Abstract:
A series structure of a chemically amplified negative tone photoresist that is not based on cross-linking chemistry is herein described. The photoresist may comprise: a first aromatic structure copolymerized with a cycloolefin, wherein the cycloolefin is functionalized with a di-ol. The photoresist may also include a photo acid generator (PAG). When at least a portion of the negative tone photoresist is exposed to light (EUV or UV radiation), the PAG releases an acid, which reacts with the functionalized di-ol to rearrange into a ketone or aldehyde. Then new ketone or aldehyde is less soluble in developer solution, resulting in a negative tone photoresist.

Description:
FIELD 
     This invention relates to the field of fabricating semiconductor devices and, in particular, to negative tone photoresists. 
     BACKGROUND 
     Modern integrated circuits generally contain several layers of interconnect structures fabricated above a substrate. The substrate may have active devices and/or conductors that are connected by the interconnect structure. As these devices become smaller the need for fine patterning through photolithography has become increasingly more important. 
     To obtain the fine patterns needed for the current generation of devices, KrF (248 nm) and ArF (193 bm) lasers are being used. The dimension of device will continue to scale down, and may require even shorter wavelengths, such as EUV (13.4 nm). To obtain smaller and finer patterns through the use of shorter wavelength light sources, a new generation of photoresists will be required. The design of the next generation of photoresist may be governed by limitation with EUV tools, such as flare. 
     When extreme ultraviolet (EUV) steppers are used to expose photoresist to radiation with a wavelength of 13.4 nm, the mid frequency roughness of the optics will cause flare. Flare is produced when the light source is reflected off the reflective optics and passes through the mask to expose the photoresist material, and will reduce the contrast of the aerial image. Small amounts of flare may be able to be corrected for by calculating the amount of flare that will occur and scaling the mask CDs accordingly; however, for higher amounts of flare, alternative strategies are necessary. One strategy is the use of a negative tone resist, especially for poly layers, because the amount of flare is proportional to the amount of light that passes through the mask. 
     A positive tone photoresist becomes more solulable to a developer solution upon exposure to light, whereas a negative tone photoresist becomes less solulable to a developer solution upon exposure to light. Consequently, when a negative photoresist is used at the poly layer, a dark field mask, instead of a bright field mask, may be used to create a dark field pattern. By using a dark field mask the impact of flare on an underlying layer may be significantly reduced. 
     Current negative tone photoresists utilize a cross-linking mechanism that makes the exposed portion of the photoresist less solulable to the base developer solution. As cross-linking occurs, the molecular weight of the polymer will decrease. However swelling may occur, since the change in solubility is governed by a change in the molecular weight, and the interactions between the resist and the developer are still very favorable. Swelling of the resist structures will prevent correct pattern transfer and affect the resolution. 
     Because of these limitations, there is a need for a negative tone photoresist that does not swell during the developing process, thereby, allowing the use of a dark field mask to reduce the effect of flare in patterning. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example and not intended to be limited by the figures of the accompanying drawings. 
         FIG. 1  illustrates a copolymer structure that may be used as a negative tone resist. 
         FIG. 2  is a cross-sectional elevation view of a photoresist layer that has been deposited on an underlying layer. 
         FIG. 3  is a cross-sectional elevation view of  FIG. 2  after at least a portion of the photoresist layer has been exposed to radiation through a mask. 
         FIG. 4  is the chemical rearrangement that occurs in the exposed portions of the photoresist layer in  FIG. 3 . 
         FIG. 5  is a cross-sectional elevation view of  FIG. 3  after the photoresist layer has been developed in a developer solution. 
         FIG. 6  is a cross-sectional elevation view of  FIG. 5  after trenches have been etched in the underlying layer from  FIG. 6 . 
         FIG. 7  is a cross-sectional elevation view of  FIG. 6  after the remaining photoresist layer has been stripped. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth such as examples of specific developer solutions in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that these specific details need not be employed to practice the present invention. In other instances, well known methods and materials, such as polymerization techniques for fabricating the polymer, spin-coating techniques, chemical amplification strategies, and stripping techniques have not been described in detail in order to avoid unnecessarily obscuring the present invention. 
