METHOD OF PROCESSING A SEMICONDUCTOR WAFER FOR CONTROLLING DRIVE CURRENT

Abstract: Transistor drive current is controlled by controllably varying light exposure across a semiconductor substrate wafer based on an integrated circuit parameter. Integrated circuit parameters upon which the light exposure is varied include gate oxide thickness, rapid temperature annealing (RTA) temperature, polyetch bias and the like.

Description: METHOD OF PROCESSING A SEMICONDUCTOR WAFER FOR CONTROLLING DRIVE CURRENT Technical Field The present invention relates to a method of manufacturing integrated circuits on semiconductor substrates. More specifically, the present invention relates to a manufacturing method which improves control of transistor drive current by varying the exposure across the wafer based on integrated circuit parameters. Background Art In a lithographic process, photoresist is applied as a thin film to a substrate and subsequently exposed through a mask. The mask contains clear and opaque features that define a pattern to be created in the photoresist layer. Areas in the photoresist layer that are exposed to the light are made either soluble or insoluble in a specific solvent called a developer. Regions of silicon oxide that are not covered by photoresist following developing are removed by etching, replicating the mask pattern in the oxide layer. The photoresist has two functions, responding to the exposing radiation to replicate the mask image in the photoresist and protecting the underlying substrate in d e remaining areas of photoresist during subsequent processing. Performance of transistors that are fabricated in an integrated circuit strongly depends on the precision of structures formed using the lithographic process and the application of various physical parameters during processing. For example, variability of gate oxide thickness, polysilicon etch bias and rapid thermal annealing (RTA) temperature all lead to variations in drive current of a transistor. Drive current, the current from source to drain of a transistor, is indicative ofthe speed performance of a transistor. A uniform drive current in transistors across a semiconductor wafer results in a uniform speed of transistors across the wafer and a low leakage current even in conditions when the drive current is high. On the other hand, a drive current that is nonuniform across the wafer results in a high I C c standby current, a high leakage current and high power consumption by the integrated circuit. Traditionally, drive current uniformity is sought by reducing the variability in parameters, such as gate oxide thickness, polysilicon etch bias and rapid thermal annealing (RTA) temperature, that give rise to drive current variations. However, the reduction in variability of these parameters is difficult. What is needed is a method of promoting the uniformity of drive current beyond what is achieved by reducing the variability of parameters that give rise to drive current variations. What is also needed is an integrated circuit with a tight distribution of drive current, in which the drive current across the entire wafer is highly uniform. What is further needed is a method of reliably producing an integrated circuit in which the drive current across an entire semiconductor wafer is uniform. Disclosure of Invention In accordance with the present invention, transistor drive current is controlled by controllably varying light exposure across a semiconductor substrate wafer based on an integrated circuit parameter. Integrated circuit parameters upon which the light exposure is varied include gate oxide thickness, rapid temperature annealing (RTA) temperature, polyetch bias and the like. In some embodiments ofthe present invention, a semiconductor wafer is partitioned into a plurality of areas and a photolithographic process is applied in which the exposure in each of the plurality of areas is individually and independently controlled. In some embodiments ofthe present invention, a photolithographic process is applied in which the exposure is controlled to change the exposure intensity in a radial pattern so that the exposure is changed moving from the center towards the edge of a wafer. In accordance with one embodiment of the present invention, a method of controlling a drive current spatial distribution on a semiconductor wafer during fabrication of an integrated circuit includes the steps of determining a spatial distribution on the semiconductor wafer of a parameter that effects the drive current, determining the effect ofthe parameter on the drive current and exposing the semiconductor wafer to illumination, variably within the spatial distribution on the semiconductor wafer so that the effect ofthe parameter is counteracted. The described semiconductor processing method achieves numerous advantages. One advantage is that the current drive is made substantially constant across a semiconductor wafer. This advantage increases in importance as semiconductor technology advances and wafers become larger. As semiconductor wafers increase in size from 8 to 10 to 12 inch diameter wafers, the variability across the wafer in parameters such as gate oxide thickness, polysilicon etch bias and RTA temperature will also increase, resulting in larger variations in drive current. Brief Description of Drawings The features ofthe invention believed to be novel are specifically set forth in the appended claims. However, the invention itself, both as to its structure and method of operation, may best be understood by referring to the following description and accompanying drawings. Figure 1 is a flow chart which illustrates steps of a typical photoresist process. Figure 2 is a cross-sectional view of a semiconductor wafer including a MOS transistor. Figure 3 is a top pictorial view of a semiconductor wafer which shows a distribution of gate oxide thickness on the wafer. Figure 4 is a cross-sectional view of a semiconductor wafer which shows a distribution of gate oxide thickness on the wafer. Figure 5 is a graph illustrating a distribution of gate oxide thickness on a wafer and a distribution of lithographic light exposure applied for controlling drive current uniformity across the wafer. Figure 6 is a top pictorial view of a semiconductor