Abstract:
The invention relates to a flexible and cost-effective method for fabricating customized rerouting metallization of the circuit contact pads. Localized depositions of insulating as well as conducting paths are provided with the capability for manufacturing multi-layered networks of interconnection. In a gas-filled chamber, either a focused laser, or an unfocussed lased impinging through a mask, is used to locally heat selected areas of the chip surface. The gas decomposes on the heated areas, depositing insulating or conducting material precisely on the heated surface areas, respectively. With this additional flexibility for product design and assembly, a number of interesting new products can now be fabricated which are in demand in both commercial and military markets.

Description:
This invention relates to the fabrication and assembly of semiconductor chips and modules, and more particularly to methods and apparatus for manufacturing customized rerouting metallization. Commercial and military systems today are placing increasing demands on flexible application as well as simplified manufacturing. 
     BACKGROUND OF THE INVENTION 
     Semiconductor devices have been prepared in the past using various combinations of metallization processes for rerouting connections to the circuit contact pads. They generally consist of first depositing thin layers of metal and later removing those portions which are not needed for the desired network. This add-and-subtract method is generally costly and involves hazardous materials and often chemical waste; it also tends to lower the process yield due to repeated handling of the chips, and may also generate stress in the chips themselves. The feature sizes achievable for rerouting remain severely limited, and the choice of metals which can be processed is restricted. When insulating layers have to be deposited, existing technology requires extra care for protecting those parts of the chip, which should not receive any deposition, such as the circuit contact pads, amounting to a cumbersome and time consuming deposition process. On the other hand, commercial and military systems urgently require flexible, cost-effective methods for mass producing rerouted semiconductors chips which are compatible with the increasing demands for more signal input/outputs and power handling, and are able to hold pace with quickly changing design rules and feature sizes. 
     Techniques have been investigated to use laser energy for direct deposition of metals and other solids from the gas phase. The incident laser energy causes photodecomposition or photolysis of the metal-containing component in the gaseous phase. Selective heating of substrate areas definded by incident focussed laser beams has been used to initiate reactions of gaseous precursors, resulting in the deposition of the desired solid reation product. The studies investigated not only the composition of the gases and the properties of different lasers, but also the effect of inorganic and organic substrates, adhesion, the possible need of seeding before deposition, and the required temperatures. Most of these investigations have been for research or specialty product development purposes. For example, the local deposition of aluminum and silicon nitride has been descibed in U.S. Pat. No. 4,340,617, July 1982, Deutsch et al.; deposition of palladium in U.S. Pat. No. 4,574,095, March 1986, Baum et al.; deposition of silicon dioxide, tungsten, molybdenum and titanium in U.S. Pat. No. 4,699,801, October 1987, Ito et al . . . The deposition of gold has been investigated by T. H. Baum in “Laser Chemical Vapor Deposition of Gold”, J. Electrochem.Soc. vol. 134, pp, 2616-2619, 1987; the deposition of copper by F. A. Houle, et al, in “Laser Chemical Vapor Deposition of Copper”, Appl. Phys. Lett. vo. 46, pp. 204-206, 1985; by J. Han et al., in “Combined Experimental and Modeling Studies of Laser-assisted Chemical Vapor Deposition of Copper . . . ”, J. Appl. Phys. vol 75 (4), pp. 2240-2250, 1994; the deposition of tungsten and copper lines has been described by R. F. Miracky in “Laser Advance into Microelectronics Packaging”, Laser Focus World vol. 27, pp.85-98, 1991. 
     It has been demonstrated that achievable laser focus is compatible with the feature sizes in semiconductor assembly and packaging (10 to 20 μm), and that the cost of laser application is lower than comparable mature mechanical machines. The goal, however, of offering for the commercial and military markets cost-effective, reliable, rerouted semiconductor products, manufacured in high volume and with flexible, low-cost production methods, has remained elusive, until now. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a plurality of semiconductor devices for application in memory, digital signal processing, microprocessor and other commercial and military products requiring flexibility, cost-effectiveness, and high reliability; secondly a process aiming at high flexibility, reduced number of process steps and process time, and low-cost manufacturability; and thirdly an apparatus for simplifying selected steps of the process. 
     It is an object of the present invention to provide a flexible and low-cost method and system for rerouting the connections to the circuit contact pads. 
     Another object of the present invention is to provide a method of expanding the scope of chip application by generating a network of multilevel interconnections. 
     Another object of the present invention is to maximize device characteristics by selectively depositing localized paths of insulators, and of conductors of various sheet resistance. 