       FIG. 1  illustrates the chemical structure of copolymer  100  that may be used as the basis of a negative tone photoresist. Copolymer  100  includes first aromatic monomer  105  copolymerized with cycloolefin monomer  110 . First aromatic structure  105  may be any aromatic structure, such as benzene and may have a functional group R1 bonded in any position to aromatic structure  105 . R1 may be altered to change resist properties, such as the adhesion and/or dissolution characteristics of the photoresist. R1 may be a hydrogen atom, an alkyl group, or a hydroxyl group. 
     Cycloolefin  110  may be any cycloolefin, such as a second aromatic structure or a norbonene structure. The chemical structure of cycloolefin  110  may be varied by changing the functional group X. As an example, X may be no atom (i.e. an aromatic structure), a carbon atom (norbornene), an alkyl, an oxygen atom, or a sulfur atom. 
     Cycloolefin  110  may be functionalized with di-ol  115 . Di-ol  115  may be an alkyl group with two hydroxyls groups that are bonded to cycloolefin  110 . 
     In addition, di-ol  115  may have additional functional groups, R2, R3, and R4 bonded to it. The selection of functional groups R2, R3, and R4, will affect the resist properties, such as dissolution, adhesion, and etch resistance. For example, R2, R3, and R4 may be individually selected from any of the following: hydrogen, alkyl, aromatic, or cage groups. As another illustrative example, if a cage is used for either one or all of R2, R3, and R4, the etch resistance of the photoresist material may be increased. 
     It is readily apparent that the photoresist may include other elements and structures. For example the negative tone photoresist may include a photoacid generator (PAG) to facilitate chemical amplification. Chemical amplification is discussed in more detail in reference to  FIGS. 5 and 6 . As illustrative example, the PAG may be an iodonium, sulfonium, or a non-ionic PAG. The PAG may be used to release the necessary acid, such as an H+ acid, when exposed to light. 
     Turning to  FIGS. 2-7 , an illustrative method of how a photoresist comprising copolymer  100  may be used is depicted. As illustrated in  FIG. 2 , a photoresist layer  210  is deposited on an underlying layer, such as underlying layer  215 , in step  205 . Photoresist layer  210  may contain copolymer  100 , illustrated in  FIG. 1 , casting solvents, such as propylene glycol methyl ether acetate (PGMEA), and/or ethyl lactate (EL), and the other necessary components such as PAGs, base quenchers and/or surfactants. Photoresist layer  210  may be deposited by a spin-coating technique onto underlying layer  215 . Underlying layer  215  may be any substrate that is used at different layers in the manufacturing of devices, such as silicon, or polysilicon, and it may contain other structures such as gates, local interconnects, metal layers, or other active/passive device structures or layers. 
     In reference to  FIG. 3 , in step. 305 , exposed portions  515  and  520  of photoresist layer  210  are exposed to radiation, which may be EUV radiation (13.4 nm), but may also be any other wavelength radiation, such as 248 nm or 193 nm. As described later, step  305  may also include baking and other processes to facilitate chemical amplification. Radiation exposure may occur through mask  310  so that only some sections of the photoresist are exposed, such as exposed portions  315  and  320 . Exposed portions  315  and  320  may undergo a pinacol rearrangement, as described in reference to  FIG. 6 . 
     As mentioned above, exposure chemistry in step  305  may occur via a chemical amplification strategy. To increase the sensitivity of the photoresist (i.e. reduce the amount of radiation needed to cause the necessary chemical reaction in the exposed portions  315  and  320 ) the photoresist may be chemically amplified. Chemical amplification occurs when a PAG reacts during exposure to produce an acid catalyst. The acid catalyst then mediates a cascade of reactions as it diffuses through the resist. A post exposure bake (PEB) step may occur after the exposure step to increase the diffusion length of the acid catalyst. 