wafer which shows a distribution of polysilicon etch bias on the wafer. Figure 7 is a graph illustrating a distribution of polysilicon etch bias on a wafer and a distribution of lithographic light exposure applied for controlling drive current uniformity across the wafer. Figure 8 is a top pictorial view of a semiconductor wafer which shows a distribution of rapid thermal annealing (RTA) on the wafer. Figure 9 is a graph illustrating a distribution of rapid thermal annealing (RTA) temperature on a wafer and a distribution of lithographic light exposure applied for controlling drive current uniformity across the wafer. Figure 10 is a graph which illustrates a distribution of drive current values for a plurality of transistor devices on a semiconductor wafer. Figure 11 is a graph illustrating a plurality of distributions of parameters that affect current density for determining a distribution of lithographic light exposures applied to control drive current uniformity across the wafer. Figure 12 is a top pictorial view of a semiconductor wafer which illustrates a scanning pattern of an apparatus for applying light exposure. Figure 13 is a top pictorial view of a semiconductor wafer which shows partitioning ofthe wafer into a plurality of areas. Figure 14 is a flow chart which illustrates a method of controlling a drive current spatial distribution on a semiconductor wafer during fabrication of an integrated circuit. Modes For Carrying Out the Invention Referring to Figure 1, a flow chart of a typical photoresist process 100 is shown which includes an optional substrate cleaning step 110, a spin coating step 112, a pre-bake step 114, an exposure step 116, an optional post-exposure treatment step 118, a develop step 120, an optional de-scum step 122, a post-bake step 124, a polysilicon etch step 126 and a photoresist stripping step 128. The optional steps are not used for the processing of certain materials. Many problems that arise in photoresist processing are attributed to a dirty or contaminated semiconductor wafer surface. The substrate cleaning step 110 is used to clean the wafer surface and thereby to avoid dirty substrate surfaces that adversely affect a lithographic process. Problems that arise from a dirty substrate surface include poor adhesion, defects such as pinholes and opaque spots, loss of line width control, or the entire loss of some pattern elements. The substrate cleaning step 110 is optional for most surfaces in VLSI fabrication since most surfaces are formed by either thermal oxidation or reduced pressure deposition techniques so that the surface is cleanest immediately following formation. If processing begins immediately upon surface formation, substrate cleaning step 110 is not necessary. In some cases, the substrate cleaning step 110 is combined with dehydration baking to remove moisture from the atmosphere that is rapidly absorbed by the substrate surface. Dehydration baking is carried out at atmospheric pressure and three temperature plateaus, 150- 200°C to remove surface water molecules, 400°C to evolve loosely held water of hydration and about 750°C for total dehydration. Also associated with the substrate cleaning step 110 is a priming step of applying primer with a preresist coating of a material designed to improve adhesion, such as hexamethyldisilazane (HMDS), typically by spin coating or vapor priming. In spin coating step 112, substrate wafers are spin-coated with photoresist to produce a uniform, adherent, defect-free polymeric film of a desired thickness over the entire wafer. Photoresist is applied to a circular substrate wafer and the wafer is rapidly spun until the photoresist is essentially dry. Spin coating typically distributes the photoresist film with a uniformity across the wafer of approximately plus or minus 100 angstroms. The spin coating step 112 includes three stages, dispensing the resist solution onto the wafer, angularly accelerating the wafer quickly (for example, about 20,000 rpm sec) to a final rotational speed, and spinning at a constant angular velocity (for example, 3000 to 7000 rpm for 20-30 seconds) to establish a desired thickness and to dry the photoresist film. Photoresist viscosity and film tiύckness depend on temperature so temperature is controlled to plus or rninus 1 °C during spin coating with the substrate wafers, photoresist and spinner hardware maintained in thermal equilibrium. Pre-bake step 114 is a soft-bake procedure in which photoresist-coated wafers are subjected to a predeterrnined temperature. The pre-bake step 114 is performed to remove solvent from the spun-on photoresist, improve adhesion ofthe photoresist and anneal stresses caused by the shear forces encountered in the spinning process. The degree of soft-baking depends on the temperature, time of baking and the baking equipment and determines the residual solvent content ofthe photoresist. The rate of attack ofthe photoresist by a developer is significantly dependent on the solvent concentration. In general, the more residual solvent, the higher the dissolution rate in developer so that wafers with insufficient soft-baking are attacked by the developer in both exposed and unexposed regions ofthe wafer. Soft-baking is closely linked to the exposure and development. Soft baking is typically performed in convection, infrared (IR) and hot-plate ovens. Soft baking attempts to produce uniform, controlled temperatures at a high cleanliness. The exposure step 116 is performed after photoresist coating ofthe wafer and soft baking by exposing the wafer to some form of radiation to create a latent image in the resist. The degree of exposure is controlled by adjusting the amount of energy impinging on the photoresist. The exposure energy is the product ofthe intensity ofthe radiation source and the time of exposure. The exposure step 116 is an important step in photoresist processing for several reasons. First, exposure is a step in an imaging sequence in which the wafers are individually processed so that a lengthy exposure process may lead to a throughput restriction relative to other photoresist processing steps. Also, attempts to increase processing throughput during exposure typically results in decreased resolution. Longer exposure times and consequential