Patent Publication Number: US-2006003271-A1

Title: Basic supercritical solutions for quenching and developing photoresists

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to the field of photolithography to form integrated circuits and more particularly to the field of developing an irradiated photoresist.  
      2. Discussion of Related Art  
      Photolithography is used in the field of integrated circuit processing to form the patterns that will make up the features of an integrated circuit. A photoresist is employed as a sacrificial layer to transfer a pattern to the underlying substrate. This pattern may be used a template for etching or implanting the substrate. Patterns are typically created in the photoresist by exposing the photoresist to radiation through a mask. The radiation may be visible light, mid ultraviolet (G-line, I-line), deep ultraviolet (248 nm, 193 nm), extreme ultraviolet (EUV) light, or an electron beam. In the case of a “direct write” electron beam, a mask is not necessary because the features may be drawn directly into the photoresist. Most photolithography is done using either the “i-line” method (non-chemically amplified) or the chemical amplification (CA) method. In the i-line method, the photoresist becomes directly soluble when irradiated and may be removed by a developer. In the chemical amplification method the radiation applied to the photoresist causes the photo-acid generator (PAG) to generate a small amount of a photo-generated acid throughout the resist. The acid in turn causes a cascade of chemical reactions either instantly or in a post-exposure bake. In a positive tone photoresist the photo-generated acid will deprotect the compounds used to form the photoresist to make the photoresist soluble. If a PEB (Post-exposure bake) is not used the developer will serve to stop the acid from causing further reactions. In either situation, there is typically a time lag in between the initiation of the reactions by the photo-generated acid and the quenching of the acid by the developer. As illustrated in  FIG. 1 a , during this time lag the photo-generated acid in an irradiated region  1   10  of the photoresist  120  may diffuse into the regions  130  of the photoresist  120  that were not irradiated and cause a reaction in the regions  140 . The width of the opening  150  formed by developing the photoresist  120  will be greater than desired due to the migration of the photo-generated acid during the lag time into the regions  140  of the non-irradiated portion  130  of the photoresist  120 . The migration of the photo-generated acid into the non-irradiated portion  130  of the photoresist  120  may cause line roughness and loss of control of the critical dimensions of the features patterned by the photoresist. A chill plate may be used to minimize the migration of the photo-generated acid after a post-exposure bake. But, as the critical dimensions of the structures formed by photolithography become smaller, and particularly as the technology passes into the 45 nanometer node, a chill plate may no longer provide the control of the acid migration necessary to achieve the critical dimensions in this node.  
      The photoresist may be removed by a developer after the photoresist is deprotected by the photo-generated acid. The deprotection by the photo-generated acid increases the solubility of the resist so that it may be removed by a basic developer.  FIG. 1   b  illustrates a basic developer  160  that has been applied to a photoresist  120  to develop the irradiated portion  110 . An organic aqueous base such as tetramethylammonium hydroxide (TMAH) may be used as the developer  160  to remove the photoresist from the irradiated areas. But, as the technology moves to the 45 nanometer node, the dimensions of the structures patterned by a photoresist mask will become so narrow that the traditional aqueous base developer may not be able to access the narrow features with high aspect ratios of 2 or higher and may fail to fully develop the irradiated portions of the photoresist.  FIG. 1   b  illustrates the incomplete development of a photoresist  120  by the developer  160  by the area  170  of the irradiated portion  110  that was not accessed by the developer  160 . Additionally, even when the developer  160  is able to fully access the irradiated area  110  of the photoresist  120  the developer  160  may cause line collapse due to the high surface tension of the aqueous developer  160 , also as illustrated in  FIG. 1   b.  The aqueous base developers therefore also affect critical dimension control. Another drawback to using aqueous base developers is that copious amounts of the aqueous developer and water rinses to remove the aqueous developer are used, thus creating a large amount of waste.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIGS. 1   a  and  1   b  are illustrations of a cross-sectional view of prior art processes of quenching and developing a photoresist.  
       FIGS. 2   a - 2   k  are illustrations of a process of forming vias within an integrated circuit employing a basic supercritical solution as a quencher and as a developer.  
