Patent Number: 061987921
Section: description

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 schematically depicts an apparatus for EUV lithography that comprises a radiation source 11, such as a synchrontron or a laser plasma source, that emits x-rays 12 into condenser 13 which in turn emits beam 14 that illuminates a portion of reticle or mask 15. The emerging patterned beam is introduced into the imaging optics 16 which projects an image of mask 15, shown mounted on mask stage 17, onto wafer 18 which is mounted on stage 19. Element 20, an x-y scanner, scans mask 15 and wafer 18 in such direction and at such relative speed as to accommodate the desired mask-to-image reduction. As described in further detail herein, the wafer is housed in a wafer chamber that is separated from the other elements of the photolithography system located upstream as illustrated in FIG. 1. These other elements can be housed in one or more chambers which are preferably maintained in vacuum to minimize attenuation of the x-rays. EUV radiation projected from the mask and translated by the camera travels through an aperture in the wafer chamber. With the present invention, this aperture does not employ a solid filter window, e.g., one that is fabricated from soft x-ray transmitting materials. The EUV lithography device of the present invention is particularly suited for fabricating integrated devices that comprise at least one element having a dimension of .ltoreq.0.25 .mu.m. The process comprises construction of a plurality of successive levels by lithographic delineation using a mask pattern that is illuminated to produce a corresponding pattern image on the device being fabricated, ultimately to result in removal of or addition of material in the pattern image regions. Typically, where lithographic delineation is by projection, the collected radiation is processed to accommodate imaging optics of a projection camera and image quality that is substantially equal in the scan and cross-scan directions, and smoothly varying as the space between adjacent lines varies. In a preferred embodiment, projection comprises ringfield scanning comprising illumination of a straight or arcuate region of a projection mask. In another preferred embodiment, projection comprises reduction ringfield scanning in which an imaged arcuate region on the image plane is of reduced size relative to that of the subject arcuate region so that the imaged pattern is reduced in size relative to the mask region. The individual elements that form the EUV lithography device as shown in FIG. 1 can comprise conventional optical devices, e.g., condensers, cameras, and lens, for projection EUV lithography. Preferably the EUVL device employs a condenser that collects soft x-rays for illuminating a ringfield camera. A particularly preferred EUVL device that employs a condenser having a diffraction grating on the surface of a mirror upstream from the reflective mask that enhances critical dimension control is described in Sweatt et al., U.S. patent application Ser. No. 09/130,224 entitled "Diffractive Element in Extreme-UV Lithography Condenser" filed on Aug. 6, 1998, which is incorporated by reference. The condenser illustrated therein has the ability to separate the light from a line or quasi point source at the entrance pupil into several separated lines or transform point foci that are still superimposed on each other at the ringfield radius, thus maximizing the collection efficiency of the condenser and smoothing out any inhomogeneities in the source optics. FIGS. 2A and 2B illustrate the vacuum enclosure housing the elements depicted in FIG. 1, and the associated devices for the gas curtain. The partition 40 (shown as the partial cut-away) stretches horizontally across the enclosure and divides it into a lower chamber 41 and an upper chamber 42. This partition 40 serves to help prevent the contaminants in the lower chamber 41 from entering the upper chamber 42. Contaminants of primary concern are gaseous hydrocarbons that formed when a wafer typically having a layer of photoresist thereon is exposed to EUV radiation. The partition 40 has an aperture 43 to allow the EUV radiation 44 to pass through from the upper chamber 42 into the lower chamber 41. This aperture 43 is preferably formed at the center directly above the middle of the wafer stage; the aperture is of sufficient size to allow the EUV ray bundle to pass through to the wafer. The aperture 43 is the only potential path for contaminants from the lower chamber 41 to enter the