Abstract:
A processing method for use in the fabrication of fabrication of nanoscale electronic, optical, magnetic, biological, and fluidic devices and structures, for filling nanoscale holes and trenches, for planarizing a wafer surface, or for achieving both filling and planarizing of a wafer surface simultaneously. The method has the initial step of depositing a layer of a meltable material on a wafer surface. The material is then pressed using a transparent mold while shining a light pulse through the transparent mold to melt the deposited layer of meltable material. A flow of the molten layer material fills the holes and trenches, and conforms to surface features on the transparent mold. The transparent mold is subsequently removed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application is a continuation-in-part of co-pending U.S. patent application Ser. No. 10/390,406 filed on Mar. 17, 2003, herein incorporated by reference. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
   Not Applicable. 
   BACKGROUND OF THE INVENTION 
   The present disclosure is related generally to methods for the preparation of a wafer surface, and in particular, to methods for the processing of nanostructures, for filling nanoscale holes and trenches, for removing voids in the fillings, and for planarizing a wafer surface 
   Both filling of nanoscale hole and trenches, and the planarizing of wafer surfaces are very important steps in integrated circuit fabrication, as well as in other nanoscale devices manufacturing. Currently, the filling of via holes and trenches is by chemical vapor deposition (CVD), physical depositions (such as sputtering, evaporation), atomic layer deposition (ALD), and chemical fluid deposition (CFD), etc. For example, in the CVD metal plug process for the interconnects of integrated circuits, the via holes and trenches are etched in a dielectric material, and subsequently filled up with a metal. Clearly, any voids left in the via holes or trenches caused by poor step coverage will create a serious problem for the interconnects. The step coverage of tungsten CVD is still limited by the low volatility of the precursor gas WF6 that leads to a low vapor pressure. The result is a mass transport limited deposition rate, and the hole openings receive faster deposition and may be closed before the volume of the holes are completely filled. Another disadvantage of CVD is its relatively high thermal budget. 
   Atomic layer deposition can achieve excellent step coverage in certain conditions. It is a modified form of CVD with gas precursors introduced one at a time and pump/purge in between, so that a film is deposited at the rate of one atomic layer per cycle, with a typical deposition rate of order 0.5 nm/min. Not only is the slow deposition rate a manufacturing issue, but also in ALD, voids may be formed if the via holes or trenches have sidewalls with negative angles. 
   Another candidate for filling future high aspect ratio via holes and trenches is chemical fluid deposition (CFD) which uses supercritical fluids like CO 2  as a carrier for organometallics. As the supercritical fluid CO 2  retains its gas nature and can flow into deep holes, the material deposition rate is limited by chemical reaction rate, giving conformal step-coverage. The drawbacks of CFD include high process pressure on the order of 100 bar, and limited choices of precursors having high solubility in the supercritical fluid. Moreover, like ALD, voids may be formed when via holes or trenches have negative sidewall angles. 
   In wafer planarization a traditional approach is to utilize a chemical mechanical polishing (CMP) method where chemical slurry is deposited on a wafer surface and subsequent polishing removes any non-flatness of the wafer. The process is messy, generating significant chemical and water waste. 
   Therefore, there is still a need for a new method capable of filling nanoscale via holes and trenches having high aspect ratios, regardless of the sidewall angles, and which has a high throughput. Similarly, there is a need for an efficient and clean method for wafer surface planarization. 
   BRIEF SUMMARY OF THE INVENTION 
   Briefly stated, the present disclosure provides methods that fill via holes and trenches having nanoscale openings and high-aspect-ratios, and which will planarize a wafer surface. In the method, a filling material (e.g. metals or Si) is first deposited onto the nanoscale via holes and trenches by a deposition method. The deposition may have poor step coverage and hence fills the holes partially and creates voids; or it may have good coverage and fill the via holes and trenches. In either case, after the deposition a mirror-flat transparent plate (mold) is pressed against the substrate, while radiation (e.g. an excimer laser pulse (XeCl, 308 nm wavelength, 20 ns pulse duration) or pulsed lamp) shines through the mold. The radiation melts the filling material, and the flat mold presses the molten materials into the holes, filling them completely without voids. The flat mold also planarizes the wafer surfaces. The methods has been successfully used to fill a hole array having 100 nm diameter, 500 nm depth (aspect ratio 5 to 1) and 200 nm inter-hole spacing (pitch) with e-beam evaporated silicon and copper. Besides superior step coverage and negligible thermal budget, the techniques of the present disclosure are fast, simple, and do not require a seed layer. Furthermore, the method can planarize the wafer surface in addition to filling the holes. This technique can be extended to other materials important for electronics devices and integrated circuits, and will have many applications in optical, magnetic, biological, and nanofluidic devices. 
