Patent Number: 
Section: description

The following is a detailed description of the presently preferred embodiments of the present invention. However, the present invention is in no way intended to be limited to the embodiments discussed below or shown in the drawings. Rather, the description and the drawings are merely illustrative of the presently preferred embodiments of the invention. The present invention is a photolithography optical system designed for use with extreme ultraviolet (EUV) radiation. FIG. 3 schematically depicts the exemplary inventive apparatus for semiconductor EUV lithography. The apparatus comprises a radiation source 301 that emits EUV radiation 303. The EUV radiation 303 may be processed by a condenser 305 to produce an EUV beam 307 to uniformly illuminate a portion of a mask 309. The radiation reflected from the mask 309 produces a patterned EUV beam 311, which is introduced into an optical system 313. The optical system 313 projects a reduced image 315 of the mask 309 onto a wafer 317. EUV radiation has a wavelength (xcex) between about 4 to 20 nm and may be produced by any suitable means including a laser produced plasma, synchrotron radiation, electric discharge sources, high-harmonic generation with femto-second laser pulses, discharge-pumped x-ray lasers, and electron-beam driven radiation devices. The sources may be continuous or pulsed. Laser-produced plasma (LPP) sources focus an intense pulsed laser beam onto a target. Suitable targets are metals and noble gases. Targets of noble gas molecule clusters in a supersonicjet produce a bright xe2x80x9csparkxe2x80x9d with a broad emission spectrum ranging from visible light to EUV radiation. High-repetition-rate (3,000-6,000 Hz) pulsed laser drivers deliver 1,500 W of focused power to the target regions. A LPP gas source converts the incident laser power into EUV light in the required spectral bandwidth. Condenser optics typically collect EUV radiation from the LPP source and condition the radiation to uniformly illuminate the mask. The condenser illuminates a narrow ring field at the mask with the EUV radiation, where the illumination must have a spatial nonuniformity of less than 1% in the cross scan dimension. The condenser further directs the EUV beam into the entrance pupil of the optical system with a partial coherence of approximately 0.7. Separate collection channels each act in concert to direct radiation across the entire ring field and the optical system entrance pupil. Since EUV radiation is absorbed by all materials, reflective optical elements rather than refractive elements are best suited for EUV optical systems. The inventive optical system comprises four reflective optical elements (mirrors) listed in order from mask to wafer: M1, M2, M3, and M4. The optical system is placed in a vacuum or other suitable atmosphere. In the lithographic process, the EUV radiation is collected and illuminates a mask, producing an image that can be projected to the wafer. The object end of the inventive optical system departs enough from the telecentric condition so that the light rays incident upon the reflective mask have sufficient clearance to prevent vignetting or clipping by mirror edges. Referring to FIG. 4, there is shown an exemplary optical system for EUV semiconductor lithography. The optical elements are all arranged in a coaxial configuration such that the vertex of each surface of revolution lies on a common centerline 414. Only off-axis sections of each mirror are used. Because this is a ring field optical configuration, only off-axis sections of the parent mirrors are utilized as shown in FIG. 4. Thus, the off-axis section of the first optical element (M1) 405, the off-axis section of the second optical element (M2) 409, the off-axis section of the third optical element (M3) 413, and the off-axis section of the four optical element (M4) 421 are exposed to EUV radiation. The use of off-axis sections, which are smaller in area than the full aspheric parent mirrors, facilitates the deposition uniform reflective multilayer coatings. The ability to achieve the required uniform layer thickness is enhanced as the reflective area of the mirror is decreased. EUV Beam 1403 diverges from a reflective mask 401 onto concave aspheric mirror M1405. Beam 2407 is reflected from mirror M1405 in a divergent cone to a convex aspheric mirror M2409. Beam 3411 is reflected from mirror M2409 in a divergent cone to a concave aspheric mirror M3413. Beam 4415 is reflected from mirror M3413 in a convergent cone back to convex aspheric mirror M2409. At this location, the aperture stop 412 is place on the mechanical centerline 414 of the optical system. Beam 5419 is reflected from mirror M2409 in a divergent cone to a concave aspheric mirror M4421. Beam 6423 is reflected from mirror M4421 in a convergent cone forming a reduced image of the mask 401 pattern onto a wafer 425. The projected EUV aerial image enables a chemical reaction in a photoresist layer on the wafer 425 forming the latent image in the photoresist. This latent image is then subsequently processed by well-known means to form the patterned wafer. Concave mirrors have positive optical power and convex mirrors have negative optical power. Using this convention, the optical power configuration of the inventive system from object to image (including the double bounce from mirror M2409 ) can be described as: positive, negative, positive, negative and positive, corresponding to mirrors M1405, M2409 (first bounce), M3413, M4409 (second bounce), and M4421, respectively. This configuration of alternating positive and negative optical powers allows the optical system to achieve a Petzval sum that is approximately zero, while enabling correction of both