Self-aligned alignment marks for phase-shifting masks

Abstract: A phase-shifting optical lithographic mask has a set of phase shifting features (11) and a set of alignment marks (13), all having a common thickness and being made of a common material, such as chromium oxynitride, that is partially transparent to optical radiation used in an optical lithographic system. Both of these sets are located on a slab of quartz (10). An alignment shutter (12) layer laterally intervenes between the alignment marks and the phase-shifting features, in order to suppress optical radiation leakage from the phase-shifting features to the alignment areas. A portion of the top surface of the alignment shutter layer and the entire top surface of the reinforced alignment marks are reinforced by an opaque layer (32, 33), such as chrome. In addition, another similarly reinforced layer, a chip shutter layer (14, 34), can be laterally located at an extremity of the reinforced alignment marks, in order to suppress optical radiation leakage from one chip area to another in step-and-repeat lithography.

Description: Background of the Invention This invention relates to optical lithography, such as that used for fabricating semiconductor integrated circuit and other devices. More particularly, the invention relates to optical lithographic masks and to methods of making such masks, for use in optical lithographic systems for fabricating such devices. These masks are also called "reticles" particularly when they are used in optical systems having magnifications different from unity. In fabricating integrated circuits, it is desirable, for example, to have as many transistors per semiconductor wafer as possible. Hence, it is desirable to make transistor sizes as small as possible. Similarly it is desirable to make as small as possible any other feature size, such as the feature size of a metallization stripe--i.e., its width--or of an aperture in an insulating layer that is to be filled with metal, for example, in order to form electrical connections between one level of metallization and another. According to geometric optics, if it is desired to print on a photoresist layer, which is located on the semiconductor wafer in an optical lithographical system, the corresponding feature having a width equal to W, a feature having a width equal to C must be located on the mask (reticle). Further, according to geometric optics if this feature of width equal to C is a simple aperture in an opaque layer, then the ratio W/C = m, where m is known as the "lateral magnification". When diffraction effects become important, however, instead of a sharp black-white image a diffraction pattern of the object feature C is formed on the photoresist layer, whereby the edges of the image become indistinct. Consequently, the resolution of the features of the reticle, as focused on the photoresist layer and transferred to the workpiece, deteriorates. In prior art this diffraction problem has been alleviated by such techniques as using phase-shifting portions ("phase-shifting features") on the mask. Therefore the mask is then known as a "phase-shifting mask" --and hereinafter the mask will therefore likewise be called "the phase-shifting mask". These phase-shifting features impart a phase shift ϕ to the optical beam emanating from the optical source, with ϕ typically equal to approximately π (=180°). Phase-shifting features can be either opaque (i.e., with an optical intensity transmission coefficient, T, approximately equal to 0) or partially transparent (i.e., with the value of T advantageously being in the approximate range of 0.05 to 0.15, typically T being approximately equal to 0.10)--all as measured with respect to the optical radiation of wavelength λ. Thus, in a phase-shifting mask, the features such as that shown in the form of a simple aperture C in the mask will become more complicated than just simple apertures and will include the above-mentioned phase-shifting features. Typically, for use in the optical lithographical system, the phase-shifting mask has two basic areas: (a) feature areas, and (b) alignment areas. That is to say, the phase-shifting mask has the following basic areas: (a) feature areas whereby the images formed by them on the photoresist layer correspond to device features of the semiconductor wafer or other workpiece (such as selected areas of the workpiece where impurity ions are implanted or selected areas of the workpiece where portions of the workpiece are removed), and (b) workpiece fabrication alignment areas thereof whereby the images formed on the photoresist layer correspond to workpiece fabrication alignment marks. As known in the