   The foregoing features, and advantages set forth in the present disclosure as well as presently preferred embodiments will become more apparent from the reading of the following description in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     In the accompanying drawings which form part of the specification: 
       FIGS. 1A through 1D  illustrate the sequence of the present invention for filling of via holes and trenches having nanoscale opening and high aspect ratio: (a) material deposition, (b) place a flat mold on top, (c) melting the deposited material using a light pulse while pressing the molten material, and (d) the molten material is pressed into the vias and trenches. 
       FIGS. 2A through 2D  illustrate the sequence of the present invention for planarizing a wafer surface: (a) deposition of a material (optional), (b) place a flat mold on top, (c) melting the deposited material using a light pulse while pressing the molten material, and (d) the molten material is planarized. 
       FIGS. 3A and 3B  illustrate scanning electron micrographs of (a) SiO2 nanotrench array with a material (silicon) deposited on the top, before the melting and pressing, and (b) after the melting and pressing, the Si filled the SiO2 trenches completely without voids and the top surface is planarized. 
       FIGS. 4A and 4B  illustrate scanning electron micrographs of (a) SiO2 nano-pillar array with a material (silicon) deposited on the top, before the melting and pressing, and (b) after the melting and pressing, the Si filled the SiO2 pillars and the top surface is planarized, showing all holes were completely filled by α-Si without void. Laser fluence was 1.1 J/cm2. 
       FIGS. 5A and 5B  illustrate scanning electron micrographs of (a) a material (silicon) is deposited on a non-planar surface of SiO2 grating (bright part) before the melting and pressing, and (b) after the melting and pressing, the Si filled the gaps between the SiO2 trenches and the top surface is planarized; and 
       FIG. 6  shows a scanning electron micrograph of a laser-assisted via-hole filling by Cu. The Cu plugs were broken after wafer cut. Laser fluence was 1.1 J/cm2. 
   

   Corresponding reference numerals indicate corresponding parts throughout the several figures of the drawings. It is to be understood that the drawings are for illustrating the concepts set forth in the present disclosure and are not to scale. 
   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The following detailed description illustrates the invention by way of example and not by way of limitation. The description enables one skilled in the art to make and use the present disclosure, and describes several embodiments, adaptations, variations, alternatives, and uses of the present disclosure, including what is presently believed to be the best mode of carrying out the present disclosure. 
   Turning to  FIGS. 1A-1D  and  FIGS. 2A-2D , a method of the present invention is a fabrication process that either fills nanoscale holes or trenches  10  with a filling material  12 , planarizes a wafer surface  14 , or achieves both filling and planarization in the same process. The method first deposit a filling material  12  (e.g. metals or Si) onto the holes and trenches  10  by any conventional deposition method. The deposition may have poor step coverage and hence fills the holes and trenches  10  partially and creates voids  16 ; or a good coverage and fills the holes and trenches  10  completely. In either case, after the deposition a mirror-flat transparent plate (mold)  18  is pressed against the substrate  20 , while a light pulse  22  (e.g. an excimer laser pulse (XeCl, 308 nm wavelength, 20 ns pulse duration) or pulsed lamp) shines through the mold  18 . The light pulse melts the filling material  12 , and the flat mold  18  presses the molten filing materials  12  into the holes and trenches  10 , filling them completely without voids. The flat mold  18  also planarize the wafer surfaces  14 . To facilitate a filling (such as reduce the needed pressure and enhance adhesion) a thin layer of a wetting material or composite may be deposited onto the sidewall of holes and trenches  10 . If only planarization is needed, the deposition process in the above is not necessary, and only the melting and pressing with the mold  18  will be used. In some applications, a uniform etch back will be used to remove certain materials to expose the substrate  20  in certain locations. 