astigmatism and distortion. Since the focal length of the inventive optical system can be scaled to accommodate a variety of packaging concepts, it is useful to describe the inventive optical system relative to this quantity. In the preferred embodiment, the absolute radii of the mirrors M1405, M2409, M3413, and M4417, relative to the system focal length, are listed in Table 2. Referring to Table 3, the relative positions of the mirrors M1405, M2409 (first bounce), M3413, M2409 (second bounce), and M5421 for the preferred embodiment are listed. For a 4-to-1 reduction, the distance of the mask 401 to M1405 is 388.845 mm. EUV multilayers are constructed using alternating layers of two materials with different optical properties. These materials need to have low intrinsic absorption at EUV wavelengths and provide an optical impedance mismatch at the layer interfaces so that reflected waves can be generated. Common material pairs with desirable reflectance characteristics include molybdenum/silicon (Mo/Si) for wavelengths near 13.4 nm and molybdenum/beryllium (Mo/Be) for wavelengths near 11.3 nm. Since the optical impedance between these material pairs is low, many layer pairs are required to achieve a useful reflectance. The multilayer mirror may be designed for specific radiation wavelengths and incidence angles. As the reflectance of the multilayer stack is maximized with the addition of multilayer pairs, the reflectance of the multilayer mirrors may be affected by variations in the radiation wavelength and angle of incidence. The multilayer mirror may only have a narrow wavelength bandwidth that produces maximum reflectance. This narrow spectral bandwidth means that the multilayer reflectance is, for a fixed angle of incidence, sensitive to shifts in the incident radiation wavelength. The multilayer mirror reflectance may decrease if the incident radiation wavelength deviates outside the mirror""s maximum reflectance bandwidth. In an EUV optical system, the multilayer mirror reflectance may also be affected by the radiation incidence angle. Although multilayer mirrors may be designed to maximize reflectance for a specific incidence angle, this maximum reflectance may only exist over a narrow range proximate to the design incidence angle. A multilayer mirror optimized for perpendicular radiation may produce maximum reflectance over a wider range of incidence angles than a mirror specifically designed for more acute incidence angles. The reflectance can decrease significantly when the incidence angle deviates outside the multilayer""s angular bandwidth. The ability of mirrors to equally reflect radiation over a range of incidence angles can be important because rays passing through an optical system may not impinge upon mirrors at the same angle across the beam width. The projected beam of an EUV optical system converges and expands, the individual rays of the beam do not travel in parallel paths. Maximum reflectivity over a wider range of ray angles can be maintained by configuring all mirrors so that the mean incidence angles are close to perpendicular. An embodiment of the present invention utilizes a mirror system configuration having near perpendicular or low mean incidence angles. Low mean incidence angles at each mirror ensures that the optical system transmission, which is described by the formula Tsys=R1xc3x97R2xc3x97 . . . xc3x97Ri, where Ri represents the reflectivity of the ith mirror, will be maximized. Low mean incidence angles also help to ensure that multilayer amplitude and phase effects as measured in the exit pupil of the projection system have little or no impact on imaging performance. These amplitude and phase effects can substantially alter the partially coherent imaging characteristics of the system, thus limiting the ability to control the CD across the field format. Table 4 shows the mean incidence angles of each mirror surface for a preferred embodiment of the present invention. Multilayer coatings that have either a uniform or graded thickness can be designed and applied to each of the mirror surfaces to maximize the EUV transmission of the inventive five bounce system. The maximum EUV transmission of the inventive optical system may be greater than 17% if the reflectance of the mirrors is greater than 70%, which is close to the maximum theoretical reflectance of a Mo/Si multilayer mirror. Table 5 shows the maximum aspheric departure from a best-fit spherical surface centered on the off-axis section of each mirror of the preferred embodiment. The table has two entries for mirror M2, one corresponding to each reflection surface of M2, since there are two instantaneous clear apertures on this surface. The inventive optical system is designed using low aspheric departures across the off-axis section of the parent to facilitate mirror metrology using visible wavelengths. The off-axis sections of the present projection system can be designed so that the aspheric departures are small relative to a visible wavelength. Mirrors having small aspheric departures can be tested at their centers of curvature without the need for null optics that adversely impact the absolute accuracy of metrology testing. In another embodiment, the inventive optical system has a physically accessible, real aperture stop on mirror M4. The physical aperture stop ensures that imaging bundles from each field point within the ring field are not clipped or vignetted and are formed in the small manner. The physical aperture stop also makes the projected imagery, setting aside the effects of the field dependent aberrations and variations in illumination from the condenser across the ring field, independent of position within the ring field. The aerial images from different field