       FIG. 4  illustrates the chemistry that will occur in the presence of the acid catalyst to change the solubility of the copolymer. A pinacol rearrangement may occur due to the presence of an acid in exposed portions  315  and  320 , shown in  FIG. 3 . The general pinacol rearrangement chemistry is well known and discussed in, “H. Bosshard, M. E. Baumann and G. Schetty, Helv. Chim. Acta., 53, 1271 (1970),” and “T. E. Zalesskaya and I. K. Lavorva, JOC, USSR, 4, 1999 (1968).” Diol  115  reacts with an H+ acid to form a carbonate containing material, such as a ketone or an aldehyde  410  and a water by-product  415 . They H+acid  405  may be regenerated after the pinacol rearrangement, and may continue to diffuse through the resist to mediate further reactions. After rearrangement, functional groups R2, R3, and R4 will be bonded to the resulting ketone or aldehyde  410 . The R3 functional group will be bonded to a different carbon after the reaction, consistent with the chemistry of pinacol rearrangement. 
     Step  305  may also include the generation of water  415 . If the pinacol arrangement deprotection occurs while the resist is still under vacuum in the EUV tool, the water may outgas. Outgassing is in general considered very problematic, because it can contaminate the optics. However early research shows that the presence of water or oxygen in the chamber may actually help to clean EUV optics. However high concentrations of water can cause oxidation of the optics, which will damage the optics. The optics may be protected from oxidation by the use of capping layers that are known in the art. 
     Another method for protecting the optics from oxidation is to design copolymer  100  so that the pinacol rearrangement deprotection has a high activation energy. For reactions with high activation energies, the pinacol rearrangement deprotection will only occur at high temperatures. In this case the acid will be generated under exposure, but the deprotection will not occur until after the wafer has been removed from the exposure toll and is subjected to a PEB. Water that is generated during the PEB may outgas during the PEB, but this should not be problematic since water will not cause damage to the bake tools. The polymer can be designed to have a high deprotection energy for the pinacol rearrangement by selecting the correct functional groups R2, R3, and R4, such as alkyl groups. 
     As shown in  FIG. 5 , the newly generated ketone or aldehyde  410 , shown in  FIG. 4 , may be less soluble in an aqueous base developer solution. In step  505 , the less soluble exposed portions  315  and  320 , when developed in developer solution will remain, while the rest of the photoresist layer  210 , depicted in  FIG. 3 , will be removed by the developer solution. As an illustrative example, 2.38% tetra-methyl ammonium hydroxide (TMAH) may be used as the developer solution. Therefore, a pattern on a mask, such as mask  310  in  FIG. 3 , may be transferred onto underlying layer  305  with the use of the aforementioned negative tone photoresist. 
       FIGS. 6 and 7  illustrate a single example of how this negative tone photoresist may be used. However, it is readily apparent that many other processing steps may be used to transfer the pattern from the photoresist to the substrate, such as ion implantation instead of/in addition to the steps depicted in  FIG. 6  and  FIG. 7 . Turning to  FIG. 6 , in step  605 , common etchants may be used to etch via openings and/or trenches, such as trenches  610 , while exposed portions  315  and  320  remain. Exposed portions  315  and  320  may protect the portions of underlying layer  215  that are beneath exposed portions  315  and  320 . 
     In reference to  FIG. 7 , exposed portions  315  and  320 , shown in  FIG. 6 , may be stripped away leaving underlying layer  215 , with trenches  610 . Common photoresist stripping methods may be used to remove exposed portions  315  and  320 . 
     Therefore, as shown above, a negative tone photoresist that is not based on cross-linking chemistry may be made and used. Negative photoresist may have applications at EUV wavelengths to reduce flare. Negative tone resists will reduce flare, because a dark field mask can be used at the poly layer, which reduces flare compared to a bright field mask. The deprotection chemistry may show improved performance over current negative resists based on cross-linking chemistry because swelling may be lower. When portions of the photoresist are exposed to light, the PAGs may generate acids. These acids may react with the di-ol deprotecting group to form ketone/aldehyde and water. The newly generated ketone or aldehyde is less soluble in a developer solution, such as 2.38% TMAH, than the original diol. Consequently, the photoresist that was not exposed will dissolve in the developer solution, while the exposed portions will not dissolve leaving the exposed portions of the photoresist. 
     In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.