   The structures that the method can be applied to fill include the holes, trenches, curved patterned, and any patterns that can be generated by a patterning method. The lateral size of the patterns may have a broad range from 1 nanometer to 1 centimeter. The vertical size (i.e. depth) of the patterns is in the range of 1 nm to 1 millimeter. 
   The materials  12  that may be used to fill a hole or trench  10  or are planarized include metals, semiconductors, dielectrics, their compounds, mixtures or multilayers, and alike. The metals may include gold, copper, aluminium, platinum, silver, magnetic materials, nickel, iron, alloys and composites and alike. The semiconductors may include the elements and compounds from the group IV (e.g Si, Ge, SiGe, SiC), III-V (e.g. GaAs) and II-VI (e.g. CdS, ZnO) of the periodic table. The dielectrics may include silicon dioxide, silicon nitride, many others, and their mixtures and compounds. 
   While ultraviolet (UV) laser radiation sources are advantageous, depending on the substrate  20 , other sources can be used. Infrared lasers can be used to soften or liquefy many metals. Heat lamps of different radiation spectrum also can be used to melt or soften the materials  12  to be filled or planarized. The radiation spectrum of the laser and lamp can vary from the wavelength from 1 nanometer to 100 microns. The pulse duration of the radiation  22  can be selected upon the filling and planarization process. The pulse can be short enough so that the only a surface layer of the substrate  20  is heated, while the rest of the substrate  20 , as well as the mold  18  is not heated significantly. Multiple pulses can be used. 
   The substrate  20  may be formed from a semiconductor, metal or dielectric, their compounds, mixtures, and multilayers. 
   The mold  18  can be any material harder than a molten fill material  12 , including quartz, glass, or other materials which are transparent to the radiation  22 , even silicon or semiconductors whose band gap is larger than the radiation  22  of the wavelengths. The thickness of the mold  18  can be as thin as in micrometer or as thick in several millimetres or centimetres. 
   The optional wetting layer may include metals, dielectrics, and semiconductors, such as titanium, chromium, silicon, etch, depending upon the materials  12  to be filled. 
   The methods of the present invention may be used to make electronic devices (such as contact holes, contact lines, gates for transistors, microwave antennae, surface acoustic wave devices), optical devices, (such as subwavelength optical elements, filters, polarizarers, waveplates, and photonics crystals), magnetic devices, (such as random access memory, patterned media, quantized magnetic disks where the bits of the devices is very small), biological devices, (such as DNA, protein, virus or other biological analyzing devices), and fluidic devices of different materials needed to fill up certain parts of the trenches of holes. 
   As a first example to demonstrate the method of filling nanoscale via holes  10  by laser assisted direct imprint (LADI), we fabricated in a Si substrate  20 , a hole array having 100 nm diameter, 500 nm depth (aspect ratio 5 to 1) and 200 nm inter-hole spacing (pitch). The hole array was patterned by nanoimprint lithography and etched by Cl2/Ar RIE. As silicon was a good heat sink, a 15 nm thermal oxide layer was grown to reduce heat loss to the surrounding bulk Si, and to shrink the hole size at the same time. Finally, 200 nm Si or Cu was deposited on top of the hole array by e-beam evaporation. As expected and shown in  FIG. 3A , evaporation deposition has very poor step coverage, so the holes were only partially filled, with the sidewall of the holes receiving little deposition before the hole opening was closed. 
   During the hole filling by LADI, we pressed a UV grade quartz plate  18  with a mirror flat surface against the substrate  20  coated with the filling material  12 , and vacuumed the sample assembly below 200 mTorr. Then a single excimer laser pulse  22  (XeCl, 308 nm wavelength, 20 ns pulse duration, 2.5×2.5 mm2 beam size) of fluence 1.1 J/cm2 melts the filling material  12  momentarily. When in molten state, Si and Cu have very low viscosity (see the next section), so they could flow into the holes  10  and reach the hole bottoms within some 100 ns.  FIG. 2  shows via filling by Si, indicating a complete filling without voids  16 . 
