80 has counted to this predetermined time value, it then may generate a signal to turn off the RF power supply 14. In the alternative, an integrating logic circuit (not shown) could be connected directly to line 38 and could be utilized, after the comparator 40 has determined that the first stud has been exposed, to integrate the value of the signal on line 38 to determine when the amplitude of that signal has reached a constant value indicating that all of the studs 72 have been exposed. Example 4 The final example relates to the detection of the etching endpoint through a doped subcollector region. Fig. 5 is a schematic cross-sectional diagram of a wafer with such a subcollector structure. More specifically, a top layer 90 of N- doped silicon is shown, with a subcollector layer 92 of N+ doped silicon disposed therebelow. This N+ subcollector layer 92 comprises the given layer to be etched through. A second layer 94 set below the given layer 92 comprises P- doped silicon. It is desired to etch a trench 96 through the subcollector given layer 92. In the wafer in Example 4, the silicon doping for the layers 90, 92 and 94 is typically either P or AS. A typical doping concentration for the subcollector given layer 92 is 10²⁰ atoms/cm3. Likewise, a given doping concentration for the second layer 94 therebelow is 10¹⁵ - 10¹⁶ atoms/cm3. With respect to the structure of Fig. 5, it is important that the trench 96 be etched completely through the N+ subcollector given layer 92 in order to avoid electrical leakage. Current techniques for monitoring the etch rate through such a subcollector region include laser interferometry on the wafer surface and the measurement of the etch time as a means of estimating etch depth. However, because of the varying thicknesses of the oxides and photoresists utilized in forming the wafer, the depth of the subcollector given layer 92 below the surface varies. Accordingly, since both of these techniques monitor the etch depth, they are inherently unreliable for determining the etch endpoint through the subcollector given layer 92. Note that in this structure the major species of Si does not change during the trench etch. In order to monitor and detect the endpoint of the etch through the given layer 92 in accordance with the present invention, the concentration of a minor species from the subcollector is monitored in the plasma. In the Example of Fig. 5 with an As-doped subcollector given layer 92, etching in a halogen plasma (i.e., CCl2F2 or CClF3) produces a trace quantity of AsCl or AsF in the plasma when the trench etch reaches the subcollector given layer 92. Note that very small amounts of As of on the order of 10¹⁴ atoms/cm3 are in layer 90 and in layer 94. Accordingly, very small amounts of AsCl or AsF may be generated during the etching of the epi layer 90 above the subcollector given layer 92, or during the etching of the substrate layer 94 below the subcollector given layer 92. However, since the dopant concentration in the subcollector given layer 92 is 10⁶ times greater than in the surrounding layers, a signal tuned to an appropriate transition for AsCl or AsF will increase rapidly as the subcollector given layer 92 is reached, and then drop precipitously as the given layer 92 is etched through. In Example 4, a tunable dye laser made by Laser Science, No. DCM-01, was focussed on a point approximately 1cm above the target in the etch reactor. The laser may be tuned to a wavelength corresponding to an electronic state of the minor species being monitored. In the example for As doping, a laser pumping wavelength of 244,63nm may be utilized to detect a minor species of AsCl at the 249,98nm emission wavelength. Alternatively, a laser pumping wavelength of 239,88nm may be utilized to detect a minor species of AsF at the 248,03nm emission wavelength. As a further alternative, minor species As atoms may be used for the endpoint detection. In this regard, during etching the As volatilizes as AsF and AsCl x , and a portion of these species is fragmented back to As in the plasma via plasma dissociation. It has been discovered that the As atom 4p³ ⁴S 3/2 ---> 5s ⁴P 3/2 transition at 193.75927nm is coincident with the 193nm ArF excimer laser output. The second frequency of detection for this state would then be the 245.6529nm (5s ⁴P3/2 ---> 4p3 2 D 5/2 transition) or the 303.2847nm (5s ⁴P 3/2 ---> 4p³ ²P 3/2 transition). The As atoms present in the plasma can be detected with extremely high sensitiviy using these transitions. Table 1 suggests pump wavelengths for various plasma constituents, arising from either Si or GaAs etching. A suggested emission wavelength is also provided for each minor species dopant. It should be noted that Table 1 lists only the strongest suggested LIF wavelengths. Besides those wavelengths mentioned in the Table, other wavelengths in the UV, the visible, or the IR regions may be employed for greater convenience. Substrate Dopant Detect Suggested Laser Pump,nm Suggested Emission Observe,nm Si B (p type) BCl 272,0* 284,75 BF 195,74* 201,10 P (n type) PCl 241,86* 248,69 PF 445,35 381,95 GaAs Zn (p type) ZnCl 380-470 380-470 ZnF 258,77* 267,31 S (n type) SF 357,1 360-370 Si (n type) SiCl 294,22* 301,76 SiF 436,82 453,59 With reference