METHOD AND APPARATUS FOR ETCH RATE STABILIZATION

Abstract: A method of consecutively processing a series of semiconductor substrates with minimal etch rate variation. The method includes steps of (a) placing a semiconductor substrate on a substrate support in a plasma etching chamber, (b) maintaining a vacuum in the chamber, (c) etching an exposed surface of the substrate by supplying an etching gas to the chamber and energizing the etching gas to form a plasma in the chamber, (d) removing the substrate from the chamber; and (e) consecutively etching additional substrates in the chamber by repeating steps (a-d), the etching step being carried out by maintaining an amount of reactive gas in contact with an exposed surface of the substrate at a level sufficient to obtain a repeatable rate of etching for each of the substrates processed during repeated step (c). An apparatus useful for carrying out the method includes a plasma etching chamber having a substrate support in an interior thereof and a dielectric member facing the substrate support, a gas supply supplying etching gas into the interior of the chamber, an antenna passing RF energy through the dielectric member, a vacuum pump separated from the interior of the chamber by an adjustable gate valve, and a controller maintaining an amount of reactive gas in contact with an exposed surface of the substrate at a level sufficient to obtain a repeatable rate of etching for each substrate processed in the chamber during consecutive single substrate etching of batch of substrates.

Description: METHOD AND APPARATUS FOR ETCH RATE STABILIZATION Field of the Invention The invention relates to improvements in a plasma processing chamber and to a method of plasma etching metal, polysilicon or dielectric layers on substrates such as semiconductor wafers. Background of the Invention Plasma processing systems wherein an antenna coupled to a radiofrequency (RF) source energizes gas into a plasma state within a process chamber are disclosed in U.S. Patent Nos. 4,948,458; 5,198,718; 5,241,245; 5,304,279; 5,401,350; and 5,571,366. In such systems, the antenna is located outside the process chamber and the RF energy is supplied into the chamber through a dielectric wall or window. Such processing systems can be used for a variety of semiconductor processing applications such as etching, deposition, resist stripping, etc. When such systems are used for plasma etching a batch of consecutively processed semiconductor substrates, the etching rate may change during the course of etching the batch of substrates. Such variation in etch rate is undesirable since the features etched into the substrate may fall outside acceptable product parameters. Accordingly, there is a need in the art for a plasma processing chamber and plasma etching method which allows consecutively processed semiconductor substrates to be processed with better control of the etch rate. Summary of the Invention The invention provides a method of consecutively processing a series of semiconductor substrates with minimal etch rate variation. The method includes steps of (a) placing a semiconductor substrate on a substrate support in a plasma etching chamber, (b) maintaining a vacuum in the chamber, (c) processing the substrate by supplying an etching gas to the chamber and energizing the etching gas to form a plasma in the chamber, (d) removing the substrate from the chamber; and (e) consecutively processing additional substrates in the chamber by repeating steps (a-d), the processing step being carried out by maintaining an amount of reactive gas in contact with an exposed surface of the substrate at a level sufficient to obtain a repeatable rate of etching for each of the substrates processed during step (c). According to a preferred feature of the invention, the semiconductor substrate can include a layer of polysilicon which is etched with HBr and Cl 2 during step (c) and/or the chamber is maintained at a vacuum pressure of less than 100 mTorr during step (c). The chamber is preferably evacuated by a vacuum pump separated from the interior of the chamber by an adjustable gate valve which is maintained in a fixed position during step (c). During step (c) an RF bias can be applied to the substrate by the substrate support and/or the flow rate of at least one component of the etching gas is increased when step (c) is repeated. For instance, the flow rate of at least one component of the etching gas can be adjusted to maintain constant pressure within the chamber during step (c) and/or a flow rate of at least one component of the etching gas can be adjusted based on real time analysis of one or more reactive species inside the chamber during step (c). In the preferred embodiment, the etching gas is energized into a plasma state by a planar or non-planar antenna separated from the interior of the chamber by a dielectric member, the dielectric member is at least coextensive with the substrate support, and the substrate support has a capacitance to ground of about 600 pF or less. More preferably, the dielectric member includes a coating of polytetrafluoroethylene facing the substrate support and the substrate support has a capacitance to ground of about 200 pF or less. The invention also provides a plasma etching apparatus comprising a processing chamber having a substrate support in an interior thereof and a dielectric member facing the substrate support, a gas supply supplying etching gas into the interior of the chamber, an antenna passing RF energy through the " ^ dielectric member and energizing the etching gas into a plasma state, a vacuum pump evacuating the interior of the chamber and separated from the interior of the chamber by an adjustable gate valve, and a controller maintaining