Abstract:
A patterned layer over a wafer is produced by depositing a print-patterned mask structure. Energized particles of a target material are deposited over the wafer and the print-patterned mask such that particles of said target material incident on the mask structure enter the mask structure body and minimally accumulate, if at all, on the surface of the mask structure, and otherwise the particles of target material accumulate as a generally uniform layer over the wafer. The print-patterned mask structure, including particles of target material therein, is removed leaving the generally uniform layer of target material as a patterned layer over the wafer.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention is related to methods of forming a fine-featured printed layer such as in semiconductor device manufacturing, and more specifically to methods and apparatus employing a print-patterned phase-change material for region masking during material deposition. 
         [0003]    2. Description of the Prior Art 
         [0004]    There are today many well known processes for selective material deposition in wafer processing, such as in the manufacture of patterned layers in semiconductor wafer processing. One such process of interest herein is referred to as a liftoff process. In a typical liftoff process, a resist structure is formed over a region of a wafer in order to block the deposition of material in that region. The material of interest is then deposited over at least portions of the wafer, including over the resist structure. The resist structure is then dissolved for example by a solvent, removing both the resist structure and the material of interested deposited thereover. In this way, a definition of a pattern on the wafer surface may be obtained without etching. Since the liftoff process is an alternative to the more common photolithographic etching processes, the liftoff process is often used to define geometry of materials which are difficult to etch, such as gold. 
         [0005]    One requirement for a liftoff process is that in the process of forming the resist structure, means for introducing the solvent underneath the deposited material of interest must be provided so that the solvent may dissolve the underlying resist structure. This typically requires that the resist structure be taller (i.e., thicker) than the thickness of the deposited layer of material of interest. Furthermore, the resist structure is typically patterned during or after deposition so that the solvent may contact as much of the resist structure as possible and so that it dissolves the resist structure as quickly as possible, for example by providing the resist structure with re-entrant sidewalls. An example of a structure used in this process is illustrated in  FIGS. 7A and 7B . With reference to  FIG. 7A , substrate  50  has formed thereon a resist structure  54 , patterned to have reentrant sidewalls, and a target material layer  56  formed thereover such that a first portion  56   a  overlies resist structure  54  and a second portion  56   b  directly overlies the substrate  52  (or alternatively, intermediate layers, not shown). A solvent may be introduced into regions  58  due to the reentrant sidewall profile of resist structure  54  to thereby dissolve and remove resist structure  54  and with it remove portion  56   a  of layer  56 . The device following the liftoff step is shown in  FIG. 7B . 
         [0006]    While liftoff is an effective process for wafer patterning, the process has several limitations. First, the resist structure must be formed to be significantly taller than the target material layer, or conversely the thickness of the target layer must be made thin relative to that of the resist structure. Second, the resist structure must be patterned during or after deposition so as to have a reentrant sidewall profile. Each of these limitations result in a relatively high cost and complexity of this wafer patterning process. Furthermore, there is a limit to the width of a useful resist structure and hence to the width of the masked region. If the mask structure is too wide, the solvent takes a significant time to fully undercut the structure, resulting in unwanted damage to other portions of the structure by the solvent. Thus, there is a need in the art for a process which provides a patterned wafer without requiring etching and without limitations on thickness of a target material layer or width of the masked region. 
       SUMMARY OF THE INVENTION 
       [0007]    Accordingly, the present invention is directed to systems and methods for producing a patterned wafer which do not require that the masking structure be significantly taller than the target layer of material which is to be patterned. Additionally, the resist sidewall profile does not need to be re-entrant. Furthermore, the present invention does not require the patterning of the mask structure following deposition. In addition, the present invention does not limit the width of the region to be masked. 
         [0008]    In addition, the removal of the mask structure is dependent only on the thickness of the structure, and not its areal dimensions. The top of the mask structure is exposed, as opposed to being covered by a layer of target material. Therefore, undercutting is not required, and the mask can be attacked from top. Thus, the thickness of the mask will determine rate of removed. Removal of the mask structure according to the present invention is therefore less complex and less expensive as compared to prior art mask structures. Therefore, the present invention overcomes a number of the limitations of the previously-described liftoff process. 