points in the ring field will look the same and variations in projected feature size will be minimized. This type of projected imagery is known as xe2x80x9cstationary imageryxe2x80x9d. Tables 7, 8, and 9 contain constructional data and other relevant information for the currently preferred configuration of mirrors M1, M2, M3, and M4. The inventive as a 4:1 reduction ratio, a numerical aperture of 0.15, and a 0.5 mm ring is capable of 50 nm resolution and depth of focus of approximately 0.6 xcexcm. Referring to Table 7, parameters describing the mirror surfaces of the preferred embodiment are listed. Note that surface 2 and surface 4 describe the first and second bounces from mirror M2, respectively. The radius of curvature refers to the radius of curvature of each optical element, and the thickness refers to the vertex-to-vertex thickness between the optical surfaces. For example, the thickness of the object is 388,8452 mm and represents distance from the mask to the vertex of mirror M1. Referring to Table 8, the aspheric parameters A(1)-A(4) for the optical elements M1, M2, M3, and M4 are set forth for a preferred embodiment. The aspheric profile of each mirror is uniquely determined by its K, A, B, C, and D values. The sag of the aspheric surface (through 10th order) parallel to the z-axis (z) is a function of radial coordinate (h) given by Equation (1) wherein h is the radial coordinate, is the curvature of the surface (1/R), and A, B, C, and D are the 4th, 6th, 8th, and 10th order deformation coefficients, respectively. Mirrors M1, M3, and M4 are all oblate spheroids with 6th, 8th, and 10th order polynomial deformations. The reflective surfaces of M2 are oblate spheroids with 6th and 8th horder polynomial deformations.   z  =                    ch        2                    1        +                              1            -                                          (                                  1                  +                  k                                )                            ⁢                              c                2                            ⁢                              h                2                                                          +          Ah      4        +          Bh      6        +          Ch      8        +          Dh      10       Table 9 gives first order data for the preferred embodiment of the inventive optical system. Another advantage of the present invention is that the centroid distortion is balanced across the ring field width. As shown in FIG. 5, xe2x80x9cbalancedxe2x80x9d means that the variation in static distortion across the width of the ring field is quadratic. In scanning lithography, the entire wafer field is covered by synchronously scanning both the mask and wafer across the projected ring field. Excessive or unbalanced static distortion can cause the time-averaged printed image to be blurred or smeared along a field-dependent trajectory. This leads to image properties that are field invariant in the scan direction, but which vary in the perpendicular or cross-scan dimension. By creating a static distortion field that varies quadratically across the ring field (i.e., xe2x80x9cbalancingxe2x80x9d the static distortion), both the blurring (smearing) and placement (dynamic distortion) are minimized. Table 10 shows the performance of the system as described by the root mean square (RMS) wavefront error and corresponding Strehl ratio. Table 11 shows the deviation (distortion) of the image centroid at the wafer from its ideal location. Since the inventive optical projection system has an odd number of reflections, the mask and wafer are located of the same side of the imaging system. This introduces a limitation on the wafer travel. In the preferred configuration, the separation of the mask and wafer in the scan plane is 263.75 mm. The skilled artisan will readily appreciate that the entire optical system can be scaled by a constant greater than 1.0 to increase the separation between the mask and wafer. For example, if the inventive optical system were scaled by a factor of 1.5xc3x97, the mask to wafer separation would be almost 400 mm. When the optical system is scaled, the incidence angles remain the same and the compatibility of the design with multilayer coatings is unaffected. However, the distortion, wavefront error measured in waves, and the mirror asphericity increases proportionally to the scale factor. The limits imposed by mirror fabrication technology and the associated mirror metrology may limit the scale factor that can be used to increase the mask to wafer separation. While the present invention has been described in terms of a preferred embodiment, those skilled in the art will readily appreciate that numerous modifications, substitutions and additions may be made to the disclosed embodiment without departing from the spirit and scope of the present invention. For example, although an optical system has been described above for use with a semiconductor photolithography system, those skilled in the art will readily appreciate that the inventive optical system may be utilized in any similar lithography device and that the present invention is in no way limited to the mechanisms described above. Similarly, the skilled artisan will readily appreciate that the optical system shown in FIG. 4 is in no way limited to use with a particular type of lithography system or a particular lithography machine. Those skilled in the art will also readily appreciate that the optical system may be used with any similar lithography mechanism. It is intended that all such modifications, substitutions and additions fall within the scope of the present invention, which is best defined by the claims below.