an amount of reactive gas in contact with an exposed surface of the substrate at a level sufficient to obtain a repeatable rate of etching for each substrate processed in the chamber during consecutive single substrate etching of a batch of substrates. According to a preferred embodiment of the apparatus, the controller continuously adjusts a flow rate into the chamber of at least one component of the etching gas and/or the controller maintains the gate valve in a fixed position during etching of each substrate processed in the chamber. The substrate support can include an electrode applying an RF bias with the controller adjusting the RF bias to each substrate during consecutive single substrate etching of a batch of substrates. For instance, the controller can adjust the flow rate of one or more components of the etching gas to be higher for a later group of substrates than for an earlier group of substrates. According to one embodiment of the invention, the controller adjusts the flow rate of one or more components of the etching gas to provide a constant vacuum pressure in the chamber during etching of the substrates. For instance, the controller can adjust the flow rate of one or more components of the etching gas based on real time analysis of one or more reactive species in the chamber during etching of the substrates. During etching of an individual substrate having a layer of polysilicon to be etched by HBr and Cl, supplied into the chamber at adjustable flow rates, the controller can control the vacuum pump to maintain a fixed pumping speed, control the gate valve to maintain a fixed position, control the gas supply to maintain the flow rate of the HBr at a constant rate, and control the gas supply to vary the flow rate of the C^ in an amount sufficient to compensate for adsorption of reactive species of the etching gas by interior walls of the chamber. Brief Description of the Drawings The invention will be described in greater detail with reference to the accompanying drawings wherein: Figure 1 shows a schematic cross sectional view of a plasma etching chamber which can be used to carry out the method of the invention; Figure 2 is a graph showing the position of an adjustable gate valve versus wafer number for a series of 18 wafers processed in a plasma etching chamber; Figure 3 is a graph of endpoint signal versus time for a process of etching polysilicon in a LAM 9400SE™ plasma etching chamber operated at 10 mTorr, 350 watts power to the antenna, 100 watts to the chuck, 20 seem C , 100 seem HBr, and 60 °C chuck temperature; Figure 4 is a graph of efficiency versus load resistance for a LAM 9400™ chamber configuration compared to a LAM 9600™ chamber configuration; Figure 5 is a graph of Cl 2 flow rate versus wafer number during processing of a series of 12 wafers; Figure 6 is a graph of polysilicon etch rate versus Cl, flow rate during etching of polysilicon in a chamber operated at 10 mTorr, 350 watts power to the antenna, 100 watts to the chuck, variable Onflow rate and 100 seem HBr, Figure 7 is a graph of voltage, power, and flow rates of HBr and Cl, versus wafer number during processing of a series of 12 wafers wherein the total amount of HBr and Cl 2 is varied while maintaining a ratio of 5 : 1 of HBr to O ; Figure 8 is a graph of polysilicon etch rate versus wafer number during etching of polysilicon with variable HBr and Cl, supplied in a ratio of 5 : 1 ; Figure 9 is a graph of voltage, power, and flow rates of HBr and CL, versus wafer number during processing of a series of 12 wafers wherein the flow rate of Cl 2 is maintained constant at 20 seem and the flow rate of HBr is varied; Figure 10 is a graph of polysilicon etch rate versus wafer number during etching of polysilicon with variable HBr while maintaining Cl 2 fixed at 20 seem; ~ ~ Figure 11 is a graph of voltage, power, and flow rates of HBr and Cl 2 versus wafer number during processing of a series of 12 wafers wherein the flow rate of Cl 2 is varied and the flow rate of HBr is fixed at 100 seem; Figure 12 is a graph of polysilicon etch rate versus wafer number during etching of polysilicon with variable Cl, and HBr fixed at 100 seem; Figure 13 is a graph of voltage, power, and flow rates of HBr and Cl 2 versus wafer number during processing of a series of 12 wafers wherein the flow rate of Cl 2 is varied, the flow rate of HBr is fixed at 100 seem and the pumping speed is increased; Figure 14 is a graph of polysilicon etch rate versus wafer number during etching of polysilicon with variable Cl 2 , HBr fixed at 100 seem and increased pumping speed; Figure 15 is a graph of voltage, power, and flow rate of Cl, versus wafer number during processing of a series of 12 wafers wherein the flow rate of Cl χ is varied and no HBr is supplied to the chamber; Figure 16 is a graph of polysilicon etch rate versus wafer number during etching of polysilicon with variable Cl 2 and no HBr, Figure 17 is a graph of chamber pressure and chlorine flow rate versus time during processing of a silicon conditioning wafer wherein the flow rate of Cl, is variable, the flow rate of HBr is fixed at 100 seem and the pumping speed is increased; Figure 18 is a graph of chamber pressure and chlorine flow rate versus time during processing of a photoresist coated wafer wherein the flow rate of CL, is variable, the flow rate of HBr is fixed at 100 seem and the pumping speed is increased; Figure 19 is a graph of chlorine flow rate versus wafer number during processing of a series of twelve wafers wherein the flow rate of C is variable, the flow rate of HBr is fixed at 100 seem and the pumping speed is increased; ~~* Figure 20 is an optical emission spectrum (OES) during etching of a bare silicon wafer in a chamber operated at 10 mTorr, 