         [0009]    According to one aspect of the present invention, a print-patterned mask structure is formed over a substrate. The mask structure may be formed directly on the substrate or on an intermediate layer formed over the substrate. The mask structure may be formed by depositing individual droplets of a phase-change material, such as a wax, using an ink-jet type print head. A target material is then deposited over the mask structure and layer the mask structure is formed on (e.g., the substrate). According to one embodiment, the target material is deposited with sufficient energy, for example kinetic energy, that particles of the material incident on mask structure actually enter the body of the mask structure as opposed to building up as a layer over the surface of the mask structure. Other than over the mask structure, the target material builds up as a uniform layer. The layer of target material is therefore discontinuous in the region of the mask structure. The mask structure with embedded target material may then be removed by a solvent, etchant, and/or heating, leaving the region previously occupied by the mask structure open and free of target material. 
         [0010]    According to another aspect of the present invention, the target material may be heated such that its thermal energy permits particles of the material in the region of the mask structure to enter the body of the mask structure. Alternatively, or in addition, the mask structure may be heated to facilitate the introduction of the target material during deposition. 
         [0011]    The above is a summary of a number of the unique aspects, features, and advantages of the present invention. However, this summary is not exhaustive. Thus, these and other aspects, features, and advantages of the present invention will become more apparent from the following detailed description and the appended drawings, when considered in light of the claims provided herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    In the drawings appended hereto like reference numerals denote like elements between the various drawings. While illustrative, the drawings are not drawn to scale. In the drawings: 
           [0013]      FIG. 1  is an illustration of a system for the deposition of a print-patterned mask material as might be employed in one step of the present invention. 
           [0014]      FIG. 2  is an illustration of a wafer in the process of having a mask structure formed thereon according to one embodiment of the present invention. 
           [0015]      FIG. 3  is an illustration of a wafer having a mask structure formed thereon and in the process of having a target material applied thereto according to one embodiment of the present invention. 
           [0016]      FIG. 4  is an illustration of an arc spraying apparatus which might be employed in one step of the present invention. 
           [0017]      FIG. 5  is an illustration of a wafer having a layer of target material formed thereover and having a mask structure with incorporated target material embedded therein removed from the wafer according to a step of the present invention. 
           [0018]      FIG. 6  is a process flow diagram illustrating various steps according to the present invention, and in which steps in dashed lines indicate optional steps. 
           [0019]      FIGS. 7A and 7B  are illustrations of a wafer being patterned, and a patterned wafer, respectively, according to a liftoff process well known in the art. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    In the following detailed description of the present invention, digital lithography is employed to form a print-patterned mask. Digital lithography is a process for directly depositing material in desired patterns onto a substrate, taking the place of the delicate and time-consuming photolithography processes used in conventional wafer fabrication. Digital lithography typically uses a printer head to controllably eject individual droplets from a reservoir to form a patterned layer over regions of a substrate. The droplets are commonly of a phase change material. One application of digital lithography is the deposition of material in a printed pattern designed to act as a mask (thus referred to herein as a “print-patterned mask”). Of course, it is to be understood that other printing systems may be used to form the mask, as it will become apparent from the following description that the material forming the mask forms a more critical aspect of the present invention than the method of its deposition. 
         [0021]    A system suitable for implementing the embodiments of the method set forth below is described in U.S. Pat. No. 6,972,261, Wong, et al., the disclosure of which is incorporated herein by reference. More specifically, with reference to  FIG. 1 , there is shown therein the relevant components of a system  10 , which includes a heat source  12  that heats a reservoir  14  typically containing a phase-change material. The phase-change material is thereby heated to a temperature that is sufficient to maintain the material in a liquid state. The temperature of the reservoir is generally maintained above 50 degree centigrade and, in some situations, at temperatures above 100 degrees centigrade, a temperature sufficient to liquefy many organic materials that are in the solid phase near room temperature. 
         [0022]    The phase-change material may be an organic material that melts at temperatures higher than room temperature. Other desirable characteristics of the phase-change material include that the patterning material is non-reactive with organic and inorganic materials which are or will be deposited on the wafer and used in such deposition, and that the phase change material has a high selectivity to etchants and particularly solvents. In one embodiment, the phase-change material dissolves in a basic solution (i.e., is “base-strippable”), although in other embodiments other characteristics of the solvent and/or apparatus may be employed to remove (if desired) the phase-change material. 