     Another object of the present invention is to expand assembly options by employing various metal/solder combinations. 
     Another object of the present invention is to provide a simplified technology for covering the edge sides of the chips in preparation for flexible and reliable extension of the reroute network across the edge sides. 
     Another object of the present invention is to provide an efficient, flexible, economical, environment-friendly, mass producible technology for dense interconnection and assembly of semiconductor chips. 
     These objects have been achieved by a flexible mass-production process using a combination of sequential vapor depositions of insulators and various metals onto selected and narrowly defined areas of the chips. Various combinations have been employed for the circuit surfaces as well as edge sides of the chips, and have been successfully used for producing customized conductor patterns for assembling multi-chip cubes or chips-onto-substrate products. 
     The technical advance represented by the invention, as well as the objects thereof, will become apparent from the following description of a preferred embodiment of the invention when considered in conjunction with the accompanying drawings and the novel features set forth in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 to  4  illustrate the process flow for rerouting conductive by paths to circuit contact pads according to the invention. FIGS. designated by “a” show perspective views, and FIGS. designated by “b”show cross sectional views. 
     FIG. 5 is a perspective view of a cube assembled of rerouted semiconductor chips using adhesive polyimide film. 
     FIG. 6 illustrates a cross section of a cube assembled of rerouted semiconductor chips using adhesive polyimide film, after solder balls have been attached. 
     FIG. 7 is a perspective view of a cube assembled of rerouted semiconductor chips after assembly onto a substrate using solder balls. 
     FIG. 8 illustrates a processing system suitable for laser chemical vapor deposition of semiconductor chips. 
     FIGS. 9 a  to  9   d  illustrates the process sequence of chemical vapor deposition onto a semiconductor chip using a focussed laser beam. 
     FIGS. 10 a  to  10   d  illustrates the process sequence of chemical vapor deposition onto a semiconductor chip using a non-focussed laser beam impinging on a mask. 
     FIG. 11 illustrates the fixture for three-dimensional laser chemical vapor deposition. onto semiconductor chips. 
     FIG. 12 illustrates the fixture for the laser chemical vapor deposition after rotation of the chips and with an additional height set tool. 
     FIGS. 13 a  and  13   b  illustrate cross sections of two different chip positions in the fixture of FIGS. 11 and 12 for the operation of the laser. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 to  4  illustrate the process of rerouting a single semiconductor chip  10 , while in manufacturing, of course, many chips are processed at the same time. In the preferred embodiment, semiconductor chips  10  is made of silicon; other embodiments use gallium arsenide, or any other III-V or II-VI semiconductor material. Chip  10  has circuit contact pads  11  thereon (for example aluminum) wherein it is desired to reroute electrically conductive paths to the contact pads  11 . The area between the circuit contact pads is convered by protective, electrically insulating overcoat  12 . The rerouting is accomplished by the following process steps: First, the chip is placed on a support, described in detail later, and placed in a reaction chamber, also described in detail later, which can be evacuated (to a residual pressure 1 μTorr or less). In manufacturing, all chips face with their respective circuit sides towards the laser. As shown in FIG. 2, a focussed laser beam is then used to remove oxides and any other incidental contamination from the circuit contact pads  11 . 
     Next, the support with the chip  10  is rotated (apparatus and method are described in detail below) so that the chip turns one edge side toward the laser (in manufacturing, all chips turn their respectice edge sides toward the laser). The chamber, so far still under vacuum, is now filled with a gas of a mixture of precursors selected such that, wherever a heated surface is encountered, the precursors will react with each other and decompose, while precipitating a compound onto the heated surface to form a solid precipitate. (This process is referred to as “chemical vapor deposition”) The remaining reaction products stay volatile so that the desired deposit is the only solid reaction product. The focussed laser beam is now directed towards specified portions of the edge side of the chip so that light energy is absorbed by the semiconductor material under illumination, heating it quickly to elevated temperatures. As a consequence, the precipitation from the gas phase will now happen at those laser-heated areas (this process is referred to as “laser chemical vapor deposition”). 
     The surface temperature profile, induced by laser heating of the substrate, defines the reaction zone and controls the deposition. The optical and thermophysical properties of the substrate and deposited material are thus important process parameters. The decomposition temperature of the precursor will determine the laser power necessary for the initiation of deposition, and the vapor pressure will profoundly influence the deposition rates. 