to Fig. 1, the RF power supply 14 should be turned off when the minor species being monitored drops in concentration by at least 50%. In order to detect this 50% drop in concentration, the reference level on line 42 is set to a level which is approximately 50% less than the amplitude of the signal on line 38 when it is etching in the subcollector given layer 92. Accordingly, when the amplitude of the signal on line 38 drops below the 50% reference level on line 42, the comparator 40 generates a control signal on line 44 to turn off the RF power supply 14. It can be seen from the above Examples that the present endpoint detection apparatus and method can be utilized for both Si wafers, GaAs wafers, or other III-V semiconductor wafers. With this technique, it is possible to monitor selected minor species which have been converted from the bulk layers being etched into the gaseous plasma. This technique is highly sensitive to the chemical composition of each etch layer and does not rely on any form of measurement of the etch depth. The apparatus set forth in Fig. 1 exercises control over the etch reactor chamber 12 by turning off the RF power supply 14. It should be noted, however, that etch control may also be exerted by controlling the flow of etch gases into the etch reactor chamber 12. Typically, the laser beam will be directed through the etch chamber 12 at a height of approximately 5cm above the wafer being etched. The location of the laser beam at this height above the wafer will provide a generalized picture of the plasma which is very useful for batch type reactors. However, it may be desirable to direct the laser beam at a height which is very close to the wafer surface, i.e., approximately 2mm or less, in order to obtain a localized monitoring of the plasma above a particular region on the wafer. It should be noted that for high pressure single wafer reactors where the electrode plates are separated by only 3mm, this relatively low height for the laser beam may be necessary. In contrast, low pressure wafer reactors typically have electrode plates which are separated by 50mm, and thus permit a variety of laser beam heights above the wafer. It should be noted that it may be desirable to scan the laser beam across the wafer in order to get detection samples of the plasma fluorescence at various points therein. Such laser beam scanning may be necessary if the laser beam is disposed very close to the surface of the wafer in order to monitor the plasma over a variety of separated localized etch regions. Such a technique would provide extremely high sensitivity for each etch region being monitored. Laser beam scanning can be implemented simply by mechanically varying the position of the beam optics used to direct the beam through the window 20 of Fig. 1. In general, laser beam focusing is not necessary for etch endpoint detection. However, such focusing does improve spacial resolution and would be useful where localized plasma monitoring is desired. The present technique may also be utilized to adjust the etching rate in the etch reactor 12. The etching rate may be controlled by controlling the gas inlets into various points in the etch reactor chamber or by varying the RF intensity at various points in the reactor chamber 12 to thereby change the etching at those points in the chamber. In one embodiment, the etch reactor chamber 12 could include at least 2 closable gas inlets into the chamber, each for providing a suitable gas mixture into a different predetermined volume of the chamber. These gas inlets would then be regulated to control the amount of gas being applied to the reactor chamber to thereby control the etching therein. In order to monitor the etching uniformity in the reactor chamber 12, the laser beam 18 would include a focusing lens 100 for focusing the laser beam to a predetermined point in the plasma in the chamber 12. A standard scanning apparatus would then be utilized to mechanically scan this lens 100 and any other focusing optics to thereby scan the focused point over a plurality of predetermined volumes in chamber 12. The detecting apparatus would then include means for mechanically moving the detecting optics to thereby monitor the fluorescence from this scanned focused point in the chamber 12. The detecting means would further include means for comparing the intensity of the second frequency of radiation obtained for each of the focused points (or any integrated value therefor) in a given one of the plurality of predetermined volumes in the plasma to a reference value, and then generating a signal indicative of this comparison as a control signal. This control signal would then be applied to regulate the amount of gas provided through the at least two gas inlets to thereby obtain etching uniformity. This apparatus could also be used to provide a profile of the etch concentration in the reactor chamber 12. While the invention has been illustrated and described with respect to preferred embodiments of this invention, it is to be understood that the invention is not limited to the precise construction herein disclosed, and the right is reserved to all changes and modifications coming within the scope of the invention, as defined in the appended claims.