         [0023]    Wax is an example of a phase-change material with the previously described characteristics. Kemamide 180-based waxes from Crompton Corporation of Middlebury, Conn., are but one example of a suitable wax for use as a phase-change patterning material. 
         [0024]    Returning to  FIG. 1 , in this system, one or more droplet sources such as droplet source  16  receives the liquid phase-change material from reservoir  14  and outputs droplets  18  for deposition on a substrate  20 . The substrate  20  may be maintained at a temperature such that the droplet cools rapidly after deposition. 
         [0025]    When increased coalescence between adjacent droplets is required, such as in the formation of masked areas having dimensions great that the dimensions of a single droplet (roughly 30-40 micrometers in diameter), the substrate temperature can be increased to increase droplet spreading and thereby increase coalescence. When printing lines of Kemamide-based wax from an acoustic ink-jet printer, it has been found that increasing the substrate temperature from 30 degrees to 40 degrees centigrade improves the print quality of the pattern. In the case of Kemamide-based waxes, it has been found that excellent results are achieved when the surface is maintained at 40 degrees centigrade, which is about 20 degrees centigrade below the solid phase point of the wax. At 40 degrees centigrade, the temperature of the substrate is still low enough that the droplet rapidly solidifies upon contacting substrate  20 . 
         [0026]    After a droplet of phase-change material is deposited on substrate  20 , the relative positions of substrate  20  and droplet source  16  are adjusted to reposition droplet source  16  over a second position to be patterned. The repositioning operation may be achieved either by moving droplet source  16  or by moving substrate  20 . As shown in  FIG. 1 , a control circuit  22  moves droplet source  16  in a predetermined pattern over substrate  20 . A driver circuit  24  provides energy to droplet source  16 , causing ejection of droplets when the droplet source  16  is positioned over a region of substrate  20  to be patterned. By coordinating the movement of droplet source  16  with the timing of droplet source outputs, a pattern can be “printed” on substrate  20 . 
         [0027]    The presently described method ultimately is used to produce patterned wafers and other structures. As such, it is desired to form printed mask features in specific positions on the surface of substrate  20 . Positional registration of the deposition of droplets  18  forming a print-patterned mask is routinely accomplished in digital lithographic systems by use of fiduciary marks, digital imaging and processing, and processor controlled relative motion of the droplet source and the substrate. The ability to align the formation of a mask over substrate  20  through image processing prior to and while patterning is a significant advantage of the digital-lithographic process over other masking methods. 
         [0028]    In order to control and align the movement of droplet source  16 , printed fiduciary alignment marks, such as mark  26 , may be applied or formed on a surface of the layer upon which the phase-change material is to be applied. Alternatively, the fiduciary marks may be on a carrier holding substrate  20  during the deposition process (not shown). An image processing system such as a camera  28  may be used to coordinate the orientations of the droplets and the surface on which they are applied. A processing system then adjusts for the position of the pattern layer by altering the pattern image file before actual printing of the pattern layer. Positioning adjustment are accomplished in software and translated to movements of the droplet source  16 . 
         [0029]    Each droplet source may be implemented using a variety of technologies including traditional ink-jet technology. An alternative technology well suited for generating extremely small droplet sizes is the use of sound waves to cause ejection of droplets of patterning material as done in acoustic ink printing systems, as described in, for example, U.S. Pat. No. 6,972,261, Wong et al. Examples of such systems appropriate for the ejection of droplets of phase-change material include: ink-jet systems (such as disclosed in U.S. Pat. No. 4,131,899, which is incorporated herein by reference), ballistic aerosol marking (BAM) devices (such as disclosed in U.S. Pat. No. 6,116,718, which is incorporated herein by reference), acoustic ink printer (AIP) systems (U.S. Pat. No. 4,959,674, which is incorporated herein by reference), carrier-jet ejectors (as disclosed in U.S. Pat. No. 5,958,122, which is incorporated by reference herein), deflection-controlled ink-jet systems (such as disclosed in U.S. Pat. No. 3,958,252, which is incorporated herein by reference), etc. Such systems also include pattern transfer systems, such as: xerographic, ionographic, screen, contact, and gravure printing systems, etc. 