   For via hole and trench  10  filling by Cu ( FIG. 6 ), the Cu plugs were found broken after wafer cut, and it is possible that the built-in stress caused by fast cooling of the liquid Cu has contributed to the Cu plug fracture. Nonetheless, most holes were completely filled to the bottom, as indicated by the Cu plug sections found on the bottom of the holes. 
   For both Si and Cu filling, based on the laser fluence and thermal properties of the related materials, the 15 nm SiO 2  and a very thin layer of the surrounding Si at the upper part of the hole melted. However, due to the 8-9 orders higher viscosity of SiO 2  compared to that of the filling material  12 , the flow of the molten SiO 2 /Si is negligible, leading to no apparent distortion of the holes  10 , as evidenced by the regular and distinct shape of the Si and Cu plugs with diameter corresponding to the original hole diameter. On the other hand, the 15 nm thermal SiO 2  is much thinner than the characteristic heat diffusion length (see the next section) of 260 nm for SiO 2 , leading to significant heat loss to the bulk Si. The via hole and trench  10  filling would undoubtedly be greatly facilitated in reality when the via holes and trenches are surrounded by a thick thermal insulator. During the via hole and trench filling by LADI, the wafer surface  14  is planarized due to the low viscosity of the molten fill material  12  and the flat surface of the mold  18 . 
   In the via hole and trench  10  filling process, the driving force is apparently the applied pressure on the mold  18 . The counter forces include surface tension, the viscous force and the inertial force. The minimum applied pressure necessary for squeezing the molten material  12  into the holes  10  is determined by the surface tension and has order of σ/φ with σ as surface tension and φ as hole diameter. Liquid Si and Cu has surface tension one order higher than that of water, necessitating order of 100 bar for filling holes  10  with 100 nm diameter. In practice, this restriction could be relieved by coating the walls of the holes  10  with a thin lining layer that wets the molten filing material. 
   The effect of viscous force can be estimated by assuming that a steady flow develops momentarily at t=0 (i.e. ignore inertial force). Then the liquid filling material will travel, before it solidifies, to a critical depth having order of 
   
     
       
         
           
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   where P eff  is the effective pressure (order of 10 bar) taking into account the surface tension, μ is the viscosity, and τ is the melting duration (˜100 ns) at the propagating plug front that depends on the thermal diffusivity D (=κ/ρCp, with κ as thermal conductivity, Cp as specific heat and ρ as density) and the laser pulse duration tp. Due to the low viscosity of liquid Cu and Si that is comparable to that of water, the theoretical filling depth L c1  is calculated to be several μm, one order higher than the hole depth in the experiment. 
   Similarly, the effect of inertial force, which decides how fast the steady flow develops, can be estimated by assuming an inviscid liquid (μ=0). The corresponding critical filling depth has order of: 
   
     
       
         
           
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   L c2  is estimated to be several μm, the same range as L c1 . Therefore, both the viscous and the inertial force is important in determining the maximum filling depth. 
   As for substrate heating, an upper limit of the maximum temperature experienced at the supposed dielectric/semiconductor interface can be estimated as following: For a homogeneous material, the maximum temperature reached at depth equal to its characteristic heat diffusion length δ (=2√Dtp) is roughly ⅕ that of the maximum temperature experienced at the top surface. Now suppose that the dielectric material below depth δ is replaced by a semiconductor (i.e. assume dielectric thickness equal to δ, or 260 nm for SiO 2 ) that conducts heat better, then the maximum temperature at the dielectric/semiconductor interface would be &lt;⅕ that of the maximum temperature at the dielectric surface, or 400 oC if assuming a maximum surface temperature of 2000° C.; 
   and the heating lasts for only order of 10 tp (=200 ns here). As a consequence, unlike tungsten CVD, the via hole filling by LADI would have a negligible thermal budget. 