art, in a step-and repeat movement procedure for forming images on the photoresist layer, the workpiece is subdivided into chip ("die") subregions: Each subregion typically is defined and encompassed by one resulting step-and-repeat position of the workpiece (overlain by the photoresist layer ). Each corresponding subregion of the photoresist layer is successively exposed to the optical beam in the system. In order to align the system for the step-and-repeat process, step-and-repeat alignment marks are required on the mask in addition to the workpiece alignment fabrication marks. As further known in the art, in an optical lithographical system an unwanted diffraction of optical radiation causes an undesirable leakage of optical radiation from one area on the photoresist layer to another. During a step-and-repeat procedure, this leakage will expose areas of the photoresist layer overlying adjacent chip subregions of the workpiece to an undesirable optical radiation background if the adjacent chip subregions are located in close proximity. Therefore this leakage causes undesirable deterioration of the sharpness of definition of features of adjacent chip subregions. In addition to this (step-and-repeat) leakage, an unwanted optical radiation leakage from the feature areas to the alignment areas on the photoresist layer undesirably can reduce the sharpness of definition (contrast ratio) of the images of the alignment marks on the photoresist layer ; hence, this leakage can cause a deterioration of the sharpness of definition of the positions of the features ultimately formed in the workpiece. To minimize these optical radiation backgrounds--instead of increasing the distance between chips (as defined by the step-and-repeat procedure) or the distance between feature areas and alignment areas, whereby precious feature area on the workpiece would be sacrificed-in prior art an opaque shutter layer is introduced on the mask between the alignment areas and the boundaries of the chip subregions, as well as between the feature areas and the alignment areas. In the interest of economy of processing, workers in prior art formed these shutter layer simultaneously with forming the opaque layers (T = 0) of the phase-shifting areas. However, in cases where the phase-shifting features are partially transparent (i.e., in cases where T ≠ 0), such a method would produce alignment marks and shutter layers that would likewise be partially transparent, whereby the alignment of the photoresist layer during the step-and-repeat procedure as well as during exposure to the optical beam in the system would be compromised, and an undesirable amount of leakage would persist. Summary of the Invention This invention involves a phase-shifting mask that can be used in the optical lithographical system. The inventive mask has partially transparent phase-shifting features together with alignment marks that are reinforced (rendered opaque) and are self-aligned with respect to the phase-shifting features. The thus self-aligned reinforced alignment marks are either workpiece fabrication alignment marks, step-and-repeat alignment marks, or both. The optical radiation leakage is reduced by virtue of an opaque shutter layer located between feature areas and alignment areas of the mask. The mask can also have a reinforced (opaque) shutter layer that prevents optical radiation leakage from the feature areas to the alignment areas on the photoresist layer 101. In some embodiments the mask further has a reinforced shutter layer to prevent leakage of optical radiation from one chip area to another. The invention also involves a method for making such masks. In a specific embodiment, a phase-shifting mask in accordance with the invention comprises: (a) a substrate that is transparent to optical radiation of wavelength λ; features in the form of partially transparent, patterned layer elements 11 ("mask imaging features 11"); a second region comprising a partially transparent, localized layer element 12 ("alignment mark shutter 12"); a third region comprising partially transparent, patterned layer elements 13 ("alignment marks 13"); and a fourth region comprising a partially transparent, localized layer element 14 ("chip shutter"). These four regions are coated with a photoresist layer 15, typically AZ5206 supplied by Hoechst Celanese Corporation, whose address is Electronics Products Division, 3070 Highway 22 West, Somerville, NJ 08876. As further indicated in FIG 1, a direct writing electron beam e - is directed on the resist layer 15 at regions thereof overlying only the first region and an adjacent portion of the second region. Thus the photoresist layer 15 is bombarded