slower throughput are typical of high resolution photoresists. During the exposure step 116, photochemical transformations occur within the photoresist. The goal of an optimized exposure process is production of desired photochemical effects in the shortest amount of time while maintaining a high reproducibility. Optimization of the exposure step 116 is highly correlated with other steps of the photoresist process 100. In particular, the exposure step 116 optimization depends on determining an optimum resist thickness and producing this optimum thickness uniformly across the wafer and among the multiple wafers produced. The exposure step 116 also depends on choosing a soft-bake process that complements exposure conditions and determining line-width specifications and dimensional tolerances with respect to a mask and exposure tool. The exposure step 116 further depends on establishing the developer concentration, temperature, agitation requirements and selecting post-bake conditional based on other steps ofthe photoresist process 100. The photoresist process 116 further depends on the specification of a photoresist etching process. Variations of photoresist thickness result in changes in the line width. For example, a variation of 20% in photoresist thickness may result in a line width variation of 0.25 μm, which may be tolerable in practice. However, typically other parameters, for example lamp intensity and developer concentration, that impact line width also have a variability. If all such parameters were to show worst-case variation, a larger cumulative line width would result so that a 20% variation in photoresist thickness is unacceptable as part of a set of allowable photoresist process variations. Results ofthe exposure step 116 are made further variable by standing waves, which are caused when actinic light waves propagate through a photoresist film down to the substrate, reflect off the substrate back through the photoresist. The reflected waves constructively and destructively interfere with the incident waves so create zones of high and low exposure with a separation of λ/4n, where λ is wavelength ofthe illumination and n is die index of refraction ofthe photoresist. Standing waves are disadvantageous because the periodic variation of light intensity in the photoresist causes d e photoresist to receive nonuniform doses of energy throughout the layer thickness. Standing waves also disadvantageously cause line width variations as the photoresist crosses a step, due to variation ofthe total energy coupled to the photoresist by interference effects at different photoresist thickness. Both the periodic variation in light intensity and the line width variations contribute to resolution loss of photoresists. Photoresist line width variations are most difficult to control in die vicinity of steps, where large photoresist thickness changes take place. These variations result from differences in d e energy that is coupled into the photoresist at different photoresist thickness, scattering of light at steps due to diffraction and reflections, and standing wave effects. One technique for reducing line width variations over steps involves usage of anti-reflective coatings (ARC) which absorbs most of d e radiation diat penetrates the photoresist. Standing wave effects are substantially reduced because much less radiation is available to reflect off the substrate. Scattering from topographical features is also suppressed. Also, ARC partially planarizes the wafer topography, further improving line width variation over steps. However, usage of ARC also has some disadvantages. In particular, two extra process steps are incurred through usage of ARCs: spin-coating of ARC before spinning on the photoresist and prebaking the ARC before spinning on the photoresist. The optional post-exposure treatment step 118 is incurred if ARC is used. The post-exposure treatment step 118 involves transferring the pattern to the ARC. The develop step 120 develops the photoresist film to leave behind an image which subsequently serves as a mask for etching, ion-implantation, lift-off and the like. The development step 120 attempts to leave the original thickness of unexposed resist without measurable reduction while precisely producing specified patterned dimensions. Development should cause minimal pattern distortion or swelling and the development time should be short, a minute or less. Development is typically accomplished by immersion, spray or puddle developing. Immersion developing involves immersing a wafer into a developer bath and agitating the wafer for a specified time at a specified temperature. Spray developing involves spraying developer on rotating wafers, allowing a controlled fraction ofthe developer to drain off the wafers. The puddle technique is carried out by dispensing a fixed amount of developer onto a static wafer. The develop step 120 is typically optimized in terms of developer concentration, developer temperature and developer agitation method. The optional de-scum step 122 is a mild plasma treatment u at is typically used to remove very small quantities of photoresist in small unwanted areas, for example a few hundred angstrom areas. The de-scum step 122 is used to remove photoresist that could not be removed during the develop step 120. During the post-bake step 124, the photoresist is subjected to an elevated temperature after development but before etching so that residual solvents are removed to improve adhesion and to increase the etch resistance ofthe photoresist. Polysilicon is etched in etch step 126 and the photoresist is removed in the photoresist stripping step 128. Referring to Figure 2, a cross-sectional view of a semiconductor wafer 200 shows a MOS transistor 210 including a semiconductor substrate 220, a source diffusion 222, a drain diffusion 224 and a gate 226 formed from a gate oxide layer 228. The MOS transistor 210 has spacers 230 formed from spacer oxide 232 on the sides of the gate 226. Lightly doped drain (LDD) regions 234 are diffused into die substrate 220 underlying the spacers 230. Referring to Figures 3 and 4, respectively a top pictorial view and a cross-sectional view of a semiconductor wafer 300 illustrate a gate oxide thickness distribution 310 on