Patent Publication Number: US-6340603-B1

Title: Plasma emission detection during lateral processing of photoresist mask

Description:
TECHNICAL FIELD 
     The present invention relates to a method of manufacturing a semiconductor device wherein a multilayered structure is etched to form a pattern therein. The present invention is particularly applicable to methods for monitoring the etching of a polysilicon layer in a semiconductor device. 
     BACKGROUND ART 
     The escalating requirements for high density and performance associated with ultra large scale integration (ULSI) semiconductor devices require smaller design features, increased transistor and circuit speeds, high reliability and increased manufacturing throughput for competitiveness. As the devices and features shrink, and as the drive for higher performing devices escalates, problems are generated that require resolution in terms of new methods of fabrication or new arrangements or both. 
     In general, semiconductor devices comprise a substrate and elements such as transistors and/or memory cells thereon. Metal-oxide-semiconductor (MOS) type devices are typical of modem integrated circuits and typically comprise a pair of ion implanted source/drain regions in a semiconductor substrate and a channel region between the source/drain regions. A gate arrangement is then formed by depositing a thin gate oxide above the channel region followed by the deposition of a composite layer on the gate oxide to function as a gate electrode. The composite layer typically comprises one or more conductive layers which typically includes a layer of polysilicon. 
     Of particular importance in memory and logic devices is the formation and dimensional control of the gate electrode. Gate dimensions are critical since the gate electrode of the device controls the flow of electrons and is vital to proper device operation. As discussed above, a gate structure is formed by depositing a conductive layer at a uniform thickness, which is then typically followed by the deposition of an antireflective film on the conductive layer. A patterned photoresist or mask is then formed by depositing and patterning a photoresist material on the antireflective film followed by etching through the film and multiple layers to ultimately form the desired gate structure. 
     The basic photolithography system conventionally employed in forming these features consists of a light source, a stencil or photomask containing the pattern to be transferred to the semiconductor substrate, a collection of lenses, and a means for aligning existing patterns on the substrate with patterns on the mask. The resolution of an optical projection system such as a photolithographic system is limited in practice and can range from 0.8 down to 0.3 micron for standard exposure systems. The highest resolution in optical lithography is currently achieved with deep ultra violet (DUV) systems operating at 248 nm wavelengths but mid ultra violet (MUV) systems with a wavelength of 356 nm are also in widespread use. 
     The limits of photolithography are now being challenged, however, by the manufacture of high-performance logic and memory technologies. These technologies are entering development cycles with immediate requirements for sub-quarter micron, or less, printed gate lengths and tight dimensional control on the gate structures across large chip areas. 
     Since fabrication of a gate arrangement requires the further step of etching through the patterned photoresist, the gate arrangement is approximately limited to the resolution obtained in the photolithographic process. Resolving narrower line widths than those formed by the photolithographic process requires additionally demanding processing steps. Moreover, resolving the spaces between photoresist lines and other small areas using conventional lithographic techniques is considerably more difficult. 
     A common technique that compensates for the inherent deficiencies of optical photolithography to produce finer line widths is accomplished by etching the sidewalls of a formed photoresist mask prior to etching through the photoresist mask to form the gate structure. This trimming process is achieved by adjusting the etching parameters to etch the sidewalls of a photoresist mask to reduce the width of the elements defined by the photoresist mask. In accordance with conventional techniques, a patterned photoresist layer is placed in a plasma etching environment for a fixed period of time under prescribed etching conditions to etch the sidewalls of the photoresist mask thereby reducing the line width of the patterned photoresist. 
     The time and etching parameters necessary to achieve the appropriate width reduction is empirically determined by adjusting various etch parameters and, in a separate step, measuring the resulting line widths, as by scanning electron microscopy. The procedure is repeated until the appropriate time and parameters have been determined for a given pattern density. Then, batches of semiconductor substrates with an overlaying photoresist mask are repeatedly processed with the empirically determined parameters. Conventional trimming processes, however, fail to account for variations during the individual wafer processes, such as fluctuating chamber conditions, process parameter fluctuations etc. or for a changed pattern density on the substrate to be etched. 