350 watts power to the antenna, 100 watts power to the chuck and 100 seem Cl,; Figure 21 is an optical emission spectrum during etching of a bare silicon wafer in a chamber operated at 10 mTorr, 350 watts power to the antenna, 100 watts power to the chuck and 100 seem HBr, and Figure 22 is an optical emission spectrum during etching of a polysilicon coated wafer in a chamber operated at 10 mTorr, 350 watts power to the antenna, 100 watts power to the chuck, 20 seem Ct 2 and 100 seem HBr. Detailed Description of the Preferred Embodiments The invention provides an improved plasma processing reactor and plasma etching method capable of providing more repeatable results when etching a batch of consecutively processed semiconductor substrates. In particular, it has been found that during the course of etching a batch of semiconductor wafers, the variation in etch rate may take the form of a sudden increase or decrease of as much as 30% or of a slow change taking place over several hours. The change in etch rate appears to be associated with the history of the plasma etch chamber and, in particular, with the types of substrate that have previously been processed. A commonly employed method to stabilize the etch rate of a film is to process 25 (or more) dummy wafers (generally plain silicon) after which the real product wafers are processed. Although this technique does lead to some initial stabilization, as various types of product wafers are processed, the etch rate can soon become quite variable. In addition, processing dummy wafers in this way is time consuming and leads to an increase in the cost-of-ownership of the tool. In a production environment, the users of plasma etch tools prefer to have stable and reproducible performance so that they can be more assured that all process steps are within a specified process window. — * The etching of polysilicon with Clχ or Cl 2 lHBr gases shows a significant drop in etch rate after processing wafers coated with photoresist. This reduction in etch rate can be as much as 20-30% of the original rate. After etching SiO^-coated wafers, the polysilicon etch rate usually increases by several percentage points. In either case, the etching of a number of conditioning wafers is required to restore the etch rate and this results in lost tool time and reduced wafer throughput. Experiments have been designed and standardized to quantify these effects. A typical process sequence consists of etching five (or more) plain silicon conditioning wafers, a polysilicon coated wafer, five photoresist coated wafers and one more polysilicon coated wafers. The difference in etch rate of the initial polysilicon coated wafer compared to the later polysilicon coated wafers is noted and is a measure of the disturbance of the process. Ideally, one would prefer that the etch rates should be the same. During the following tests, the lower RF bias match network on the substrate support (i.e. , chuck with mechanical or electrostatic clamping) was equipped with a radio frequency probe which measured the voltage, current and phase of the RF signal at the process chamber. These measurements were used to compute the load (chamber plus plasma) impedance and the load power delivered to the chamber. Other variables which were evaluated to identify the sources of variability in the etch rate were optical signals (two wavelengths) emitted by the plasma and the pumping speed at the chamber as indicated by the position of the gate valve (e.g. , variable gate valve manufactured by VAT located in Switzerland and hereinafter referred to as "VAT valve") located between the turbo pump and the chamber. The method and apparatus of the invention can be carried out in any suitable plasma etch reactor wherein individual substrates are consecutively processed. A preferred plasma etching reactor is an inductively coupled plasma reactor such as shown in Figure 1. According to the invention, the plasma etching reactor is modified to include a controller suitable for achieving etch rate stabilization during batch processing of individual semiconductor substrates. The reactor can include any suitable source for generating plasma in the chamber and — any suitable gas supply for delivering etching gas into the chamber (e.g., a gas ring, showerhead, gas injector mounted in an opening extending through the dielectric window, etc.). In the embodiment shown in Figure 1, the vacuum processing chamber 10 includes a substrate holder 12 providing an electrostatic clamping force to a substrate 13 as well as an RF bias to a substrate supported thereon and a focus ring 14 for confining plasma in an area above the substrate while it is He backcooled. A source of energy for maintaining a high density (e.g. 10 11 -10 12 ions/cm 3 ) plasma in the chamber such as a planar antenna 18 powered by a suitable RF source and associated RF impedance matching circuitry 19 inductively couples RF energy into the chamber 10 so as to provide a high density plasma. The chamber includes suitable vacuum pumping apparatus including an adjustable gate valve (not shown) connected to outlet 15 for maintaining the interior of the chamber at a desired pressure (e.g., below 50 mTorr, typically 1-20 mTorr). The reactor shown in Figure 1 includes a substantially planar dielectric window 20 of uniform thickness between the antenna 18 and the interior of the processing chamber 10. However, other antenna and/or widow configurations can be used such as a non-planar antenna and/or a non-planar dielectric window. Further, while any suitable gas supply arrangement can be used, in the embodiment shown a gas distribution ring 22 surrounding the focus ring 14 includes a plurality of gas outlets (e.g. , 18 circumferentially spaced apart outlets) such