Claims: A method for automatically controlling the etching of an integrated circuit wafer in the chamber of a reactive ion etching reactor when a given layer (50) of said wafer is etched through to a second layer (52) therebelow, wherein said given layer and said second layer have different concentrations of a minor species, with the larger concentration thereof being less than or equal to 10% of either of the wafer layers by weight, the method comprising the steps of: etching said wafer (16) in said etching reactor with an etching ion plasma suitable for etching said given layer through to said second layer and for causing said minor species to enter said plasma, wherein when said given layer is etched through to said second layer, the concentration of said minor species in said plasma changes; directing a laser beam through said plasma in said chamber, with said laser beam having a first frequency suitable for pumping said minor species in said plasma to an excited energy state thereof, with said pumped minor species in said excited energy state then emitting a second frequency of radiation as said pumped minor species decays to a lower energy state; detecting said second frequency of radiation in said chamber; generating a control signal when a change in an intensity parameter of said second frequency of radiation is detected, thereby indicating that said given layer has been etched through to said second layer; and changing the plasma generation in said chamber in accordance with said control signal. A method as defined in claim 1, wherein said etching step comprises the step of generating said suitable plasma by applying RF energy into a suitable gas in said etching reactor; and wherein said plasma generation changing step comprises stopping the application of said RF energy to said suitable gas in said etching reactor in accordance with said control signal, wherein said laser beam directing step comprises pulsing said laser beam through said plasma; and wherein said detecting step comprises detecting for said second frequency of radiation only for a predetermined time period after initiation of each laser beam pulse. A method as defined in claim 2, wherein said etching step comprises the step of etching a wafer with a given layer of Al film with a Cu minor species, and with a second layer which does not contain Cu, wherein said etching step includes the step of etching with a plasma containing Cl, wherein said laser beam directing step includes the step of directing said laser beam with a first frequency which is suitable for pumping a Cu minor species of [CuCl] X to an upper energy state, and wherein said control signal generating step includes the step of generating said control signal when the intensity parameter of said second frequency of radiation decreases to less than 5% of its initial value. A method as defined in claim 2, wherein said etching step comprises the step of etching a given layer of SiO2 without Al doping and with a second layer there-under including a plurality of Al alloy studs therein which contact the bottom surface of said SiO2 layer, with Al comprising said minor species to be monitored, wherein said etching step comprises the step of etching with a plasma containing F, wherein said laser beam directing step comprises the step of directing said laser beam with a first frequency which is suitable for pumping an Al minor species of AlF to an upper energy state, wherein said control signal generating step comprises the step of generating said control signal when the intensity parameter of said second frequency of radiation increases to a constant value. A method as defined in claim 2, wherein said etching step comprises the step of etching a given layer of Si with As as said minor species by generating a plasma which causes said As minor species to form a species in the plasma from the group consisting of AsCl and AsF, wherein said control signal generating step comprises the step of generating said control signal when the intensity parameter of said second frequency of radiation decreases by at least 50%, A method as defined in claim 2, wherein said etching step comprises the Step of etching a given layer of polycrystalline silicon heavily doped with a minor species B and with said second layer thereunder containing a B minor species therein with a concentration that is at least 50% less than that in said given layer, and wherein said control signal generating step comprises the step of generating said control signal when the intensity parameter of said second frequency of radiation decreases by at least 50%. A method as defined in claim 2, wherein said laser beam directing step includes the step of focusing said laser beam to a predetermined point in said plasma and scanning that focused point over a plurality of predetermined volumes in said chamber, wherein said detecting step includes the step of detecting said second frequency of radiation emitted from said scanned point in said plasma. wherein said etching step includes the step of directing a suitable gas for generating said plasma into said etching reactor by means of at least two closable gas inlets thereinto, with each gas inlet providing said suitable gas into a different one of said plurality of predetermined volumes in said chamber for forming said plasma therein, wherein said detecting step includes the step of comparing the intensity of said detected second frequency of radiation of said focused points in a given one of said plurality of predetermined volumes in said plasma to a reference value, wherein said control signal generating step includes the step of generating a signal indicative of the comparison as said control signal, and wherein said plasma generation changing step includes the step of regulating the amount of gas provided to said given one of said plurality of predetermined volumes by its respective gas inlet in accordance with said control signal. A method as defined in claim 2, wherein said etching step comprises the step of etching a given layer chosen from the group consisting of Si with P as said minor species, Si with B as said minor species, GaAs with Zn (p type) as said minor species, GaAs with S (n type) as said minor species, and GaAs with Si (n type) as said minor species.

Inventors: BENNETT, REID STUART, EPHRATH, LINDA MERO, SCHWARTZ, GERALDINE COGIN, SELWYN, GARY STEWART
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1987
Main classification: H01J37
IPC codes: H01L21, C23F1, G01N21, H01J37