    
    
     DETAILED DESCRIPTION OF THE PRESENT INVENTION  
      Described herein are compositions formulated with at least one supercritical fluid to quench and develop a photoresist and methods of using these compositions. In the following description numerous specific details are set forth. One of ordinary skill in the art, however, will appreciate that these specific details are not necessary to practice embodiments of the invention. While certain exemplary embodiments of the invention are described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative and not restrictive of the current invention, and that this invention is not restricted to the specific constructions and arrangements shown and described because modifications may occur to those ordinarily skilled in the art. In other instances, well known semiconductor fabrication processes, techniques, materials, equipment, etc., have not been set forth in particular detail in order to not unnecessarily obscure embodiments of the present invention.  
      A basic supercritical solution may be used to quench a photo-generated acid within a photoresist as well as develop the photoresist. The basic supercritical solution may be a combination of a supercritical fluid and a base or a supercritical base. A supercritical fluid is a state of equilibrium between a liquid and a gas, that is above the critical temperature (T c ) and critical Pressure (P c ) A basic supercritical solution formulated to include at least one supercritical fluid has a low viscosity and surface tension and is capable of penetrating narrow features having high aspect ratios and the photoresist material due to the gas-like nature of the supercritical fluid.  
      A basic supercritical solution may be used to quench and develop photoresists that are applied to various substrates to create patterns for the formation of many structures used in integrated circuits. In one embodiment, a photoresist developed by a basic supercritical solution may be used to form lines for transistor gates. In another embodiment, a photoresist developed by a basic supercritical solution may be used to form trenches or vias for interconnect lines. In one embodiment the patterned photoresist may be used to form both vias and trenches by a conventional dual damascene method. Other applications for forming microelectromechanical machines (MEMS), microfluidics structures, or other small structures are also comprehended. For the sake of simplicity a process of forming only vias will be described.  
      In  FIG. 2   a,  substrate  200  is provided. Substrate  200  may be any surface generated when making an integrated circuit upon which a conductive layer may be formed. In this particular embodiment the substrate  200  may be a semiconductor such as silicon, germanium, gallium arsenide, silicon-on-insulator or silicon on sapphire. A dielectric layer  210  is formed on top of substrate  200 . Dielectric layer  210  may be an inorganic material such as silicon dioxide or carbon doped oxide (CDO) or a polymeric low dielectric constant material such as poly(norbornene) such as those sold under the tradename UNITY™, distributed by Promerus, LLC; polyarylene-based dielectrics such as those sold under the tradenames “SiLK™” and “GX-3™”, distributed by Dow chemical Corporation and Honeywell Corporation, respectively; and poly(aryl ether)-based materials such as that sold under the tradename “FLARE™”, distributed by Honeywell Corporation. The dielectric layer  210  may have a thickness in the approximate range of 2,000 and 20,000 angstroms.  
      In  FIG. 2   b,  after forming the dielectric layer  210 , a bottom anti-reflective coating (BARC)  215  may be formed over the dielectric layer  210 . In embodiments where non-light lithography radiation is used a BARC  215  may not be necessary. The BARC  215  is formed from an anti-reflective material that includes a radiation absorbing additive, typically in the form of a dye. The BARC  215  may serve to minimize or eliminate any coherent light from re-entering the photoresist  220 , which is formed over the BARC  215  in  FIG. 2   c,  during irradiation and patterning of the photoresist  220 . The BARC  215  may be formed of a base material and an absorbant dye or pigment. In one embodiment, the base material may be an organic material, such as a polymer, capable of being patterned by etching or by irradiation and developing, like a photoresist. In another embodiment, the BARC  215  base material may be an inorganic material such as silicon dioxide, silicon nitride, and silicon oxynitride. The dye may be an organic or inorganic dye that absorbs light that is used during the exposure step of the photolithographic process.  