         [0030]    Described next are specific steps for the formation of a print-patterned mask, and the production of a patterned wafer formed with said mask.  FIGS. 2 through 5  illustrate a first embodiment of a device at several intermediate stages of its production according to a process illustrated in  FIG. 6 . While the following description makes specific reference to the device illustrated in  FIGS. 2 through 5 , without making more specific reference thereto the description is following the sequence illustrated in  FIG. 6 . 
         [0031]    With reference to  FIG. 2 , the process of forming a mask on a substrate for the production of a patterned wafer is illustrated. Droplet  18  ejected from droplet source  16  impacts the surface of substrate  30  to form a mask feature  32 . While the present description assumes that droplet  18  is deposited directly onto the surface of substrate  30 , it will be understood that it is within the scope of the present description and invention that droplet  18  may also be deposited onto an intermediate layer (not shown) formed on or over the surface of substrate  30 . While the dimensions of mask feature  32  may vary depending on the volume of material ejected from droplet source  16 , the material comprising droplet  18 , the nature of the surface of substrate  30  (affecting the wetting of the droplet), the temperature of substrate  30 , etc., typically mask feature  32  will have a diameter of 30-50 microns. A wider mask feature may be obtained by coalescing adjacent droplets, as previously described. Mask feature  32 , together with a plurality of similar such features (not shown), forms a print-patterned mask. Substrate  30  together with the print-patterned mask (and any intermediate layer or layers) form structure  34 . 
         [0032]    A target material is next deposited over structure  34 . The print-patterned mask formed of feature  32 , together with a plurality of similar such features, represents the regions over substrate  30  which will be free of such deposited material. Deposition of the target material is illustrated in  FIG. 3 . The target material can be one of a wide variety of materials, elemental or alloyed. One specific example of target material  36  is aluminum (Al). While the actual target material may vary, the method and nature of its deposition forms an important aspect of the present invention. 
         [0033]    According to a first embodiment of the present invention, the target material is deposited by an arc spraying process. An apparatus  40  for arc spraying is illustrated in  FIG. 4 . In the arc spraying process a pair of electrically conductive wires  42   a ,  42   b  are melted by means of an electric arc at  44 . The molten material is atomized by compressed air and propelled towards the surface of substrate  30 . The energetic molten particles impact the substrate and solidify thereon to form a coating. 
         [0034]    Returning to  FIG. 3 , atomized particles  36  of the target layer material are directed toward structure  34 . Two different results are simultaneously obtained. First, in regions where the surface of substrate  30  is exposed (i.e., other than where mask feature  32  is located) the atomized particles accumulate to form layer  38  of the target material. Second, particles  36  are caused to be sufficiently energetic by the arc spraying process that in the region of mask feature  32 , the particles actually enter the body of material forming mask feature  32 . If the majority of such particles are sufficiently energetic, very few if any of the particles accumulate on the surface of mask feature  32 , but rather become embedded therein. In this way, layer  38  is actually discontinuous in the region of mask feature  32 . 
         [0035]    It will be understood that while it is possible that a small portion of the target material particles do settle on the surface of mask feature  32 , the number of such particles will be relatively very small, and the thickness of the layer of such particles over mask feature  32  will be relatively very thin. Such a thin layer is easily separated from the relatively much thicker layer  38  of target material (often of its own accord, for example by cooling-induced contraction), effectively rendering layer  38  discontinuous. 
         [0036]    The energy required for target material particles  36  to enter the body of mask feature  32  may take one or more forms. Particles  36  may, for example, have sufficient kinetic energy to enter feature  32 . For example, the propellant for the arc spraying process transfers kinetic energy in the form of the momentum of the particles in the direction of feature  32 . Given a sufficient momentum (and permeability of the material forming feature  32 ), particles  36  may embed themselves in feature  32 . 
         [0037]    Particles  36  may also have sufficient thermal energy to enter feature  32 . For example, in the process of atomizing the conductive wire material, the electric arc heats said material. While the particles may lose some of this thermal energy to the environment, they may retain sufficient thermal energy to cause a local softening or even melting of the material forming feature  32  such that they become embedded therein. 