     Systems for determining the completion of an etching process through an underlayer are known (see, e.g. S. Wolf and R. N. Taulber “Silicon Processing for the VLSI Era”, vol. 1, Latice Press 1986, pp. 447-458 and pp. 565-567). For example, laser interferometry and optical emission spectroscopy have been used to measure the time for complete etching through an underlayer, particularly complete etching through an underlying silicon oxide, polysilicon or aluminum layer. However, it is believed that the art has not recognized a method of determining the completion of an etch step by measuring or monitoring the actual lateral, e.g. width, dimensions of a photoresist element during etching. 
     As the gate electrode critical dimensions shrink, process variations become increasingly intolerable. Thus, there is a need for a method of determining the dimensions of a line width during lateral processing of a photoresist mask. Further, it would be highly advantageous to develop a process capable of controlling the line width dimensions, to minimize processing variations and to precisely control gate electrode dimensions during photoresist mask etching. 
     SUMMARY OF THE INVENTION 
     An advantage of the present invention is a method of laterally processing a photoresist mask having a plurality of elements by accurately etching and monitoring the width of the elements of the photoresist mask. 
     Another advantage of the present invention is a method for etching a feature in a semiconductor substrate by accurately controlling the lateral dimensions of an overlaying photoresist mask. 
     Additional advantages and other features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the invention. The advantages of the invention may be realized and obtained as particularly pointed out in the appended claims. 
     According to the present invention, the foregoing and other advantages are achieved in part by a method of laterally processing a photoresist mask on a substrate, which method comprises: depositing a layer of photoresist material on the substrate; patterning the photoresist layer to form a photoresist mask having at least one element with a first prescribed width; laterally processing the photoresist mask having the element to a second prescribed width in a plasma chamber having an etchant mixture; detecting a change in the emission spectrum of at least one specie in the plasma while simultaneously laterally processing the photoresist mask; and stopping the lateral processing when the detected change in the emission spectrum of the specie corresponds to the second width. 
     By monitoring the emission spectrum, particularly the intensity of a selected plasma specie formed in the plasma, for changes in the plasma emission spectrum of the selected plasma specie, the present invention advantageously provides an in-situ method for precisely determining the optimum time for complete lateral processing of the photoresist mask element. 
     Another aspect of the present invention is a method of etching a feature on a semiconductor substrate. The method comprises: forming a dielectric layer on the semiconductor substrate; forming a conductive layer on the dielectric layer; forming a photoresist mask having at least one element having a first prescribed width on the conductive layer; laterally processing the photoresist mask having the element to a second prescribed width in a plasma chamber having an etchant mixture; detecting a change in the emission spectrum of at least one specie in the plasma while simultaneously laterally processing the photoresist mask; stopping the lateral processing when the detected change in the emission spectrum of the specie corresponds to the second width; and etching through the photoresist mask having the element with the second width to the underlying layers to form the feature. 
     Additional advantages of the present invention will become readily apparent to those having ordinary skill in the art from the following detailed description, wherein the embodiments of the present invention are described, simply by way of illustration of the best mode contemplated for carrying out the present invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The various features and advantages of the present invention will become more apparent as a detailed description of the embodiments thereof is given with reference to the appended figure described below. 
     FIG. 1 is a diagram illustrating a cross-sectional view of a portion of a composite structure having a photoresist mask thereon prior to etching. 
    
    
     DESCRIPTION OF THE INVENTION 
     The present invention addresses and solves the problem of random and systematic variations in a photoresist mask trimming and deposition processes caused by variations in the plasma etching chamber, fluctuations in process parameters, varied pattern densities, etc. by achieving an in-situ monitoring technique preformed during the processing procedure. The present invention enables the manufacture of semiconductor devices, particularly logic and semiconductor memory devices having submicron features, with improved critical dimensional control of a gate electrode in a cost effective manner with an attendant increase in device performance and uniformity of device performance. Embodiments of the present invention include assessing the removal or deposition on the sidewalls of elements comprising a photoresist mask and determining the precise time that a desired reduced or increased dimension has been achieved, thereby improving the overall efficiency, quality and uniformity of the resulting device. 
     An embodiment of the present invention is illustrated in FIG. 1 with respect to forming a gate electrode of either a memory or logic type device. As illustrated in FIG. 1, substrate  10  has dielectric layer  12  thereon, conductive layer  14  overlays dielectric layer  12 , antireflective coating (ARC)  16  overlays conductive layer  14  and a photoresist mask having as elements photoresist lines  18  is on ARC  16 . The photoresist elements, shown as lines, have a first width designated as LW 0  and open spaces between lines designated as SW 0 . In very large scale integrated circuit applications, substrate  10  has several million (soon to be more than one billion) of such gate electrodes. 