upper chamber 42, as the partition is sealed along the perimeter. The gas curtain is positioned to prevent or minimize the amount of contaminants flowing through this path. In addition, the area of the aperture 43 should be kept to a minimum to help reduce the amount of gas that enters the upper chamber 42. Preferably, the aperture has an opening with an area of 1 cm.sup.2 to 5 cm.sup.2. In one embodiment, the opening is a slit having a width of 0.4 cm to 1.5 cm and a length of 2.6 cm to 3.7 cm. Lower chamber 41 houses a wafer stage 45, and the upper chamber 42 houses the other devices, e.g., camera and condenser, that are upstream of the wafer stage as shown in FIG. 1. It is understood that instead of employing the single upper chamber 42, additional chambers can be employed since it may be more convenient or desirable to isolate the various devices of the projection photolithography system in separate chambers with different environments. An inert gas curtain is created immediately below the aperture 43 to prevent or at least minimize the flow of hydrocarbon gases through the aperture to the upper chamber 42. Inert gas flows from a source 46 through a duct 47 which connects with the inert gas inlet duct 48 in the lower chamber 41. The inert gas inlet duct 48 extends horizontally to a point as close to the EUV radiation path as practical without actually interfering with this path, as shown in FIG. 2B. The inert gas flows through the inlet duct 48, from left to right, and exits the right end of the duct which is adjacent to the path of the EUV radiation. The inert gas flows horizontally across the path of the EUV above the wafer and entrains the hydrocarbon gas coming from the wafer. The resulting gas mixture continues to flow horizontally in the lower chamber 41 to the right, as shown by path 49. This gas mixture exits the lower chamber 41 through outlet duct 50 which is connected to a vacuum device 51. A vacuum typically in the range of 0.005 torr to 0.5 torr is established within the lower chamber 41. In general, the inlet duct 48 has some clearance above a wafer that is positioned on wafer stage 45. The clearance is preferably 2 mm to 10 mm, and preferably there is a similar clearance beneath the partition 40. A `chimney` or conduit 52 bridges the gap between the inlet duct 48 and the partition 40 such that it prevents the hydrocarbon gas coming from the wafer from flowing upward and around the inert gas inlet duct and through the aperture 43 to the optics in the upper chamber 42. In general, the pressure in the lower chamber 41 is larger than the pressure in the upper chamber 42. This results in some of the inert gas exiting the inert gas inlet duct 48 from flowing up through the chimney 52 and into the upper chamber 42. Gases in the upper chamber 42 which may come from the gas curtain or may be out gassed from various sources in the upper chamber are evacuated from the upper chamber through one or more outlet ducts 54 connected to vacuum devices 53. A vacuum typically in the range of 10.sup.-6 torr to 0.005 torr is established within the upper chamber 42. As illustrated in FIG. 2B, aperture 43 for the EUV has an elongated contour with width w, that matches the outer contour of the EUV beam that is projected from the mask and translated by the camera into the wafer chamber. The width of the inert gas inlet duct 48 is preferably the same as the aperture 43 in the partition 40. The vertical dimension of the inert gas inlet duct is h.sub.i, as shown in FIG. 2B. The inert gas can comprise any gas that does not interfere with the operation of the projection photolithography device, e.g., gas(es) with good x-ray transmission in vacuum conditions. Suitable gases include, for example, hydrogen, helium, argon, oxygen and mixtures thereof. Preferably, the flow of inert gas over the wafer is at a rate such that the mass transfer Peclet number (a non-dimensional quantity given by Vh.sub.i /D, where V is the gas velocity, h.sub.i is the vertical dimension of the inert gas inlet duct, and D is the diffusivity of the hydrocarbon gas in the inert gas) has a value between 20 and 35. This can be achieved by using the proper the gas flow rate, duct size, type of gas, or any combination thereof. A typical set of values for these are: (i) a gas flow rate of 0.025 g/sec to 0.05 g/sec and preferably about 0.03 g/sec, (ii) a duct with a vertical height of 1 cm to 5 cm and preferably 4 cm and a width of 2 cm to 4 cm and preferably 2.92 cm, and argon for the inert gas. Modeling was employed to determine the