as circular holes (not shown) for delivering process gas supplied by the gas supply 23 to the processing chamber 10. The substrate holder 12 can include conventional features such as a lift pin arrangement (not shown) for lifting the substrate 13. Under normal operating conditions, the gas flow to the chamber is fixed and the chamber pressure is held constant (typically about lOmTorr) by continuously adjusting the VAT valve opening under computer control. For example, during the processing of forty conditioning wafers, it has been observed-* that the polysilicon etch rate slowly increases and the VAT valve slowly opens. Figure 2 shows the VAT valve slowly opening during conditioning by the first ten conditioning wafers and first polysilicon coated wafer. However, as photoresist- coated wafers are etched, the valve rapidly closes and the polysilicon etch rate decreases. Table I summarizes the polysilicon etch rate decline after processing five photoresist coated wafers in two different chamber configurations, i.e. , a LAM 9400™ and a LAM 9600™. TABLE I Analysis of these experiments has led to the following explanation for the polysilicon etch rate variations. The decrease in etch rate after processing resist- coated wafers results from the chamber walls being coated with etched photoresist fragments. This coating then adsorbs a large quantity of the incoming gases until the walls are fully passivated. This means less chlorine is available to etch the wafer and also causes a reduction in chamber pressure. This in turn means a drop in polysilicon etch rate and the VAT valve closes somewhat to maintain constant chamber pressure. The optical signal of the etched silicon species is also considerably reduced as shown in Figure 3. The subsequent desorption of chlorine and other molecular residue can then take several hours even if no resist-coated ~~ * wafers are processed. If a contaminated chamber is pumped overnight, much of the etching gas is desorbed by the walls and when processing is recommenced the etching gases are again strongly adsorbed by the residue on the walls and the etch rate is again low. If oxide-coated wafers are etched in chlorine, the silicon probably leaves in the form of S C/ 4 releasing free oxygen. The oxygen vigorously cleans the photoresist fragments from the chamber walls which do not then need to be so heavily passivated with chlorine and so the subsequent polysilicon etch rate is higher. One way to evaluate these effects is to consider the chamber to have three pumps and three sources of gases. The pumps correspond to the turbo pump, the wafer and the chamber walls. The gas sources correspond to the inlet mass flow controller, the wafer and the chamber walls. A Mathcad model has been developed along these lines to calculate partial pressures and etch rates as gases are consumed at different speeds by the various pumping mechanisms. The results indicate that large quantities of the incoming gas are "pumped" by the chamber walls when they are coated with resist residue but that molecular desorption occurs at a much slower pace. Thus there is a net pumping effect of the walls and the VAT valve closes to compensate. When oxide wafers are processed, deposits on the walls are released and there is a net generation of gaseous products. In this case, the VAT valve is observed to open slightly as wafers are processed. The above-described chemical and pumping effects are only part of the etch rate problem. When using a LAM 9400™ tool, and changing substrates from silicon wafers to resist-coated wafers, the load resistance and load power (which cannot be measured with accuracy due to the large chamber capacitance of this particular chamber design) are observed to drop. The drop in load resistance results from the change in the chemical constituents in the chamber as the photoresist is etched. The drop in resistance moves the operating point down the power efficiency curve and the delivered power is reduced. This results in a further drop in polysilicon etch rate comparable in magnitude to the chemical and—" pumping effects described above. The power drop in a LAM 9600™ tool is less since, with its lower chamber capacitance, the operating point is higher up the efficiency curve where the dependence of load power on load resistance is less. Figure 4 shows the relative placement on the efficiency curve and the effect of photoresist coated wafers on the load resistance and delivered power. As shown in Table I, there is a drop in delivered power for the two chamber configurations. A third effect is that of chamber temperature which may rise during processing. However, temperature effects are believed to be small compared to the chemical and RF power effects. Experiments have been conducted in an attempt to obviate the chemical and RF effects. The first experiment (conducted on a LAM 9400™ tool) changed the servo loop from adjusting the VAT valve position to maintain constant pressure at constant gas inlet flow. In the new loop, the VAT valve's position was fixed and constant pressure was maintained in the chamber by varying the chlorine flow rate to compensate for the pumping by the chamber walls. As shown in Table I, this resulted in a change in polysilicon etch rate of 12.9% (compared to 22.5% with the standard servo loop) but was still accompanied by a drop in delivered power of ~ 15% after processing five resist-coated wafers. This result suggested that the chemical effects had been compensated but that the RF effect had not. The chamber configuration was then changed to that of the LAM 9600SE™ tool having a reduced chamber capacitance ( ~ 200pF vs. ~ 600pF for the LAM 9400™ tool) in an attempt to minimize the drop in RF power. A repeat of the experiment showed a drop