     Dielectric layer  12  can comprise inorganic layers such as silicon dioxide, silicon nitride, silicon oxynitride, etc. Conductive layer  14  typically comprises polysilicon with one or more conductive layers such as a silicide layer or other similar low electrically resistant materials, ARC  16  typically comprises a nitride of silicon or a nitride of a metal such as titanium. 
     Additional device dependant layers can be further deposited depending on the particular device contemplated. For example, a silicide layer can further be formed on conductive layer  14  using conventional silicide forming techniques to a thickness of about 100 to about 1,000 Angstroms. In the manufacture of memory devices, an interpoly dielectric layer overlying on conductive layer  14  would be formed along with a second conductive layer overlying on the interpoly dielectric layer and then the photoresist mask overlying the second conductive layer. 
     The layers of the present invention are configured for the formation of submicron gate electrodes and have a height of between about 1,500 Angstroms to about 5,000 Angstroms. For example, dielectric layer  12  can be a gate oxide layer of less than about 50 Angstroms thick, thermally grown on substrate  10 . Conductive layer  14  can be a doped polysilicon layer formed on the tunnel oxide using conventional chemical vapor deposition (CVD) or plasma enhanced chemical vapor deposition (PECVD) techniques to a thickness of about 900 to about 2,000 Angstroms. Antireflective coating (ARC) film  16 , used for many reduced-size critical dimension semiconductor devices can then be deposited overlying the polysilicon layer using conventional CVD or PECVD techniques. In an embodiment of the present invention, a silicon oxynitride film serves as the antireflective coating film. Although an embodiment uses silicon oxynitride as the antireflective film  16 , alternative ARC materials can be used that include a controllable atomic percentage of nitrogen, for example, silicon oxime. The ARC  16 , typically a silicon oxynitride film, is formed to a thickness of about 300 Å. 
     The photoresist is then patterned on ARC  16  to generate a photoresist mask at a thickness of about 5,400 Angstroms typically having a plurality of elements (e.g. a plurality of lines separated by spaces). To achieve high density, the photoresist mask is typically formed by a deep ultraviolet (DUV) radiation sensitive photoresist layer capable of achieving line width resolutions of about 0.30 to about 0.20 micron. 
     During the photolithographic process, radiation is passed through a photomask defining a desired pattern to imagewise expose a layer of photoresist material on the substrate. After exposure to radiation, the photoresist layer is developed, typically by a liquid developer, to form a relief pattern therein having a plurality of elements. The plurality of elements can comprise various geometrical shapes including lines, pads, columns, etc. 
     As shown in FIG. 1, the plurality of elements comprises a series of lines separated by spaces. The sidewalls of a photoresist element define a first width, shown as LW 0  in the figure, and spaces between elements also capable of having a first dimension, designated as SW 0 . The initial width dimensions of the elements achieved after development of the photoresist layer would otherwise be the resolution of the underlying layer unless further processing of the photoresist mask is undertaken. Hence, resolution of finer line widths or smaller spaces between elements (smaller spaces can be achieved by increased line widths) requires additional processing of the photoresist mask. 
     According to the present invention, the semiconductor substrate, as shown in FIG. 1, is inserted into an etch chamber and the photoresist mask having plurality of elements  18  is then etched under plasma conditions to a second width. The plasma etch chamber has an etchant mixture, the composition of which can be adjusted to either increase or decrease the width of the photoresist elements. Additionally, the plasma chamber parameters, such as gas flow, pressure and applied power, can be adjusted to modulate the second width of the photoresist mask elements. 
     As is known in the art, plasma etching of a photoresist mask involves competing etch and deposition processes. By adjusting the ratio of the etchant composition, the line width of the photoresist can be controlled during etching of the photoresist mask in a plasma etching environment thereby overcoming the resolution limit of current DUV techniques. The line width can be controlled to either an increased or decreased horizontal dimension, i.e. laterally processed, with the attendant control over the resolution of spaces and ultimate control over the gate electrode dimension. 