the wafer 300. Several methods of measuring gate oxide thickness are known in the semiconductor processing art. For example, gate oxide d ickness is measured using a scanning electron microscope (SEM) micrograph. An electron microscope uses a beam of electrons to investigate a sample to achieve a higher magnification than is possible using an optical microscope. In other embodiments, gate oxide thickness is measured using an optical interference method in which a spectrophotometer supplies an incident light beam in the UV to visible range (480-790nm). The incident wavelength of the light is varied as the incident angle is held constant. Reflected light intensity is measured as a function of wavelength. Gate oxide thickness may also be measured using ellipsometry which furnishes a nondestructive optical technique for measuring oxide thickness as well as the optical index of refraction at the measuring wavelength. The ellipsometry technique uses die change in state of d e polarization of light when light is reflected from a surface. The state of polarization is determined by die relative magnitude of parallel and peφendicular components ofthe radiation and by the phase difference between the parallel and perdendicular components. The polarization change depends on die optical constants ofthe silicon, the angle of incidence of the light, the optical constants ofthe oxide film and the oxide film thickness. If die optical contants ofthe substrate are known and the film is non-absorptive at the test wavelength, then the state of polarization ofthe reflected beam depends on the optical contstants and the thickness ofthe gate oxide film. In omer embodiments, the gate oxide thickness is measured using a capacitance method but requires the fabrication of a MOS capacitor. Gate oxide thickness is determined as a function of measured oxide capacitance, area of die gate, dielectric constant of die silicon dioxide of the gate and the free space permittivity. In still other embodiments, gate oxide thickness may be measured using an oxide film color chart in which various oxide d icknesses are grown and measured by ellipsometry or optical interference. A color chart is prepared from the thickness samples and compared to gate oxide samples. Growth of defect-free, thin (15-100nm), high-quality, noncontaminated, uniform thickness gate oxide is important for device operation. Drain current in a MOS transistor is inversely proportional to gate ickness so that gate oxide is normally made as thin as possible considering oxide breakdown and reliability considerations. However, gate oxide thickness is typically not constant across the wafer 300 but instead has a pattern that is determined by multiple various oxidation growth parameters. These parameters include the crystallographic orientation of silicon in the wafer, the silicon doping level, the presence of halogen impurities (Cl, HCl, TCA, TCE, for example) in the gas phase ofthe oxidation, growth pressure, die presence of plasma during growth and d e presence of photon flux during growth. These parameters are greatly dependent on die type of semiconductor processing equipment, the various chemicals and physical parameters such as temperatures and pressures that are used during processing. Thus, fhe gate oxide thickness distribution 310 may take various different forms. In the present example, shown in Figures 3 and 4, various parameters such as growdi pressure and temperature distributions within the circular disk wafer may result in a radial pattern of gate oxide thickness. A radial distribution of gate oxide thickness results from various nonuniformities that occur during processing, including variations in gas flow, temperature and pressure. A second parameter that influences the form ofthe gate oxide thickness distribution 310 is temperature inhomogeneities arising from convection currents at high pressures resulting in poor diickness uniformity. This poor thickness uniformity is illustrated by topographical thickness lines 312 shown in Figure 3 that intersect fhe radial distribution 310. In one processing apparatus, the diagonal topographical thickness lines 312 are created by a vertically disposed furnace which produces a gas flow on one side of d e wafer 300. In some processing systems, the topographical thickness lines 312 predominate over the radial gate oxide thickness distribution 310. Referring to Figure 5, a graph illustrates a distribution 510 of gate oxide thickness on the semiconductor wafer and a distribution 520 of lithographic light exposure. In this example, gate oxide thickness varies across the semiconductor wafer 300 approximately from a thickness of 70 angstroms at the center of e semiconductor wafer disk to a diickness of about 74 angstroms at the outer edge ofthe disk. The drive current of a transistor device varies according to gate oxide thickness. For a iicker gate oxide layer, as compared to a thinner layer, a larger voltage is necessary to inject die same amount of charge into the channel region of a transistor, resulting in a reduced drive current. Accordingly, a wider gate oxide thickness causes die drive current to decrease, a thinner gate oxide diickness causes the drive current to increase. Thickening ofthe gate oxide is compensated by increasing the light exposure applied to the transistor, thereby reducing the channel width. Accordingly, increased light exposure is applied to regions of thick gate oxide on die semiconductor substrate so that die channel widdi ofthe transistor devices in the diick gate oxide regions is reduced, raising d e drive current in these regions so that the drive current over die entire semiconductor wafer is substantially constant. The light exposure is controlled across die semiconductor wafer 300 to improve drive current uniformity across die wafer 300. In conventional semiconductor processing, die same exposure is applied at all locations of the wafer, to every wafer in a processing lot, widiout taking into account variations in gate oxide diickness across a wafer. Figure 5 shows, in an embodiment of die present invention, that the exposure is varied across the semiconductor wafer 300 to make drive current uniform across die wafer 300. Specifically, the exposure applied to the wafer 300 is