by the electron beam e - at all regions thereof overlying the patterned mask imaging features 11 and an adjacent portion of the mask shutter 12, but not in any region overlying any of the alignment marks 13 or the chip shutter 14. Either before, after, or simultaneously with the bombardment of the photoresist layer 15 with the electron beam e - , the bottom surface of the substrate 10 is irradiated with a flood ultraviolet beam UV 1 , whereby the photoresist layer 15 is exposed to the ultraviolet beam UV 1 in those regions thereof and only those regions thereof that overlie the lateral spaces between adjacent ones of the partially transparent elements 11, 12, 13, and 14--i.e., not in those regions that overlie any of the partially transparent elements 11, 12, 13, or 14. Typically the wavelength λ 1 of the ultraviolet beam UV 1 is in the approximate range of 350-to-450 nm. The photoresist layer 15 is then baked at a sufficient temperature and for a sufficient time to cross-link the regions thereof that have been exposed to the UV 1 beam, to the e- beam, or to both. Thus (FIG. 2) the regions 31 but not the regions 21 of the photoresist layer become cross-linked. Next (FIG. 3), the top surface of the photoresist layer is irradiated with another ultraviolet beam UV 2 , whereby the non-cross-linked regions 21 of the photoresist layer--but not the cross-linked regions 31 thereof--become soluble in a standard developing solution such as a solution of TMAH (tetramethyl ammonium hydroxide) in water with a normality in the approximate range of 0.1 to 0.5. Typically, the wavelength(s) of the beam UV 2 is (are) essentially the same as that (those) of the beam UV 1 ; that is to say, the two beams UV 1 and UV 2 have essentially the same spectral content. A suitable developing solution removes the regions 21 of the photoresist, whereby only the regions 31 of the photoresist remain intact--as indicated in FIG. 4. In this way, corresponding areas of the partially transparent elements 12, 13, and 14 (but not the mask imaging features 11) become exposed. Then (FIG 5) a layer 21, 25 of chromium ("chrome"; Cr) or chromium oxide is sputtered onto the top surface of the remaining regions 31 of the photoresist layer, and at the same time a layer of chromium or chromium oxide is sputtered 22, 23, 24 on the exposed areas of the elements 12, 13, and 14, respectively. Advantageously, the thickness of this layer of chromium or chromium oxide is very thin, typically approximately 50 nm. The composition of this layer is important only to the extent that it is sufficiently opaque and that it adheres well to the underlying layers 12 and 13. It is preferable that this layer of chromium or chromium oxide be thinner than approximately one-third the quantity (H - h), where H represents the height of the photoresist layer 31, and h represents the common thickness of the partially transparent elements 11, 12, 13, and 14. Next (FIG. 6) the layer 21, 25 of chromium or chromium oxide is removed by a lift-off process, i.e., by a process of dissolving the remaining regions 31 of the photoresist layer means of a resist stripping solution that attacks the remaining cross-linked regions 31 of the photoresist layer. Typically this stripping solution is a ketone, acetone, or a methyl ethyl ketone. During this lift-off process, the chromium or chromium oxide layer 22, 23, 24 is not removed: i.e., the portion of the chromium or chromium oxide that overlies the elements 12, 13, and 14, respectively, remains intact as reinforcing layers 32, 33, and 34, respectively. Thus this remaining portion of chromium or chromium oxide reinforces the alignment mark shutter 12, the alignment marks 13, and the chip shutter 14, whereby there are formed the following items: A reinforced alignment mark shutter layer 12,32; reinforced alignment marks 13,33; and a reinforced chip shutter 14,34. The reinforced alignment mark shutter 12, 32 is self-aligned with respect to the mask imaging features 11 because the patterned layer 12 was defined and fabricated simultaneously with these mask imaging features 11. To achieve this self alignment, the edge of the electron beam e - (FIG. 1) located overlying the alignment mark shutter 12 obviously need not be precisely aligned with the edge of the alignment mark shutter 12 itself: it is sufficient that the edge of the beam e - falls somewhere on this alignment mark shutter 12, preferably at a location thereon remote from the alignment marks 13. Similarly, the reinforced alignment marks 13,33 are self-aligned with respect to the reinforced chip shutter 14,34. The thickness of the partially transparent layers 11, 12, 13, and 14 is illustratively in the