     In accordance with the present invention, a semiconductor substrate is placed into a plasma etch tool and the chamber conditioned. Given the guidance and objectives of the present disclosure, the optimum etching conditions can be necessarily determined in a particular situation. For example, the chamber pressure is maintained at about 3 mTorr to about 200 mTorr and the tool power set at about 200 Watts (W) to about 1,000 W. The temperature of the substrate is maintained at about 0° C. to about 65° C. The total gas inlet flow is maintained at about 50 standard cubic centimeters per minute (sccm) to about 200 sccm. 
     In an embodiment of the present invention, a photoresist mask overlaying a composite arrangement comprising a semiconductor substrate having a gate oxide thereon, a layer of polysilicon on the gate oxide and an ARC on the polysilicon is placed in a commercially available plasma etch tool having at least one window for monitoring the optical emission spectrum of the generated plasma. According to the disclosed embodiment, the photoresist mask is selectively etched in order to increase or decrease the width of photoresist elements using a single-chamber plasma process. In particular, an embodiment of the present invention uses a decoupled plasma source (DPS) poly chamber from Applied Materials, Inc., Santa Clara, Calif., for example an Applied Materials AMAT DPS Poly chamber. Normal plasma etch tools use a single power source, referred to as a “source power”, with the wafers electrically grounded; alternately, the wafers may be attached to the source power, and the chamber is electrically grounded. The plasma etch tool controls the source power and affects how much plasma is generated, and also controls the concentration of plasma formed reactive species, and hence how the reactive species lands on the wafer. 
     In the decoupled plasma source etcher, the power supply is separated into a source power and a bias power. The source power is high power supplied to coils or other such electrodes. The bias power (or “bottom power”) power is connected to the wafer. The source power ionizes the gas supplied into the chamber, and generates the reactive species in the chamber. The bias power on the wafer drives the reactive species to accelerate the reactions. Hence, there is more control of the etching process: the source power controls generation of the chemical species and therefore controls the chemical etch portion, and the bias power controls the physical part of the etch, for example the bombardment of the species onto the wafer. 
     Optical emission spectra are a consequence of the relation of excited species present in the plasma (i.e. relaxation of an excited species from a higher energy state to a lower energy state). The species present in the plasma at any given time during lateral processing of a photoresist mask are a consequence of externally applied parameters, such as gas composition, gas flow, pressure, applied power to the plasma, etc. and the chemical and/or physical reactions occurring within the plasma chamber, such as recombination reactions between the reactive species with the photoresist material to either deposit on or remove photoresist material. 
     In accordance with the present invention, optical emission spectra are used to detect the real-time progress of laterally processing a photoresist mask. Optical emission spectroscopy in the present invention relies on detecting changes in the intensity of optical radiation (i.e. light) emitted by reactant or product species in the plasma. A particular specie emits light at discrete wavelengths (emission spectrum) associated with relaxation of characteristic energy states. Moreover, the emission intensity can be associated with the relative concentration of a selected specie in the plasma. Hence, the concentration of reaction and/or product species can be determined by monitoring the plasma emission spectra for a correlated increase or decrease in the characteristic intensity associated with a selected specie, which in turn is also a function of the progress of the lateral etching or depositing of the sidewalls of the photoresist mask elements. 
     To facilitate control over the width of the photoresist elements during the lateral process step and determine the optimum time for achieving a desired change in the width dimension, changes to the lateral dimension of the photoresist mask are monitored by detecting changes in the emission intensity of selected species in the plasma. In accordance with the present invention, manipulation of the etchant mixture and/or other plasma parameters in the plasma chamber results in an increase or decrease in the width of the photoresist mask. Detecting a change in the emission spectrum of at least one specie in the plasma while simultaneously laterally processing the photoresist mask results in improved control over line width and greatly reduces process variation. 
     In an embodiment of the present invention, an array of pattern lines having an initial width of about 0.3 to about 0.2 micron is formed on a semiconductor substrate. The detected intensity of at least one species in the plasma can be calibrated to record an initial intensity prior to laterally etching the photoresist mask. The lines are then etched in a plasma chamber to a second width of less than about 0.18 micron while simultaneously monitoring the emission intensity of a selected specie. As the width of the lines change to the second width, the emission intensity of the specie detected during the etching process correspondingly changes to a second intensity. Changes in detected intensities corresponding to precise line width can be experimentally determined for a given photoresist pattern and etch process. Upon reaching a second prescribed line width, the lateral process can be immediately terminated. 