lower at the center ofthe wafer 300 where the gate oxide thickness is smaller and is increased radially in proportion to the increase in gate oxide thickness toward the periphery of the semiconductor wafer 300. In this manner, light exposure applied to the semiconductor wafer 300 is varied to compensate for differences in gate oxide diickness. The effect of gate oxide thickness on the drive current is determined by measuring die drive current of multiple transistor devices at various positions on die semiconductor wafer and correlating me measured drive current wid the measured gate oxide diickness at corresponding positions of die semiconductor wafer. Referring to Figure 6, a top pictorial view of a semiconductor wafer 600 illustrates a polysilicon etch bias distribution 610 on d e wafer 600. Polysilicon etch bias is die difference in lateral dimension between an etched structure and a mask structure. Accordingly, the etch bias is a measure of uniformity of etching. Variation in polysilicon etch bias leads to dimensional variability in transistor structures such as channel widths, source and drain diffusion position and die like, ultimately resulting in nonuniformity in drive current across a semiconductor wafer 600. Polysilicon etch bias is typically nonuniform across the wafer 600 and has a pattern diat is determined by multiple various parameters. These parameters include variations in temperature, plasma density, and gas flow. The parameters also include loading effects, as well as uniformity of polysilicon deposition on the wafer and uniformity of photoresist coating on the semiconductor wafer 600. For example, the structure and properties of polysilicon depend substantially on the deposition temperature, dopant type and concentration and subsequent thermal cycling. These parameters generally determine the crystalline or amoφhous character ofthe polysilicon. In addition, photoresist typically may have a nonuniform thickness across the semiconductor wafer 600 diat arises from spin coating ofthe wafer 600 with photoresist. In the example shown in Figure 6, the combined variation in diickness uniformity ofthe polysilicon and the photoresist result in a polysilicon etch bias variability having a radial form in which the polysilicon etch bias is greater toward the center ofthe semiconductor wafer disk 600 and decreases substantially monotonically as e radial distance from the center increases to the edge of die disk. In accordance witii the polysilicon bias etch distribution 610 shown in Figure 6, the semiconductor wafer 600 may be radially partitioned so diat a photolithographic process is applied in which exposure is controlled to change die exposure intensity in a radial pattern so that the exposure is changed moving from the center 614 towards die edge 612 of die wafer 600. In some embodiments, polysilicon etch bias is measured using a scanning electron microscope (SEM) micrograph. An elecfron microscope uses a beam of electrons to investigate a sample to achieve a higher magnification than is possible using an optical microscope. Referring to Figure 7, a graph illustrates a distribution 710 of polysilicon etch bias on die semiconductor wafer 600 and a distribution 720 of litiiographic light exposure. In this example, polysilicon etch bias varies across die semiconductor wafer 600 from a relatively high etch bias at die center of die semiconductor wafer disk 600 to a low etch bias at die periphery ofthe disk. Polysilicon etch bias controls die drive current of a transistor. In particular, a larger polysilicon etch bias causes die drive current to be more nonuniform, a smaller polysilicon etch bias causes the drive current to be more uniform across the semiconductor wafer 600. The light exposure is controlled across die semiconductor wafer 600 to improve drive current uniformity across the wafer 600. Figure 7 shows, in an embodiment ofthe present invention, that the exposure is varied across the semiconductor wafer 600 to make drive current uniform across die wafer 600. Specifically, the exposure applied to the wafer 600 is lower at the center ofthe wafer 600 where the polysilicon etch bias is larger and is increased radially in proportion to the reduction in polysilicon etch bias toward die periphery of d e semiconductor wafer 600. In this manner, light exposure applied to die semiconductor wafer 600 is varied to compensate for differences in polysilicon etch bias. The effect of polysilicon etch bias on die drive current is determined by measuring the drive current of multiple transistor devices at various positions on the semiconductor wafer and correlating the measured drive current with the measured polysilicon etch bias measurements at corresponding positions of the semiconductor wafer. Referring to Figure 8, a top pictorial view of a semiconductor wafer 800 illustrates a rapid thermal annealing (RTA) temperature distribution 810 on fhe wafer 800. Semiconductor wafers are subjected to many elevated temperature steps during integrated circuit fabrication. Integrated circuit devices are fabricated by creating precisely controlled regions of dopants in the wafers. Thermal treatments can cause die dopants to diffuse, reducing control of dopant concentration and locations. Rapid thermal annealing (RTA) is short-term, high temperature processing to reduce dopant diffusion while still achieving a desired process effect. Optimal RTA processing is achieved when the temperature across the semiconductor wafer 800 is uniform and constant. However, because ofthe manner in which the semiconductor wafer 800 is mounted during RTA processing, heating and cooling ofthe wafer is variable in a radial pattern, as shown by the RTA temperature distribution 810. One consequence of me variable RTA temperature distribution 810 is that the dopant of d e lightly doped drain (LDD) diffusion (234 shown in Figure 2) diffuses further under the gate 226, causing detrimental "hot-carrier effects" in a transistor. Hot-carrier effects are caused by hot electrons that are generated by impact ionization and injected into me silicon oxide of a transistor. The hot electrons are trapped near the drain 224 and shorten the effect channel length L eff . The injected and trapped hot electrons also produce a reduction in