in delivered power of about 4% and a small increase in polysilicon etch rate of 1.1 % after processing five resist-coated wafers. This indicates that both the chemical and RF effects have been approximately compensated. Figure 5 shows that the chlorine flow more than doubles as the photoresist coated wafers are processed while maintaining a flow of HBr constant at 100 seem. The increase in chlorine flow corresponds to an increase of about 21 % of the total gas flow. The foregoing tests suggest that the wafer represents only a very small part of the whole etch process (under the conditions described above) and that the — * chamber must be viewed holistically in order to understand the several effects taking place. The test data suggests that an improvement in etch reproducibility can be made and that tool cost-of-ownership can be reduced by modifying the etch process to take into account the effects of adsorption of reactant components on chamber walls. Several experiments were conducted to investigate the feasibility of controlling the flow rates of Cl^ and HBr during the etching of polysilicon as a means of improving the reproducibility of etch rates. An improvement in etch rate repeatability can be realized by the use of the method according to the invention which takes into account the different chamber conditions encountered during the etch process. Optical emission spectra (OES) have been analyzed during the etching of silicon and photoresist with gas flows of pure Cl and HBr as well as mixtures of these gases. A standardized sequence of twelve wafers was used to quantify the etch rate variances before and after processing four blank silicon conditioning wafers and before and after processing five blanket photoresist wafers. Under conventional control conditions where the gas flow is fixed and the pumping speed at the chamber is varied to keep the pressure constant, the polysilicon etch rate falls by " 25 % after processing blanket photoresist wafers in a LAM 9400PTX™ tool. However, this percentage drop can be approximately halved by processing in a chamber having a capacitance to ground of about 200 pF (e.g. , processing in a LAM 9600SE™ tool rather than the 9400 tool having a capacitance to ground of ~ 600 pF). The reason for this is that the RF power delivered to the chamber falls by ~ 15 % in a 9400PTX™ tool after processing photoresist-coated wafers but only by 0 to 4% in a 9600SE™ chamber. The drop in power occurs because of a drop in load resistance which in turn results from an increase in plasma resistance. The latter probably arises from chemical changes in the plasma and perhaps a drop in electron temperature as photoresist wafers are processed. Measurements were made on a LAM tool which had a quartz TCP™ window and a ceramic gas ring around the electrostatic chuck. The capacitance of~^ the chuck to ground was ~ 200 pF. For each experiment, the same sequence of wafers was run and the polysilicon etch rates measured. Table II shows the wafer sequence and also the baseline process parameters around which the measurements were made. Before each run the chamber was plasma-cleaned to end-point using SE 6 and O 2 feed gases. The starting thickness of the polysilicon on each of the three etch rate wafers was determined using a Prometrix FT-650. After the twelve wafers were processed, remaining polysilicon thickness was measured and the etch rate determined. The RF power to the load was not actively controlled by a feedback loop from the RF probe located in the RF bias match network. The experiment relied on the low chamber capacitance to prevent too great a drop in power efficiency as a result of load resistance changes. The VAT valve was held in a fixed position during each experiment and the flow of one or more of the feed gases was varied to maintain a constant chamber pressure of 10 mTorr. The flow rates of the gases were recorded before the RF was turned on and during the etching of each wafer. The RF parameters from the RF probe (voltage, current, phase, delivered power, load resistance and load reactance) were also recorded. After turning on the RF power, the gas flows usually decreased initially and, in the case of photoresist- coated wafers, was followed by a steady increase for the remainder of the etch. The values of the gas flows and RF parameters correspond to a time about halfway into the etch, i.e. , about 30 seconds after turn-on of the RF power. Five tests were conducted in which the flow of one or both of the feed gases was changed while maintaining a constant pressure of 10 mTorr in the chamber. The five tests are as follows: (i) vary both HBr and Cly flows together in the ratio of 5 : 1 (ii) vary HBr flow only and maintain C/^ flow at 20 seem (iii) vary Cl 2 flow only and maintain HBr flow at 100 seem (iv) repeat the third test at a higher pumping speed (v) vary C/^ flow only and maintain HBr at 0 seem. Before the above tests were conducted, the polysilicon etch rate was — measured as a function of CI2 flow (plus 100 seem of HBr) in a clean chamber, under similar process conditions, except that the pumping speed was varied to maintain the chamber pressure at 10 mTorr. The result is shown in Figure 6. The polysilicon etch rate rises from 1750 A/min with pure HBr to ~ 2650 A/min when 50 seem of C 2 are added. The etch rate using only Cl, is about double that when using HBr only. These etch rates are useful in interpreting the results of tests (i) to (v). Table III summarizes the results of the five experiments. The first two tests gave similar results since changing both gases together in the ratio of 5: 1 resulted in only a small change in the flow of Cl so that, effectively, only the flow of HBr was being changed. The polysilicon etch rates, before and