approximate range of 20 nm to 35 nm, typically approximately 32 nm. The thickness of the chromium or chromium oxide layers 21, 22, 23, and 24 illustratively is in the approximate range of 40 nm to 60 nm, typically approximately 50 nm. The width of each of the mask imaging features 11 is typically in the approximate range of 1 µm to 10 µm . The width of the alignment shutter 12 can be anywhere from approximately 2 µm to 5 cm. The widths of the alignment marks 13 can be in the approximate range of 5 µm to 50 µm, depending on such factors as the stepper apparatus to be used in the system. The width of the chip shutter 14 is typically larger than approximately 0.5 cm. Although the invention has been described in detail in terms of a specific embodiment, various modifications can be made without departing from the scope of the invention. Instead of chromium oxynitride, the layers 11,12, 13, and 14 can be essentially a thickness of molybdenum silicide-oxide that is partially transparent to the optical radiation of wavelength λ, in which case the layers 21, 22, 23, 24, and 25 can be essentially chrome or chromium oxide having a sufficient thickness to be opaque to the radiation λ. In any event, the layers 22, 23, and 24 (and hence the layers 32, 33, and 34) need not of themselves be opaque to the optical radiation of wavelength λ: only the respective combinations of the layers 32, 33, and 34 with the layers 12, 13, and 14, respectively, need be opaque thereto.

Claims: A method of making a phase shifting lithographic mask having a semitransparent region and an opaque region comprising the steps of: (a) providing a transparent substrate on a surface of which is located a patterned first layer of first material having a first thickness and that is partially transparent to optical radiation of a wavelength λ that is the lithographic exposure wavelength, the patterned first layer having respective portions located within at least first and second areas of the surface; (b) forming a photoresist layer of a second thickness over the patterned first layer, wherein the second thickness is greater than the first thickness; (c) bombarding the photoresist layer with an electron beam at all regions thereof overlying the first area, at only a portion of the region overlying the second area, and at a region of the photoresist layer intervening between the first area and the bombarded region overlying the second area; (d) directing an optical beam everywhere on the bottom surface of the photoresist layer before, during, or after step (c) wherein the wavelength of the optical beam is not λ and wherein the patterned first layer is opaque to the optical beam; (e) developing the photoresist layer, whereby only those lateral portions of the photoresist layer are intact after the developing that either (1) were bombarded by the electron beam, or (2) are located in the second area but neither were bombarded by the electron beam nor overlie the patterned layer; (f) depositing a second layer of second material, chemically different from the first material, everywhere on the top surface of the remaining intact portions of the photoresist layer and on all areas of the patterned first layer that are not overlain with the remaining portions of the photoresist layer, the second layer being sufficiently thick so as to be, in combination with the patterned first layer, opaque to the radiation of wavelength λ; and (g) stripping the remaining portions of the photoresist layer, whereby the second layer is removed at all regions thereof overlying the remaining portions of the photoresist layer but is not removed from any regions not overlying the remaining photoresist layer. The method of claim 1 further characterized by, subsequent to step (d) but prior to step (e), the step of baking the photoresist layer. A method of making an optical lithographic mask in accordance with claim 2 characterized by, subsequent to said baking step but prior to step (e), the step of (h) directing another optical beam everywhere on the top surface of the photoresist layer. The method of claim 3 characterized in that the optical beams of steps (d) and (h) have respective spectral contents that are essentially the same. The method of claim 1 characterized in that the first layer is deposited by sputtering chromium, oxygen, and nitrogen. The method of claim 5 characterized in that step (f) includes depositing chromium or chromium oxide, whereby the patterned layer is provided.

Inventors: DEMARCO JOHN JOSEPH, PIERRAT CHRISTOPHE, DEMARCO, JOHN JOSEPH, PIERRAT, CHRISTOPHE
Applicants: AT & T CORP, AT&T CORP.
Application year: 1995
Main classification: G03F1
IPC codes: G03F9, G03F1, H01L21