transistor tiireshold voltage V τ , an increase in transistor transconductance g„, and an increase in drive current. The variation in RTA temperature distribution 810 has a radial form in which die RTA temperature is greater toward the center of die semiconductor wafer disk 800 and decreases substantially monotonically as die radial distance from die center increases to the edge of die disk. In accordance witii the RTA temperature distribution 810 shown in Figure 6, die semiconductor wafer 800 may be radially partitioned so that a photolitiiographic process is applied in which exposure is controlled to change die exposure intensity in a radial pattern so diat die exposure is changed moving from die center 814 towards the edge 812 of die wafer 800. In one example, the light exposure is controlled across die semiconductor wafer 800 in a radial pattern so mat d e exposure in a center region 830 is, for example, 400mJ, in an incremental radial region 832 an exposure of 402m J is applied and a next incremental radial region 834 has an applied exposure of 404mJ. Referring to Figure 9, a graph illustrates a distribution 910 of RTA temperature on die semiconductor wafer 800 and a distribution 920 of littiographic light exposure. In this example, RTA temperature varies across die semiconductor wafer 800 from a relatively high etch bias at the center of me semiconductor wafer disk 800 to a low etch bias at the periphery ofthe disk. RTA temperature controls the drive current of a transistor. In particular, a larger RTA temperature reduces effective channel lengtii L.jτ of a device, reduces transistor threshold voltage V τ , increases transistor transconductance g m and increases drive current. A smaller RTA temperature increases effective channel length L eff of a device, increases transistor tiireshold voltage V τ , decreases transistor transconductance g m and decreases drive current. In some embodiments, RTA temperature is measured using several thermocouples, as are known in the art of semiconductor processing. A thermocouple uses two dissimilar metals at each end to form an electrical circuit. If the two junctions are maintained at different temperatures, an electromotive force (EMF) is developed between the junctions. The light exposure is controlled across die semiconductor wafer 800 to improve drive current uniformity across the wafer 800. Figure 9 shows, in an embodiment of die present invention, diat die exposure is varied across the semiconductor wafer 800 to make drive current uniform across fhe wafer 800. Specifically, die exposure applied to the wafer 800 is lower at die center of die wafer 800 where die RTA temperature is larger and is increased radially in proportion to die reduction in RTA temperature toward the periphery of e semiconductor wafer 800. In this manner, light exposure applied to die semiconductor wafer 800 is varied to compensate for differences in RTA temperature. The effect of RTA temperature on the drive current is determined by measuring the drive current of multiple transistor devices at various positions on die semiconductor wafer and correlating die measured drive current with the measured RTA temperature at corresponding positions of the semiconductor wafer. Referring to Figure 10, a current drive distribution graph 1000 illustrates a distribution of drive current values for a plurality of transistor devices on a semiconductor wafer. The abscissa of die current drive distribution graph 1000 represents die current drive values for a plurality of transistor devices. The ordinate of the current drive distribution graph 1000 represents the number of devices having a particular current drive value. A conventional current drive distribution 1010 shows die current drive distribution of a typical conventional semiconductor wafer. A line representing an improved current drive distribution 1020 depicts the current drive distribution of a semiconductor wafer using controlled light exposure to improve current drive uniformity. The improved current drive distribution 1020 typically has a gaussian form about a central approximation value, such as a mean or nominal value 1022. A tiiree-sigma statistical measure 1012 for die conventional current drive distribution 1010 and a d ree-sigma statistical measure 1024 for the improved current drive distribution illustrate compaction ofthe current drive distribution achieved by die controlled light exposure method. A lower specification limit 1030 and an upper specification limit 1032 are designated. Transistor devices having a current drive smaller than the lower specification limit 1030 and transistor devices having a current drive greater man the upper specification limit 1032 are discarded. The disclosed processing method is expected to typically reduce die variation in current drive distribution by one-sigma (one standard deviation). The current drive distribution is tightened, tails of d e distribution are substantially eliminated so diat essentially no transistor devices have a current drive outside an allowable range. This substantial reduction in variation in current drive distribution allows a fabricated integrated circuit to be operated at a higher speed by increasing the drive current while leakage current is maintained at a low level. In particular, the improved current drive distribution 1020 is compact about d e nominal value 1022 so that very few devices have a current drive below d e lower specification limit 1030 so d at the nominal value 1022 may be increased witiiout increasing the number of devices diat are discard due to a low current drive. In tiiis manner, the operating frequency of an integrated circuit is advantageously increased. The light exposure is varied across the semiconductor wafer so diat the current drives of devices in die entire distribution is increased, allowing a faster operating frequency. For example, die operating frequency of an integrated circuit may be increased by an amount of die order of tens of megahertz. The basis for such an increase in operating frequency is grounded in analysis of statistical parameters. One statistical parameter is a process capability index C p , which is the ratio of die allowable specification to die width ofthe distribution ofthe process characteristic to