      In  FIG. 2   c  a photoresist  220  containing a photoacid generator (PAG) is formed over the BARC  215 . The photoresist  220  may be positive tone or negative tone. In a positive tone photoresist the area exposed to the radiation will define the area where the photoresist will be removed. In a negative tone photoresist the area that is not exposed to the radiation will define the area where the photoresist will be removed. The photoresist  220 , in this particular embodiment, is a positive resist. The photoresist  220  may have a thickness sufficient to serve as a mask during an etching or implantation step. For example, the photoresist may have a thickness in the approximate range of 500 angstroms and 2500 angstroms. In general, for implant purposes the photoresist will be thickest, for contact patterning the photoresist will be thinner than for implant purposes, and the photoresist will be thinnest for gate patterning. The photoresist  220  may contain a PAG, resins, a quencher, and additives.  
      As illustrated in  FIG. 2   d,  a mask  230  is formed over the photoresist  220 . In  FIG. 2   e,  the photoresist  220  and the BARC  215  are patterned by exposing the masked layer to radiation. The radiation may be broad band exposure, 365 nm, 248 nm, 193 nm, 157 nm, extreme ultraviolet (EUV), electron beam projection, electron beam scalpel, or ion beam lithographic technologies. In one particular embodiment, the irradiation used to pattern the photoresist  220  may be EUV having a wavelength of 13.5 nm. Upon irradiation, the photo-acid generator (PAG) will receive the energy from the radiation and generate the photo-generated acid that may serve as a catalyst to deprotect and to change the solubility of the resins. The change in the solubility of the resin is to enable the solvation of the resins and the removal of a positive photoresist by a developer. In a negative tone photoresist active species will catalyze the cross-linking of the resins and the developer that is subsequently applied will remove the portions of the negative tone photoresist that were not cross-linked. A post-exposure bake (PEB) may be performed on the photoresist  220  to enhance the mobility and hence the diffusion of the photo-generated acid within the photoresist  220 . The post-exposure bake may be performed at a temperature in the approximate range of 90° C. and 150° C. and for a time sufficient for the reaction to occur, which may be in the approximate range of 30 seconds and 90 seconds. The temperature and the time of the post-exposure bake are dependent on the chemistry of the photoresist  220 . The PEB may be performed in a processing chamber that is equipped to also create or maintain supercritical solutions. Alternatively, after the PEB, the substrate on which the photoresist  220  is formed may be removed from the PEB chamber and moved to a chamber equipped to create or maintain supercritical solutions.  
      As illustrated in  FIG. 2   e,  a basic supercritical solution  235  may be applied to the photoresist  220  immediately after the PEB to quench the migration of the photo-generated acid. There could be no delay between the PEB and the application of the developer or the time lag may be up to 5 minutes. In one embodiment, the basic supercritical solution  235  is applied to the photoresist  220  by combining the elements of the basic supercritical solution in situ in the reaction chamber containing the substrate on which the photoresist  220  is formed and placing the elements under pressure and temperature conditions sufficient to create a basic supercritical solution  235 . For example, to form supercritical carbon dioxide at a temperature of 31° C. the pressure is brought up to 1072 psi. In an alternate embodiment, the basic supercritical solution  235  is applied to the photoresist  220  by first injecting the compound that will be made supercritical into the chamber and applying the necessary temperature and pressure conditions to the compound to make it supercritical. Secondly, if additional components are to be added to the basic supercritical solution  235 , those components will be injected into the chamber and mixed with the supercritical compound. The substrate  200  on which the photoresist  220  is formed may be placed into the chamber either before or after the basic supercritical solution  235  is formed and mixed.  