after the four silicon conditioning wafers, are very similar (within 0.5%) when the HBr flow is varied. However, the etching of five photoresist-coated wafers results in a drop in etch rate of 12 to 15% . This situation can be compared to the results of the third experiment in which the flow of Clη, is varied and the HBr flow fixed at 100 seem. Here, the opposite effect takes place. The etch rate falls by nearly 12% after processing the conditioning wafers but remains approximately constant (+3.5%) after etching the photoresist-coated wafers. It appears that the chamber wall and wafer conditions are such that daughter products of HBr (i.e., H, Br) are preferentially consumed when conditioning wafers are being etched and that products from Cl (Cl) are preferentially consumed during the etching of resist- coated wafers. The total gas flow before turning on the RF power remains approximately constant throughout each twelve wafer sequence. This implies that the flow is independent of the chamber wall conditions and that little or no chemistry takes place until molecular dissociation occurs. Similarly, it is likely that the roles of Br and H 2 in chamber conditioning is small. The third experiment was repeated at a higher pumping speed (VAT valve at 260 counts vs. 230 counts) because of the very low flow of Ct 2 following the four conditioning wafers. In this case (test (iv)), the Ct 2 flow starts out at 55 seem because substantial quantities of HBr are being lost to the chamber walls and the — * pump. The large Ct^ flow leads to a fairly large polysilicon etch rate of 2500 A/min. As the chamber walls become saturated with HBr products, the C^ flow drops to 13 seem to compensate and the polysilicon etch rate drops by 21 % . In contrast, during the etching of blanket photoresist wafers, Cl^ products are primarily consumed and the Ct2 flow again increases to 42 seem. The polysilicon etch rate increases by a modest 5 % . The overcompensation of the etch rate is probably due to the fact that some HBr products are also consumed during, and immediately after, photoresist etching and this is made up by the faster-etching chlorine. The compensation for depleted etch products by increasing the chlorine flow (perhaps accompanied by a small increase in HBr flow) may provide a viable etch rate stabilization technique over some operating range but it is not a universal solution as the change in etch rate after conditioning wafers shows. Also, the choice of pumping speed should be determined carefully since overcompensation of etch rate can occur at high pumping speeds. Since it appears that the wafer and chamber walls pump daughter products of the primary species at different rates depending on the wafer type or chamber condition, accurate compensation could be accomplished by measuring gaseous contents of the chamber in real time. For example, measurements by suitable analytical techniques (e.g., OES spectra) could be used for this purpose. The use of a differentially pumped RGA may also accomplish the desired result but would add cost to in a production process. In terms of economics, the compensation of C y flow may be the cheapest and easiest to implement in a production process. The fifth experiment was performed using chlorine only. This may not be a viable option for production but was done for completeness to test the flow compensation. The situation with a single gas should be straightforward since, as its products are consumed, its flow is increased to compensate for the losses. From Table III it is seen that the polysilicon etch rate increases by 1.8% after conditioning wafers and drops by 0.8% after photoresist wafers. The small changes in chlorine flow are surprising compared to the previous experiments. When using HBr and Clχ together, the flows varied by 10 to 30 seem but with — chlorine only, changes of less than 10 seem are enough to compensate the etch rate. The uniformity of etch in the five tests was generally very good, having a standard deviation of about 2.5 % . The etch profile across the etched polysilicon wafers was slightly edge-fast during the first three tests and was especially uniform for test (iv), probably because of the higher gas flow rate. Test (v) resulted in a slightly center-fast etch. It is of interest to compare the density of dissociated molecules with the 14 density of ions present in the plasma. At 10 mTorr there are about 10 atoms and 3 10 molecules per cm . If the electron density is ~ 5x10 then only one particle in 2000 is ionized. The changes in gas pressure due to dissociation and the concomitant changes in flow suggest that something like one particle in two is a dissociated fragment. The strong reactivity of the dissociated atoms leads to rapid consumption and, hence, significant changes in flow rate. Figure 7 shows the variation of load voltage, load power, Ct 2 flow and HBr flow for test (i). The etch rates for the etched polysilicon wafers are shown in Figure 8. It can be seen that the load voltage has less of an effect on controlling etch rate of polysilicon whereas load voltage has more of an effect on controlling the etch rate in a more physical process such as the chlorine etching of Si or photoresist. Similar plots for the remaining tests are shown in Figures 9-16 which illustrate the effects described. The range of chlorine flow may vary considerably while processing a wafer. Figure 17 shows the chlorine flow rate and chamber pressure for the third wafer of test (iv) as a function of time into the etch. At the left, the fixed HBr flow gives a chamber pressure of 8 mTorr and, after 3 seconds, the chlorine is turned on bringing the pressure up to 10 mTorr. After 12 seconds the RF is turned on and the pressure first rises and then falls back