six standard deviations. The process capability index C p is applicable to two-sided statistical distributions. Anotiier statistical parameter is a one-sided process capability index C pk , which is the ratio of the distance from the specification limit to the process average, divided by three standard deviations. The one-sided process capability index C pk describes how well a current drive distribution for a plurality of devices in a semiconductor wafer fits within the specification limit bounded by die lower specification limit 1030 and the upper specification limit 1032. The larger a C pk value of a distribution, d e more integrated circuit devices diat do not meet a defined specification so that more devices are discarded. Referring to Figure 11, a graph illustrates a plurality of exposure functions to compensate for variations in parameters including a gate oxide diickness function 520, a polysilicon etch bias function 720 and an RTA temperature function 920 that affect current density. These exposure functions are combined to determine a distribution of litiiographic light exposures applied to control drive current uniformity across the wafer. A light exposure distribution across a semiconductor wafer is determined to compensate for each parameter of die plurality of parameters. In particular, the light exposure is increased from the center to the edge of a wafer to compensate for each of die gate oxide thickness distribution 510, die polysilicon etch bias distribution 610 and die RTA temperature distribution 910, which are respectively illustrated in Figures 5, 7 and 9. In other embodiments of a silicon processing metiiod, different light exposure distributions may be applied to compensate for various parameters. In some embodiments, die light exposure may be increased from the edge to the center of a wafer. Also, different parameters may be compensated by opposing light exposure distributions so d at, for example, die combined changes in light exposure distribution may partially or completely cancel. In some metiiod embodiments, die pattern of light exposure may be patterns otiier than a radial pattern. Referring to Figure 12, a top pictorial view of a semiconductor wafer 1200 illustrates a scanning pattern 1210 of a scanning apparatus (not shown) for applying light exposure. The scanning apparatus, which is known in the semiconductor processing arts, includes a stepper for scanning a field in an x-y rectilinear pattern. The scanning apparatus also includes a controller diat controls positioning of an illuminator witiiin a field, die exposure intensity or energy and exposure time at each position in the scan field. The scanning apparatus is programmable so that an applied scan formula of exposure intensities, for example in mJ, and durations, for example in seconds, is easily saved and modified. A semiconductor wafer may be exposed in any manner using this scanning pattern 1210 by individually and independently setting the intensity and duration of illumination applied at each point in the scan field. In addition, more precise control is made possible by having a programmable selection of distance step size in die x-y field ofthe scanner. In various embodiments, different types of illuminators may be employed. Using a photolithography technique, a deep ultraviolet illumination exposure is applied to me semiconductor wafer. In another embodiment, an X-ray lithography system is employed in which X-rays, rati er than light beams, are used to expose appropriate photoresists. In further additional embodiments, the wafers are illuminated with electron beams in which a beam of electrons are usually emitted from a single source, such as a diermionic catiiode. Using ion-beam lithography, which is similar to electτon-beam lithography, a semiconductor wafer is exposed to a beam of ions, rather than a beam of electrons, resulting in a greater photoresist sensitivity and a reduced writing time. Referring to Figure 13, a top pictorial view of a semiconductor wafer 1300 shows partitioning ofthe wafer 1300 into a plurality of areas. The semiconductor wafer 1300 is partitioned into a plurality of areas 1302. A photolimographic process is applied in which the exposure in each ofthe plurality of areas 1302 is individually and independently controlled as a function of position on the wafer to account for variations in drive current that result from gate oxide thickness, polysilicon etch bias, rapid thermal annealing temperature and other parameters. The exposure applied to an area 1302 is controlled to account for a single processing parameter or a plurality of parameters in combination. Referring to Figure 14, a flow chart illustrates a method 1400 of controlling a drive current spatial distribution on a semiconductor wafer during fabrication of an integrated circuit. In a first step 1410, a parameter that effects drive current, such as gate oxide diickness, polysilicon etch bias or RTA temperature is measured in multiple locations on a semiconductor wafer to determine the spatial distribution of die parameter. In second step 1420, die effect of d e parameter on die drive current is determined, generally by measuring die drive current at various positions on the wafer and correlating die drive cunent of these locations with the parameter values at corresponding positions. In step 1430, die semiconductor wafer is exposed to illumination within die spatial distribution on die semiconductor wafer so tiiat the effect ofthe parameter is counteracted. While the invention has been described witii reference to various embodiments, it will be understood mat tiiese embodiments are illustrative and diat die scope of die invention is not limited to diem. Many variations, modifications, additions and improvements of die embodiments described are possible. For example, die described method employs control of exposure to account for variations in processing parameters such as gate oxide diickness, polysilicon etch bias and rapid thermal annealing (RTA) temperature. The metiiod is also applicable to control of exposure to account for variations in otiier processing parameters including variations in spacer etch, variations in temperature arising from otiier processing steps and processing devices and die like.