         [0038]    Of course, particles  36  may have a combination of kinetic and thermal energy sufficient to result in their entry into feature  32 . For example, the arc spraying process typically provides both momentum and heat to the particles as they atomize from the conductive wires. Thus, in this embodiment it is the combination of kinetic and thermal energy which results in the introduction of particles  36  into the body of feature  32 . 
         [0039]    While the foregoing has focused on imparting energy to particles  36 , it is also possible to lower the energy required of those particles by affecting attributes of feature  32 . For example, in one embodiment of the present invention feature  32  is heated such that it softens and reduces the energy required of particles  32  to become embedded therein. Such heat may be provided by heating substrate  30 , by raising the ambient temperature, etc. Furthermore, the material from which mask feature  32  is formed may be selected such that it is relatively soft, or permeable, in order to minimize the energy required of particles  36 . For example, in some implementations, the arc spray process may be operated as a “cold” process, meaning that the atomized particles have lost the majority of the thermal energy from being atomized by the time they reach the substrate. In such a case, heating of feature  32  and/or selecting a relatively soft material for feature  32 , will assist with the introduction of particles  32  therein. 
         [0040]    While the foregoing has focused on the arc spraying process, it will be appreciated that many other deposition techniques are compatible with the present invention. Physical vapor deposition (PVD), laser- and arc-assisted PVD, sputtering, and molecular beam epitaxy (MBE) are examples of physical processes that may provide sufficient kinetic or thermal energy, or both, to result in target material atoms entering the body of mask feature  32  as opposed to forming a layer thereover. Chemical vapor deposition (CVD) and plasma enhanced CVD (PECVD) are example of chemical processes that may provide sufficient chemical energy to result in target material atoms entering the body of mask feature  32  as opposed to forming a layer thereover. 
         [0041]    With reference next to  FIG. 5 , the method of the present invention proceeds with the step of removing mask feature  32  and the target material embedded therein. The process and material used for this removal step depend in large part upon the material selected for the mask feature  32 . In some embodiments, the process and material used also depend on the target material incorporated into mask feature  32  during the deposition of same. According to one embodiment, feature  32  is comprised of the aforementioned Kemamide wax. A solvent such as tetrahydrofuran or other solvent known to the art is applied to the structure, or alternatively, the structure is dipped in a bath of such solvent, which may also be heated, to remove the mask structure. Importantly, with the removal of the mask structure comes removal of the target material embedded therein. The solvent and removal conditions should be such that the deposited target material remaining after removal of the mask structure is not damaged thereby. As the target material layer  38  is discontinuous at the region of mask feature  32 , removal of the mask feature  32  does not damage layer  38  in the process. 
         [0042]    The complete process  60  for the formation of a patterned wafer according to the present invention is illustrated in  FIG. 6 . According to process  60 , following any necessary cleaning and preparation of the substrate at step  62 , a print-patterned mask is deposition at step  64 . At this point, particles of target material which shall form the patterned layer on the substrate are introduced into the vicinity of the substrate at step  66 . These particles may be introduced with the desired energy to enable to be embedded into the material forming the print-patterned mask, or at optional step  68  they may be energized as needed. Furthermore, at optional step  70  the substrate may be treated (e.g., heated) to lower the required energy required of the particles. At step  72  the particles are caused to accumulate on the substrate and to be embedded within the print-patterned mask. Finally, at step  74 , the print-patterned mask is stripped, taking with it target material particles embedded therein, leaving the accumulated target material as a patterned layer on the substrate. 
         [0043]    While a plurality of preferred exemplary embodiments have been presented in the foregoing detailed description, it should be understood that a vast number of variations exist, and these preferred exemplary embodiments are merely representative examples, and are not intended to limit the scope, applicability or configuration of the invention in any way. For example, the forgoing description has focused on the formation of a patterned layer directly on the surface of a substrate. However, the process according to the present invention may be performed over any layer or layers formed over the substrate. Furthermore, one or more layer of material may be deposited over a layer patterned according to the present invention. Indeed, those subsequent layers may themselves be patterned by the process of the present invention. Therefore, the foregoing detailed description provides those of ordinary skill in the art with a convenient guide for implementation of the invention, by way of examples, and contemplates that various changes in the functions and arrangements of the described embodiments may be made without departing from the spirit and scope of the invention defined by the claims thereto.