     For the purpose of this invention, areas  13  of FIGS. 3 a  and  3   b  consist of insulating material such as silicon nitride. Its deposition can be obtained by irradiating a mixture of silicon tetrahydride (SiH4, at 740 Torr) and ammonia (NH4, at 10 Torr) with a pulsed laser. The laser energy causes dissociation of the ammonia gas. This dissociation then initiates a surface reaction with the gaseous silane to form silicon nitride After forming the dielectric deposits in areas  13 , the gas mixture is pumped off. 
     Next, the support with the chip  10  is rotated back to its original position so that the circuit side of chip  10  is facing the laser again. The chamber is now filled with a gas of a mixture of precursors selected such that, wherever a heated surface is encountered, the precursors will react and decompose, while precipitating a metal onto the heated surface. A focussed laser beam is again used to heat selected areas of the chip surface. Since the metal is thus deposited wherever the laser is heating surface areas, the process is sometimes referred to as“direct laser writing” This process of direct metal deposition can be repeated several times with different precursors, also after rotating the chip for exposing selected areas of the respective edge side to the laser beam. For some applications, it may be desirable to deposit several metal on top of each other, or to deposit the layers in different thicknesses by varying the time of laser exposure. 
     The deposited metal may consist of a single layer, or a sequence of layers of different metals or metal alloys. For the embodiment in FIG. 4, the deposited metal layer is shown as consisting of three parts: Layer  14   a  is a refractory metal, layer  14   b  a metal of high electrical conductivity, or a noble metal, and layer  14   c  a metal particularly suited for solder attachment. Layer  14   a  consists of tungsten or a titanium:tungsten alloy; layer  14   b  consists of copper; and layer  14   c  consists of platinum. The gas mixtures filling the reaction chamber have to be selected according to the desired metal deposition. For tungsten, the preferred precursors are tungsten hexafluoride as a reactive gas, and hydrogen as as a carrier and reducing gas. Equivalent gases are used for depositing titanium. For copper; the gas consists of the vapor pressure (about 10 mTorr at room temperature) of the solid bis(1,1,1,5,5,5-hexafluoro-2,4-pentanedionate)copper(II), or short CuHF; another option is the high vapor pressure of the liquid (at room temperature) copper(I)-hexafluoroacteylacetonate trimethylvinylsilane. Layer  14   c  is platinum; the gas is then bis(2,4-pentanedionato)platinum(IIii). 
     In another embodiment of this invention, layers  14   a  and  14   b  in FIG. 4 b  are replaced by a single layer consisting of gold. While various precursors have been investigated in the literature, for this invention, consistently good metal lines have been obtained from dimethyl gold trifluoroacetylacetonate, or dimethyl(1,1,1-trifluoro-2,4-pentanedionato) gold (III) The pulsed excimer, or Nd:YAG frequency-doubled laser is an alternative to the Argon Ion laser. For this embodiment, layer  14   c  also consists of gold. 
     It is an important feature of this invention of simultaneously achieving the optimization of the electrical characteristics of deposited lines  14   b  and the solder characteristics of deposited lines  14   c , The preferred embodiment includes copper for lines  14   b , exploiting the high electrical conductivity of this metal, and platinum for lines  14   c , exploiting the minimum interdiffusion, or dissolution, of this metal with liquid lead-tin solder materials. To insure a smooth transition from one metal to the other, layers  14   b  and  14   c  will be designed to overlap a short distance. In addition, the invention allows to produce approximately equal electrical sheet resistances of the deposited layers even for different metals (i.e., different resistivities) since the thickness of the deposited layers can be controlled by the controlling the time of the laser beam exposure. 
     It is another feature of this invention to allow the manufacture of a “tree” of lines with equal sheet resistance in each branch. FIG. 4 a  gives an example of such line tree  14   d.  These line trees are particularly useful for spreading an incoming electrical power connection to a plurality of circuit contact pads. 
     After completing the rerouting of a multitude of chips using the direct-write process of the invention, the chips are removed from the reaction chamber and a number of chips (up to ten or more) can be stacked into a three-dimensional assembly, or“cube”. An example of three chips  10  assembled into a cube is given in FIG.  5 . The rereouted metallizations  14   b  and  14   c  are highlighted, as is the use of the invention for creating a distribution “line tree”  14   d.  As FIG. 5 shows, the dielectric adhesive films  50 , used for assembling the multi-chip cube, consist of precisely cut preforms so that they act as spacers between the rerouted chips  10 . Respective edge sides of the chips extend over the edge of the dielectric spacer, creating a castellated outline (composition detail of the adhesive film  50  is given in FIG. 6 below). The castellated outline in FIG. 5 readily illustrates the pad grid interface formed by the multitude of layers  14   c,  ready for the attachment of solder material in the shape of prefabricated balls for further interconnection. The solder balls consist of mixtures of lead and tin as required by the desired melting or reflow temoerature. 