effectiveness of using the gas curtain to protect the projection optics located in the upper chamber from hydrocarbon contamination. The calculations were based on a two-dimensional model of a specific vacuum enclosure illustrated in FIGS. 3A and 3B, wherein the inner width of the enclosure is 1.06 m, and the height of the apparatus is 1.37 m. The upper chamber 60 has two gas outlets 61. The partition 62 separates the upper chamber 60 from the lower chamber 63. The inlet 64 for the inert gas flow is on the left side of the lower chamber 63, and the wafer 65 is located 18 cm from the bottom of the lower chamber. The lower chamber 63 has an outlet 66 on the right side. For the calculations, the upper chamber 60 was empty, i.e., the model does not include the projection optics. The region 67 of FIG. 3A is shown enlarged in FIG. 3B. Portions of the partition 62 and wafer 65 are shown in FIG. 3B along with the aperture 68. In this model, the upper boundary wall of the inert gas inlet duct 70 is provided by the partition 62, and the lower wall 69 of the inlet duct has a 1 mm clearance above the wafer 65. The vertical height of the inlet duct 70 is h.sub.i. The calculations considered an argon gas curtain positioned directly above the wafer 65, the source of the hydrocarbon in an attempt to entrain the hydrocarbon in the argon flow and to convect it out of the vacuum enclosure. The results from the calculations show that the partial pressure of the hydrocarbon in the upper chamber decreases as the Peclet number, Pe=Vh.sub.i /D, increases, where V is the velocity of the argon above the wafer, h.sub.i is the dimension of the argon inlet duct, and D is the mass diffusivity. Calculations are carried out for the momentum, energy, and mass transport in the vacuum enclosure where a gas curtain is used to prevent hydrocarbons out gassed from the wafer from entering into the upper chamber. The compressible form of the gas dynamic equations were solved. The calculations assume that the momentum and mass transport processes are governed by the continuum equations and the transport processes are two dimensional. Equations 1-4 show the non-dimensional, steady state forms of the governing continuity, momentum, energy, and mass transport equations. EQU .gradient..multidot..rho.V=0 1 EQU Re A.rho.(V.multidot..gradient.)V=-.gradient.p-.gradient..multidot..tau. 2 EQU Pe A.rho.(V.multidot..gradient.)T=.gradient..sup.2 T-Br p(.gradient..multidot.V)+Br(.tau.:.gradient.V) 3 EQU Pe.sub.m A.gradient..multidot.(.rho.Vy)=.gradient..sup.2 (.rho.y) 4 In these equations .gradient., .rho., V, p, T, y and .tau. are the dimensionless forms of the `del-operator`, density, velocity vector, pressure, temperature, mass fraction and viscous stress tensor, respectively. The equation parameters are the aspect ratio, A=.delta..sub.y /.delta..sub.x, the Reynolds number, Re=V.sub.c.delta..sub.y.rho..sub.c /.mu., the heat transfer Peclet number, Pe=V.sub.c.delta..sub.y.rho..sub.c c.sub.v /k, the Brinkman number, Br=.mu.V.sub.c.sup.2 /(k.DELTA.T), and the mass transfer Peclet number, Pe.sub.m =V.sub.c.delta..sub.y /D. The quantities, V.sub.c, .rho..sub.c, .delta..sub.y, .delta..sub.x and .DELTA.T are the characteristic velocity, density, length scales (in x- and y-directions) and temperature difference, respectively. The fluid properties, .mu., c.sub.v, k and D are the molecular viscosity, specific heat at constant volume, thermal conductivity and the mass diffusivity, respectively. Boundary conditions must be specified in order to close the system of equations. The temperature of the argon at the inlet to the duct was set to ambient (295.degree. K.) and the inlet velocity was given various values to obtain a range of inlet flow rates. The velocity was set to zero and the temperature set to ambient at all solid walls. A range of values was considered for the pressure at the outlet in the lower chamber, p.sub.w. The pressure at the upper chamber outlets was set to a low value (0.0075 torr). This pressure is actually expected to vary depending on the type of vacuum pumps used and the flow rate entering the upper chamber. The pressure in the upper chamber is expected to be sufficiently small to result in a choked flow condition (Mach number equal to one) at the aperture in the partition. The mass flow rate into the upper chamber is independent of the pressure there when the flow is choked, so the exact value is not important. The results for the flow rate and mass fraction of hydrocarbon entering the upper chamber obtained from the calculations were used along with a candidate value for the vacuum pump throughput to obtain the hydrocarbon partial pressure in the upper chamber, as described below. The wafer out gassing rate was conservatively specified to be 10.sup.14 molecules/(second.multidot.cm.sup.2). This out gassing rate is expected to be greater than that which will occur in EUVL. The hydrocarbon was assumed to be methane, and argon was used for the gas curtain in most of the calculations. The properties used are .mu.