   Besides superior step coverage and negligible thermal budget, the current technique is fast and simple and doesn&#39;t need a seed layer. Its disadvantage, as compared to tungsten CVD, lies in the challenge to scale up due to the likely high pressure required to overcome the surface tension, and the availability of high power pulsed laser having wafer-size power uniformity. 
   In a second example of the method of the present invention for wafer planarization of a surfaces  14  with relative large variation of topology, SiO 2  gratings of different periods (5 to 30 microns) were on the a silicon wafer to create a non-planar surface and followed by deposition of amorphous Si. Then the amorphous Si was planarized by the flat mold  18  press and melt using an excimer laser pulse  22 , as shown in  FIGS. 5A and 5B  (before and after the process). 
   It may now be seen that one aspect of the invention is a method of making a product having at least one nanoscale feature comprising the steps of: (1) providing a substrate  20  of a first material having a first surface including at least one recessed region  10 ; (2) coating at least the portion of the first surface comprising the recessed region with a thin layer of a meltable second material  12 ; (3) providing a mold  18  having a molding surface; (4) pressing together the mold  18  and the substrate  20  with the molding surface pressing the layer of meltable second material  12 ; and (5) while the molding surface is pressing the layer of meltable second material  12 , exposing the layer of meltable second material to radiation  22  to melt the layer  12 , thereby increasing the penetration of the meltable second material  12  into the recessed region  10  of the substrate  20 . Advantageously, the recessed region  10  has at least one minimum lateral dimension of less than 200 nanometers. 
   In one advantageous embodiment the substrate  20  comprises an insulating material with the recessed region  10  located in the insulating material and the second fill material  12  comprises a conductive material. The substrate  20  can also advantageously comprise a semiconductor material. 
   The radiation  22  can be pulsed radiation, laser radiation, or advantageously pulsed laser radiation, preferably in the ultraviolet (UV) range. Preferably, the laser radiation is of short time duration to minimize thermal burden on the substrate  20 . 
   The mold  18  is advantageously substantially transparent to the radiation  22  so that the thin film of fill material  12  can be exposed through the mold  18 . A preferred mold material is quartz or fused silica. Alternatively, the substrate  20  could be transparent to the radiation  22  for exposure through the substrate  20 . 
   The second fill material  12  can be any meltable material but is advantageously metal or semiconductor. Preferred second fill materials  12  include copper (Cu) and silicon (Si). 
   The mold  18  preferably has very smooth, substantially planar molding surface. It can have tiny molding features, but preferably has a mirror smooth, optically smooth surface to precisely planarize the thin film  12 . 
   The recessed regions  10  in the substrate  20  can be any desired recessed regions. Advantageously is is a nanoscale recessed region having at least one minimum lateral dimension less than about 200 nanometers. Advantageously recessed features  10  include holes with effective diameters of less than 200 nanometers and trenches with widths of less than 200 nanometers. The holes can be spaced apart in a one or two dimensional array. Trenches can be spaced apart, as in a parallel array. 
   A second aspect of the invention is the product made by the above described process. The product has unique advantageous of high-aspect-ratio fill of any detailed cross-section with low thermal burden. 
   The present disclosure can be embodied in-part in the form of computer-implemented processes and apparatuses for practicing those processes. The present disclosure can also be embodied in-part in the form of computer program code containing instructions embodied in tangible media, such as floppy diskettes, CD-ROMs, hard drives, or an other computer readable storage medium, wherein, when the computer program code is loaded into, and executed by, an electronic device such as a computer, micro-processor or logic circuit, the device becomes an apparatus for practicing the present disclosure. 
   The present disclosure can also be embodied in-part in the form of computer program code, for example, whether stored in a storage medium, loaded into and/or executed by a computer, or transmitted over some transmission medium, such as over electrical wiring or cabling, through fiber optics, or via electromagnetic radiation, wherein, when the computer program code is loaded into and executed by a computer, the computer becomes an apparatus for practicing the present disclosure. When implemented in a general-purpose microprocessor, the computer program code segments configure the microprocessor to create specific logic circuits. 
   As various changes could be made in the above constructions without departing from the scope of the disclosure, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.