      The basic supercritical solution  235  may be formulated in two general ways. The basic supercritical solution  235  may be formulated to include a base 1) that is separate from the supercritical fluid or 2) that is the supercritical fluid. In the first embodiment, where the basic supercritical solution  235  is formulated to include a supercritical fluid and a base, the supercritical fluid may be a non-basic compound such as supercritical carbon dioxide (SCCO 2 ), sulfur oxide (SCSO 2 ), supercritical SF 6 , chlorofluorocarbons (CFC), or hydrochlorofluorocarbons (HCFC) compounds. The supercritical fluid in this embodiment may be a single supercritical fluid or a combination of supercritical fluids. A combination of supercritical fluids may be used to adjust polarity or base strength of the solution. The base may be ammonia (NH 3 ), an amine such as diethylamide, an amide, a urethane, a quarternary ammonium salt such as TMAH (tetramethylammonium hydroxide) or an acid salt of carboxylic acid such as potassium carbonate, potassium acetate, ammonium acetate. The size of the base may be small, such as NH 4 , or a larger molecule such as an oligomer. The base may also be a side group on a surfactant, oligomer, or a polymer. The amount of base in the developer solution may be in the approximate range of an amount greater than zero and up to 20% of the developer solution. If the supercritical fluid and the base react, the solution may still act as a quencher and a developer. The solution may also contain a co-solvent such as methanol, ethanol, acetone, methyl ethyl ketone, dimethyl formamide, sulfolane, and NMP (N-methyl-2-pyrrolidone). The co-solvent may be up to 20% of the basic supercritical solution. The solution may also contain an additive such as a copper corrosion inhibitor or a surfactant. The surfactant may be in the approximate range of 0.1% and 3% of the basic supercritical solution. The amount of supercritical fluid in the solution will be the balance of the solution, in the approximate range of 50% and 99% of the solution. All of the components of the solution are suspended in the supercritical fluid.  
      In the embodiment where the basic supercritical solution  235  is a base and a supercritical fluid, the base may be an ion and therefore may not be soluble in the supercritical fluid. For example, the base may be TMAR (tetramethylammonium hydroxide). When the base is an insoluble ion, the basic supercritical solution  235  is likely to contain a co-solvent and a surfactant to stabilize the insoluble ion, such as TMAH. In such a formulation the co-solvent may be up to 20% of the solution and the surfactant may be up to 5% of the solution and more particularly in the approximate range of 1%-2% of the solution. A basic supercritical solution  235  containing an insoluble basic compound may be changed from a homogeneous solution to a heterogeneous emulsion with a change in temperature and pressure. By changing the solution from a single phase solution to a two phase emulsion solution, the emulsion may be encouraged to deposit on the substrate and to subsequently lift off of the substrate upon another change in temperature and pressure to change the solution back to a single phase. Depositing the emulsion on the substrate may be valuable to force the chemistry to interact with the resist surface on the substrate.  
      In the embodiment where the basic supercritical solution  235  may be a supercritical base, the bases that may be made supercritical include NH 3 , CH 3 NH 2 , (CH 3 ) 2 NH, and (CH 3 ) 3 N. These bases are made supercritical by applying a particular combination of pressure and temperature that will bring the base above the critical points where there is minimal distinction between a liquid and a gas. For example, supercritical NH 3  (SCNH 3 ) is formed by a pressure of 113 Bar and 133 C. In this embodiment, the basic supercritical solution  235  may be one or a combination of different supercritical bases. By using a combination of supercritical bases the basic, nucleophilic, and protic properties of the basic supercritical solution  235  may be modified for use with different photoresist compositions. For example, polymeric resist molecules would have better solubility in a basic supercritical solution  235  having high polarity. Non-basic supercritical fluids, such as supercritical carbon dioxide, may also be combined with the basic supercritical fluid to control the concentration of the base. Supercritical bases are valuable because they can have high concentrations of base and the polarity range of the solution is tunable.  
      When a basic supercritical solution  235  is applied to the photoresist, the irradiated regions  225  of the photoresist  220  that were irradiated may be solvated by the solution. Additionally, because the basic supercritical solution  235  has gas-like properties, it may permeate the photoresist  220  as illustrated in  FIG. 2e  and quench the photo-generated acid to prevent the diffusion of the photo-generated acid into regions of the photoresist that were not irradiated and are not desired to be deprotected. The quenching of the photo-generated acid may be performed separately from the developing of the photoresist with the basic supercritical solution  235 , or the quenching and the developing may be performed consecutively. If the quenching is performed separate from the developing of the photoresist, the basic supercritical solution  235  that would be used to quench the photo-generated acid may have approximately 10% lower concentration of base than the basic supercritical solution that would be used as the developing solution. The quenching solution may contain a different base than the developing solution due to the delay time between quenching and developing and other processing concerns. The basic supercritical solution  235  therefore has the ability to quench the acid almost immediately upon application and thus prevent line edge roughness and loss of CD control.  