=2.281 e.sup.-5 Kg/(m s) C.sub.v =319 J/(Kg K), k=0.018 W/(mK) and D=3.283 Kg/m s). The calculations were carried out to a steady state. A standard, commercially available simulation software was used for all the calculations. The vacuum pump throughput is given by the product of the volumetric flow rate of gas through the pump and the pressure at the pump inlet. The throughput is approximately constant for many turbo-molecular pumps over a wide range of inlet pressures. Given the mass flow rate of gas entering the upper chamber from the gas curtain, m.sub.0, and given the pump throughput, Q, Equation 5 gives the pressure at the pump inlet, p.sub.0, where R is the gas constant for the gas mixture and T is the gas temperature assumed to be equal to the ambient value (295.degree. K.). In a preferred embodiment, two turbo-molecular pumps are positioned in the upper chamber to provide a total value of Q of 6000 torr.multidot.1/sec. The pressure in the upper chamber was assumed to be uniform and equal to p.sub.0. Equation 6 is used to obtain the hydrocarbon mole fraction, x.sub.0, from the hydrocarbon mass fraction, y.sub.0, of the flow entering the upper chamber, where W.sub.A and W.sub.HC are the molecular weights of argon and the hydrocarbon (methane), respectively. The hydrocarbon partial pressure in the upper chamber, p.sub.HC, is given by the product of p.sub.0 and x.sub.0. EQU p.sub.0 =RTm.sub.0 /Q 5  ##EQU1## FIG. 4 shows simulated streamlines in the vacuum enclosure which indicate the flow path of the argon/hydrocarbon gas mixture 80. As is apparent, some of the gas 82 flows into the upper chamber and exits through the upper outlets while the remainder of the gas 81 exits through the lower outlet. The gas flowing into the upper chamber is nearly pure argon, i.e., most of the hydrocarbon flows out through the lower outlet as intended. The gas flowing into the upper chamber is driven by a localized region of high pressure which develops in the rather confined region between the wafer and the partition. The Knudsen number of the flow over the wafer, which is given by the ratio of the molecular mean free path and the gap distance between the wafer and partition, ranged from 0.02 to 0.04 in the calculations carried out here which are very close to the continuum regime so the use of the continuum equations should result in only a small error. The flow field shown in FIG. 4 has important implications regarding particle transport. For calculations based on empirical data for the drag coefficient on particles in low pressure gases, particles are likely to follow the gas flow even at the low pressures considered here. That is, the flow field shown in FIG. 2 may be effective in sweeping particles out of the vacuum enclosure. The distribution for the hydrocarbon mass fraction in the vicinity of the wafer is shown in FIG. 5. These results show that the argon gas, which is flowing from left to right, is effective in sweeping the hydrocarbon gas to the right and out of the vacuum enclosure. A region of gas with a larger concentration of hydrocarbon (shown as the dark region(s)) develops between the argon inlet duct and the wafer on the left side. The hydrocarbon is primarily restricted to a region along the surface of the wafer. Note that this hydrocarbon gas is prevented from entering the upper chamber by the partition. Results for the hydrocarbon partial pressure in the upper chamber, p.sub.HC, as a function of the argon flow rate in the gas curtain (the corresponding value of the mass transfer Peclet number is also shown) are shown in FIG. 6 for h.sub.i =1.4 cm. p.sub.HC decreases with increasing argon flow rate, and a value of 10.sup.-10 torr for p.sub.HC (our target value) can be obtained with an argon flow rate of 12.2 torr.multidot.1/sec. Although only preferred embodiments of the invention are specifically disclosed and described above, it will be appreciated that many modifications and variations of the present invention are possible in light of the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.