to 10 mTorr. The chlorine flow has a long descent and has not fully stabilized even at the end of the etch. Prior conditioning wafers have an even slower descent of flow rate whereas subsequent conditioning wafers stabilize the flow more rapidly. This reduction in— * ■ flow rate may be contrasted with the first photoresist wafer (#7 of test (iv)) shown in Figure 18. Here, after a fairly rapid stabilization of flow, the latter increases throughout the etch and through all subsequent photoresist wafers. This course is reversed only by changing wafers from photoresist to silicon. Figure 19 shows the chlorine flow rate before RF turn-on and the range of chlorine flow for each wafer (after stabilization of the pressure) of test (iv). These changes in flow point to complex consumption mechanisms and the importance of chamber condition to the etch process. According to one embodiment of the invention, the flows of HBr and/or Cl 2 need to be controlled according to the chamber condition and wafer type. In order to control the flow of one or more process gases to obtain etch stabilization in accordance with the invention, optical emission signals may provide the necessary information on the gaseous constituents of the chamber. In assessing techniques for monitoring such emission signals, spectra were taken over the wavelength range of 240-850 ran using chlorine only, HBr only and the 5:1 mixture of HBr I Cl^. Figure 20 shows a spectrum taken during the etching of a bare silicon wafer in chlorine gas. All of the lines at wavelengths greater than 700 nm are due to atomic chlorine and the band and lines between 300-400 nm and 500-600 nm are probably due to silicon chloride etch products. When HBr only is used, the spectrum shown in Figure 21 is obtained where all the lines can be identified with atomic bromine. Lines attributable to hydrogen do not appear because of their large separation in energy. Figure 22 shows a spectrum obtained using a mixture of Ct^ and HBr. Again, etch products from the silicon wafer can be seen but atomic chlorine lines are absent with the possible exception of that at 837.6 nm which figures prominently in the pure chlorine spectrum of Figure 20. Even though the ratio of HBr: Cl 2 entering the chamber is 5:1 it appears that the optical emission of atomic chlorine is suppressed. One possible explanation for this result is that Cl 2 may be consumed at the chamber walls by the absorbed H and Br which are then pumped away. Although the tests revealed an absence of Cl 2 lines in HBr/Cl mixtures, thus making the OES technique described herein — « unsuitable for measuring the ratio of Br/Cl, it is contemplated that other measurement techniques suitable for obtaining gas ratios of component gases during real time etching of substrates could be used in accordance with the method of the invention for obtaining reduction of etch rate variation during batch processing of consecutively etched wafers. When photoresist wafers are etched, the only spectral lines which are observed are atomic bromine. However, the intensity of these lines falls by a factor of four after etching five wafers. This may be due to a drop in electron temperature caused by the presence of resist etch products in the plasma. These large fragments would be expected to have many low-lying energy states which would prevent the plasma electrons from gaining significant amounts of energy. As such, depending on the equipment and actual etch process, OES may not provide the desired degree of plasma constituent monitoring. The effect of the chamber walls (such as side walls, liner, showerhead, dielectric window, etc.) on variation in etch rate has been investigated by coating the chamber walls with a process passivating material. For example, the chamber walls can be coated with various materials such as ceramic or organic materials. Although ceramic materials have been found to provide little benefit in stabilizing the etch rate of polysilicon, organic materials such as polytetrafluoroethylene (i.e. , TEFLON™) have been found to provide surprisingly good results. When combined with control of one or more process gas flow rates for purposes of etch rate stabilization in accordance with the invention, the etch rate variation for consecutively etching batches of 10 or more wafers (e.g., photoresist, oxide, polysilicon and dummy wafers) can be reduced to less than 5 % , preferably less than 2% . Such etch rate stabilization corresponds to an improvement on the order of 50% compared to conventional processes. With or without the stabilization coating, etch rate stabilization can also be improved by using chamber configurations having low chamber capacitance of less than 500 pF, preferably less than 300 pF, and more preferably less than about 200 pF. With the improvements obtainable according to the invention, it is contemplated that etch processes can be-* carried out without the need to run silicon conditioning wafers through the plasma chamber prior to commencing a production run of product wafers. TABLE II The process parameters include the following: Total Pressure 10 mTorr, TCP™ Power 350W, Bias Power 100W, HBr Flow 100 seem, Cl, Flow 20 seem, ESC 8Torr Helium ,60 °C, Etch Duration 60 sees. TABLE III The foregoing has described the principles, preferred embodiments and modes of operation of the present application. However, the invention should not be construed as being limited to the particular embodiments discussed. Thus, the above-described embodiments should be regarded as illustrative rather than restrictive, and it should be appreciated that variations may be made in those embodiments by workers skilled in the art without department from the scope of the present invention as defined by the following claims.