      Additionally, because the basic supercritical solution  235  has a surface tension that is much lower than the surface tension of water, the developing solution will not cause the photoresist walls to collapse. For example, the surface tension of water at 25 degrees celsius is 75 dyne/cm and the surface tension of supercritical carbon dioxide at 25 degrees celsius is 1 dyne/cm. The gas-like properties of the solution and the low surface tension of the solution also may penetrate high aspect ratio openings in the photoresist. In one embodiment, the pattern formed in the photoresist by irradiation may create narrow features having high aspect ratios in the range of a ratio of height to width of 2:1-5:1. If MEMS are being formed, the aspect ratios may be in the range of 5:1-20:1.  
      The basic supercritical developing solution  235  may be applied to the substrate for a time sufficient to develop and remove the photoresist  220  from the irradiated portions  225  of the photoresist  220 , as illustrated in  FIG. 2   f.  The basic supercritical developing solution may then be removed from the chamber by changing the pressure and temperature conditions to change the solution into a gas that may be evacuated from the process chamber. To minimize emissions, the gas may be captured and recycled. After the basic supercritical solution  235  is removed from the photoresist  220  as illustrated in  FIG. 2   g,  the photoresist  220  and the dielectric layer  210  do not need to be rinsed because the basic supercritical solution will lift off and diffuse out of the photoresist  220  once the pressure is altered to change the basic supercritical solution  235  into a gaseous solution. The substrate  200  may then be moved to an etching chamber where the exposed portions of the dielectric material  210  underlying the photoresist  220  may be etched to form the intended features. Rinse and vent process schemes can be employed to remove the base and other additives from the process chamber, where applicable rinse materials may be a pure stream of supercritical CO 2 .  
      After the photoresist  220  is developed and removed, vias  240  are etched through dielectric layer  210  down to substrate  200 , as illustrated in  FIG. 2h . Conventional process steps for etching through a dielectric layer  210  may be used to etch the via, e.g., a conventional anisotropic dry etch process. When silicon dioxide is used to form dielectric layer  210 , the vias  240  may be etched using a medium density magnetically enhanced reactive ion etching system (“MERIE” system) using fluorocarbon chemistry. When a polymer is used to form dielectric layer  210 , a forming gas chemistry, e.g., one including nitrogen and either hydrogen or oxygen, may be used to etch the polymer. After vias  240  are formed through dielectric layer  210 , the photoresist  220  and the BARC  215  are removed. Photoresist  220  and BARC  215  may be removed using a conventional ashing procedure as illustrated in  FIG. 2   i.    
      A barrier layer  250  is then formed over the vias  240  and the dielectric  210  as illustrated in  FIG. 2   j.  The barrier layer  250  may comprise a refractory material, such as titanium nitride and may have a thickness in the approximate range of 100 and 500 angstroms. The barrier layer may be deposited by chemical vapor deposition (CVD), sputter deposition, or atomic layer deposition (ALD). The purpose of the barrier layer  250  is to prevent metals such as copper that expand at temperatures used in semiconductor processing from bleeding out of the vias and causing shorts. A metal layer  260  is then deposited into the vias  240 . The metal layer may be copper, copper alloy, gold, or silver. In one particular embodiment copper is deposited to form the metal layer  260 . Copper may be deposited by electroplating or electroless (catalytic) deposition that require first depositing a seed material in the vias  240 . Suitable seed materials for the deposition of copper by electroplating or electroless deposition include copper and nickel. The barrier layer  250  may also serve as the seed layer.  
       FIG. 2   k  illustrates the structure that results after filling vias  240  with a conductive material. Although the embodiment illustrated in  FIG. 2   k  illustrates only one dielectric layer  210  and vias  240 , the process described above may be repeated to form additional conductive and insulating layers until the desired integrated circuit is produced.  
      Several embodiments have thus been described. However, those of ordinary skill in the art will recognize that the embodiments are not, but can be practiced with modification and alteration within the scope and spirit of the appended claims that follow.