     Given the guidance and objectives of the present disclosure, the optimum plasma specie selected can be necessarily determined in a particular situation. For example, etching the sidewalls of the photoresist elements results in a higher concentration of carbon and/or oxidized carbon radical species in the plasma which is detectable by monitoring the optical emission intensity of the carbon and/or oxidized carbon at various wavelengths, such as at about 298, 483 and 520 nm for carbon monoxide species. Monitoring the emission intensity of the species can be achieved by a optical emission system (OES), as conventionally used in the art of emission spectroscopy, including a detector, e.g. a photomultiplier tube, capable of generating an electrical signal as a function of intensity connected to a recording device to record the generated signal. The time to terminate etching of the photoresist element can be optimally determined by monitoring for the corresponding change in carbon emission intensity. 
     Depending on the etch process, there may be a point in time during the lateral etching of the sidewalls of the photoresist mask when no further etching occurs (i.e. a saturation point). According to an embodiment of the present invention, the saturation point can be determined by monitoring the emission intensity of a selected specie. By monitoring for the saturation emission intensity, the time for complete lateral etching of the sidewalls of the photoresist can be optimally determined and the etching process stopped and the substrate either removed from the etching chamber or further processed in the same chamber. For example, as lateral etching of the sidewalls of the photoresist mask element progresses to saturation, the emission signal will rise if the emission from a reactant, such as atomic oxygen (O), is monitored, and will drop if a reaction product, such as carbon monoxide (CO), is monitored. Therefore, this technique will detect saturation which in turn indicates the optimal time that the final line width has been achieved. 
     In an anisotropic plasma etching environment the process that dominates between sidewall removal and deposition depends upon etch gas stoichiometry and the plasma reaction conditions, such as applied pressure, power and temperature parameters in the plasma chamber. An exemplary composition of the etchant mixture for etching a photoresist mask of the present invention comprises HBr, O 2  and an inert carrier gas, e.g. argon, in relative percentages of about 60, 10 and 30, respectively. By varying the concentration of etchant components, the photoresist line width can be changed from greater than zero to about 0.15 micron, e.g. from about 0.001 to about 0.1 micron. 
     In accordance with the present invention, the width of photoresist elements is controlled by adjusting the etchant mixture. The lateral gain or loss accomplished in the lateral process is a function of the relative amount of the components of an etchant mixture in a plasma etch environment. Moreover, by increasing the line width of the photoresist and subsequently etching underlying layers, smaller spaces existing between the pattern lines can be effectively resolved. 
     In an embodiment of the present invention, a photoresist mask overlaying a composite arrangement, as shown in FIG. 1, is placed in a commercially available plasma etch tool having at least one window for monitoring the optical emission spectrum of the generated plasma. An etchant mixture for etching the photoresit mask is introduced to the plasma chamber and a plasma is generated. Increasing the amount O 2  of the component mixture decreases the patterned line width of the photoresist material during exposure of the photoresist mask in the plasma chamber. It is believed that the higher concentration of oxygen causes the sidewalls of the photoresist material to be etched away rapidly thereby reducing the line width dimension of the patterned photoresist mask to a second width without substantially affecting the underlying layers. 
     Etching the remaining layers employing the photoresist mask having the second width results in a gate electrode arrangement in accordance with the dimensions of the second width. Additional processing steps include conventionally etching ARC  16  using the etched photoresist having the second prescribed width as a mask. The polysilicon layer  14  is then etched according to the dimensions of the second width of the photoresist mask and ARC using conventional techniques, such as using a poly etcher in an HBr/Cl 2  plasma stopping on dielectric layer  12  in completion of forming the gate electrode structure. Further processing steps are then conducted in a conventional manner depending upon on the type of device contemplated. 
     The present invention advantageously enables manufacturing deep-submicron memory devices, such as sub 0.1 micron devices, with improved control over the critical dimensions of a gate electrode and higher integration in a simplified manner with increased device uniformity and reliability. The present invention is applicable to the manufacturing of various types of semiconductor devices, particularly semiconductor devices having a design rule of less than about 0.12 micron. 
     The process steps and structures described below do not form a complete process flow for manufacturing integrated circuits. The present invention can be practiced in conjunction with integrated circuit fabrication techniques currently used in the art, and only so much of the commonly practiced process steps are included as are necessary for an understanding of the present invention. The figures representing cross-sections of portions of an integrated circuit device during fabrication are not drawn to scale, but instead are drawn to illustrate the features of the present invention. 
     While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.