Claims: WHAT IS CLAIMED IS: 1. A method of consecutively processing individual semiconductor substrates with minimal etch rate variation comprising the steps of: (a) placing a semiconductor substrate on a substrate support in a plasma etching chamber; (b) maintaining a vacuum in the chamber; (c) etching an exposed surface of the substrate by supplying an etching gas to the chamber and energizing the etching gas to form a plasma in the chamber; (d) removing the substrate from the chamber; and (e) consecutively etching additional substrates in the chamber by repeating steps (a-d), the etching step being carried out by maintaining an amount of reactive gas in contact with the exposed surface of the substrate at a level sufficient to obtain a repeatable rate of etching for each of the substrates etched during repeated step (c). 2. The method according to Claim 1 , wherein the semiconductor substrate includes a layer of polysilicon which is etched with HBr and Cl during step (c). 3. The method according to Claim 1 , wherein the chamber is maintained at a vacuum pressure of less than 100 mTorr during step (c), the chamber being evacuated by a vacuum pump separated from the interior of the chamber by an adjustable gate valve which is maintained in a fixed position during step (c). 4. The method according to Claim 1, wherein during step (c) an RF bias is applied to the substrate by the substrate support. — * 5. The method according to Claim 1, wherein a flow rate of at least one component of the etching gas is increased when step (c) is repeated. 6. The method according to Claim 1, wherein a flow rate of at least one component of the etching gas is adjusted to maintain constant pressure within the chamber during step (c). 7. The method according to Claim 1, wherein a flow rate of at least one component of the etching gas is adjusted based on real time analysis of one or more reactive species inside the chamber during step (c). 8. The method according to Claim 1, wherein the etching gas is energized into a plasma state by a planar or non-planar antenna separated from the interior of the chamber by a dielectric member, the dielectric member being at least coextensive with the substrate support, and the chamber having a chamber capacitance of about 600 pF or less. 9. The method according to Claim 8, wherein the dielectric member includes a coating of polytetrafluoroethylene facing the substrate support and the chamber has a chamber capacitance of about 200 pF or less. 10. A plasma etching apparatus comprising: a plasma etching chamber having a substrate support in an interior thereof, the chamber including a dielectric member facing the substrate support; a gas supply supplying etching gas into the interior of the chamber; an antenna separated from the interior of the chamber by the dielectric member, the antenna passing RF energy through the dielectric member and energizing the etching gas into a plasma state; — «■ a vacuum pump evacuating the interior of the chamber, the vacuum pump being separated from the interior of the chamber by an adjustable gate valve; and a controller maintaining an amount of reactive gas in contact with an exposed surface of the substrate at a level sufficient to obtain a repeatable rate of etching for each substrate processed in the chamber during consecutive single substrate etching of a batch of substrates. 11. The plasma etching apparatus of Claim 10, wherein the controller continuously adjusts a flow rate into the chamber of at least one component of the etching gas. 12. The plasma etching apparatus of Claim 10, wherein the vacuum pump maintains the interior of the chamber at a vacuum pressure of less than 100 mTorr and the controller maintains the gate valve in a fixed position during etching of each substrate processed in the chamber during consecutive single substrate etching of a batch of substrates. 13. The plasma etching apparatus of Claim 10, wherein the substrate support includes an electrode applying an RF bias, the controller adjusting the RF bias to each substrate during etching of each substrate processed in the chamber during consecutive single substrate etching of a batch of substrates. 14. The plasma etching apparatus of Claim 10, wherein the controller adjusts the flow rate of one or more components of the etching gas to be higher for a later group of substrates than for an earlier group of substrates during consecutive single substrate etching of a batch of substrates. 15. The plasma etching apparatus of Claim 10, wherein the controller adjusts the flow rate of one or more components of the etching gas to provide a constant vacuum pressure in the chamber during etching of the substrates. — * 16. The plasma etching apparatus of Claim 10, wherein the controller adjusts the flow rate of one or more components of the etching gas based on real time analysis on one or more reactive species in the chamber during etching of the substrates. 17. The plasma etching apparatus of Claim 10, wherein the dielectric member is at least coextensive with the substrate support and the chamber has a chamber capacitance of about 600 pF or less. 18. The plasma etching apparatus of Claim 17, wherein the dielectric member includes a coating of polytetrafluoroethylene facing the substrate support and the chamber has a chamber capacitance of about 200 pF or less. 19. The plasma etching apparatus of Claim 10, wherein during etching of an individual substrate the controller maintains constant pressure in the chamber at a fixed pumping speed of the vacuum pump. 20. The plasma etching apparatus of Claim 10, wherein a substrate having a layer of polysilicon is supported on the substrate support and the gas supply supplies HBr and C 2 into the chamber at adjustable flow rates, the controller controlling the vacuum pump to maintain a fixed pumping speed, controlling the gate valve to majntain a fixed position, controlling the gas supply to maintain the flow rate of the HBr at a constant rate, and controlling the gas supply to vary the flow rate of the Cl 2 in an amount sufficient to compensate for adsorption of reactive species of the etching gas by interior walls of the chamber.

Inventors: BALDWIN SCOTT K JR, WILLIAMS NORMAN, BALDWIN, SCOTT, K., JR., WILLIAMS, NORMAN
Applicants: BALDWIN SCOTT K JR, LAM RES CORP, WILLIAMS NORMAN, BALDWIN, SCOTT, K., JR., LAM RESEARCH CORPORATION, WILLIAMS, NORMAN
Application year: 1999
Main classification: H01J37
IPC codes: B01J19, H01J37, H01L21, H05H1