Claims: CLAIMS: 1. A method of controlling the spatial distribution of a circuit performance across a semiconductor wafer during fabrication of an integrated circuit comprising the steps of: determining a spatial distribution on the semiconductor wafer of a parameter that effects the circuit performance; determining the effect of the parameter on die circuit performance; and exposing the semiconductor wafer to illumination, variably witiiin the spatial distribution on die semiconductor wafer so that the effect of the parameter is counteracted. 2. A method in accordance witii Claim 1 wherein the circuit performance is manifest by a drive current measurement. 3. A method in accordance with Claim 1 wherein the parameter is gate oxide thickness and the step of deteirnining die spatial distribution of gate oxide thickness includes die step of: measuring gate oxide diickness at a plurality of positions ofthe semiconductor wafer using a scanning electron microscope micrograph. 4. A metiiod in accordance witii Claim 3 wherein die parameter is gate oxide thickness and die step of determining the effect of the parameter on die circuit performance includes die step of: measuring die circuit performance of a plurality of transistor devices at a plurality of positions ofthe semiconductor wafer; and correlating the measured circuit performance of die transistor devices with the measured gate oxide thickness at corresponding positions ofthe semiconductor wafer. 5. A metiiod in accordance witii Claim 3 wherein the circuit performance is manifest by a drive current measurement, die parameter is gate oxide diickness and the step of determining the effect ofthe parameter on the drive current includes the steps of: measuring the drive current of a plurality of transistor devices at a plurality of positions of the semiconductor wafer; and correlating the measured drive current of the transistor devices with die measured gate oxide diickness at corresponding positions of the semiconductor wafer. 6. A method in accordance with Claim 1 wherein the parameter is gate oxide thickness and the step of determining die spatial distribution of gate oxide diickness includes a step selected from fhe steps of: measuring gate oxide diickness at a plurality of positions of die semiconductor wafer using an optical interference method; measuring gate oxide thickness at a plurality of positions ofthe semiconductor wafer using ellipsometry; measuring gate oxide diickness at a plurality of positions ofthe semiconductor wafer using a capacitance method; and measuring gate oxide thickness at a plurality of positions ofthe semiconductor wafer using an oxide film color chart. 7. A method in accordance with Claim 1 wherein the parameter is polysilicon etch bias and the step of determining the spatial distribution of polysilicon etch bias includes the step of: measuring polysilicon etch bias at a plurality of positions ofthe semiconductor wafer using a scanning electron microscope micrograph. 8. A method in accordance with Claim 7 wherein die parameter is polysilicon etch bias and die step of determining the effect ofthe parameter on the circuit performance includes die steps of: measuring the circuit performance of a plurality of transistor devices at a plurality of positions of die semiconductor wafer; and correlating the measured circuit performance of die transistor devices with the measured polysilicon etch bias at corresponding positions of die semiconductor wafer. 9. A method in accordance witii Claim 7 wherein the circuit performance is manifest by a drive current measurement, the parameter is polysilicon etch bias and die step of determining the effect of die parameter on die drive current includes the step of: measuring the drive current of a plurality of transistor devices at a plurality of positions of die semiconductor wafer; and correlating the measured drive current of die transistor devices witii die measured polysilicon etch bias at corresponding positions ofthe semiconductor wafer. 10. A method in accordance witii Claim 1 wherein die parameter is rapid tiiermal annealing (RTA) temperature and the step of determining the spatial distribution of RTA temperature includes die step of: measuring RTA temperature at a plurality of positions of die semiconductor wafer using a thermocouple. 11. A method in accordance with Claim 1 wherein the parameter is rapid thermal annealing (RTA) temperature and the step of determining die spatial distribution of RTA temperature includes die step of: measuring an electrical parameter at a plurality of positions ofthe semiconductor wafer. 12. A method in accordance with Claim 1 wherein the parameter is rapid tiiermal annealing (RTA) temperature and the step of determining the spatial distribution of RTA temperature includes the step of: measuring resistance at a plurality of positions ofthe semiconductor wafer. 13. A method in accordance with Claim 1 wherein the parameter is rapid thermal annealing (RTA) temperature and die step of determining the spatial distribution of RTA temperature includes die step of: measuring threshold voltage at a plurality of positions of the semiconductor wafer. 14. A method in accordance with Claim 1 wherein the parameter is rapid tiiermal annealing (RTA) temperature and the step of determining the spatial distribution of RTA temperature includes the step of: measuring sheet rho at a plurality of positions ofthe semiconductor wafer. 15. A method in accordance witii Claim 1 wherein the parameter is RTA temperature and the step of deteirnining the effect ofthe parameter on the circuit performance includes die steps of: measuring the circuit performance of a plurality of transistor devices at a plurality of positions ofthe semiconductor wafer; and correlating the measured circuit performance ofthe transistor devices with die measured RTA temperature at corresponding positions ofthe semiconductor wafer. 16. A metiiod in accordance witii Claim 1 wherein die circuit performance is manifest by a drive current measurement, d e parameter is RTA temperature and die step of determining die effect of the parameter on d e drive current includes the steps of: measuring the drive current of a plurality of transistor devices at a plurality of positions ofthe semiconductor wafer; and correlating die measured drive current of the transistor devices with the measured RTA temperature at corresponding positions of die semiconductor wafer. 17. A method in accordance with Claim 1 wherein the step of exposing the semiconductor wafer to illumination includes die step of exposing the semiconductor wafer to X-ray illumination in an X-ray litiiography system. 18. A method in accordance with Claim 1 wherein the step of exposing the semiconductor wafer to illumination includes the step of exposing the semiconductor wafer to deep ultraviolet (UV) illumination in a photolithography system. 19. A method in accordance witii Claim 1 wherein the step of exposing the semiconductor wafer to illumination includes the step of exposing the semiconductor wafer to electron beam illumination. 20. A metiiod in accordance witii Claim 1 wherein the step of exposing the semiconductor wafer to illumination includes the step of exposing the semiconductor wafer to ion-beam illumination.

Inventors: FULFORD H JIM JR, WRISTERS DERICK, FULFORD, H., JIM, JR., WRISTERS, DERICK
Applicants: ADVANCED MICRO DEVICES INC, ADVANCED MICRO DEVICES, INC.
Application year: 1997
Main classification: H01L21
IPC codes: G03F7, H01L21