     Such solder balls  64 , attached to the extended edge sides  65  of the circuit chips  63 ,are illustated in FIG. 6, which represents a cross section through the chip stack of FIG.  5 . As can be seen, the adhesive films  50  (thickness range 50 to 150 pm) consists of three layers. An upilex or polyimide center  62  (thickness approximately 20 to 80 μm) carries almost polymerized polyimide or acrylic adhesive layers  61  on both sides (each layer approximately 20 to 40 μm thick). By appropriate selection of the thicknesses of layers  61  and  62 , desired electrical characteristics can be achieved for product parameters, such as capacitive coupling and cross talk between conductors of the chip or the cube. 
     FIG. 7 illustrates a three-dimensional circuit assembly in the configuration of a cube  70 , fabricated as described above, after it has been soldered onto the conductor pattern of a substrate  71 . This method permits a high number of soldered contact points due to the fine feature size of the rerouted conductors on the edge side of the chips, and the thinness of the dielectric spacers. 
     FIGS. 8,  9 ,  10 ,  11 ,  12  and  13  illustrate the manufacturing apparatus and method for rerouting conductors on semiconductor chips using laser-assisted chemical vapor deposition for direct line writing. In FIG. 8, the laser projection system, gas handling system, and deposition system are shown. Computer-controlled laser  80  (Argon Ion laser or pulsed Nd:YAG frequency-doubled laser) generates laser beam  80   a;  it is processed through the beam conditioning optics  85   a  and  85   b  and enters microscope  81 . The microscope may have a video camera and monitor attached. The laser beam is reflected by mirror  85   c  and can be focussed or unfocussed by optical system (objective)  85   d . There may be an illuminator  85   e.  Finally, laser beam  80   a  exits the microscope either focussed or non-focussed, as required by the subsequent deposition method, and enters the deposition system  82 . 
     Inside the deposition system  82  is equipment  83  for batch processing individual chips, to be described below in FIG. 11,  12 , and  13 . System  82  is positioned on computer-controlled x-y translation stages  84 . Feeding into system  82  is the gas handling system consisting of sources  86   a  and  86   b  of the reactants (including sublimation and vaporization sources), and mass flow controllers  86   c  (which also contain a cut-off valve). Exhausting from system  82  is the connection to the vacuum pump  86   d , which is computer controlled and operated by several valves and gauges. 
     The manufacturing system can operate in two different modes for depositing the materials for rerouting. The sequence of processing steps for individual line writing is described in FIGS. 9 a  to  9   d,  and for multiple line writing in FIGS. 10 a  to  10   d.  In all these figures, the reaction gas  90 , often of organometallic composition, is contained in depostion chamber  90   a;  after the deposition, gas  90  is pumped off and vacuum  90   b  appears in chamber  90   a.  Individual chips  91  inside chamber  90   a  are positioned so that they are illuminated by laser beams  92   a  and  92   b,  respectively, in substantially orthogonal manner. In laser chemical vapor deposition, the laser beam is absorbed by the substrate (chips  91 ) and utilized as a localized heat source. The heated volumes of chips  91  are marked by reference designator  93 . Molecules of gas  90 , which are adsorbed by or colliding with the surface of heated volumes  93 , undergo thermal decomposition to the desired constituent (either inorganic or metal) with the liberation of volatile reaction products. The surface temperature profile, generated by the laser heating of the chips, defines the reaction zone and controls the deposition of constituents (e.g., metal). The extent of heating is influenced by the optical and thermophsical properties of the chips; and can be adjusted by varying the intensity of the heating laser source. The decomposition temperature of the gas will determine the laser power necessary for the initiation of deposition, and the vapor pressure will influence the deposition rate. The deposition thickness will increase with illumination time. 
     In FIGS. 9 b  and  9   c,  a focussed laser beam  92   a  is heating a single surface spot (for instance to about 160° C. for gold and to at least 250° C. for copper), generating a single heated volume  93  and a single deposit  94 . Consequently, a single deposition  94  remains on chip  91  in FIG. 9 d,  after the laser beam has been discontinued, the reaction gas has been pumped off and vacuum  90   b  prevails. (residual background pressure of 1 μTorr or less). Useful thicknesses for layer  94  vary between 100 and 1000 nm, dependent on the material (e.g., silicon nitride, copper, platinum, gold, etc.). 
     It is a major object of this invention to provide an economical, mass production technology by creating multiple depositions in one laser exposure by means of mask  95  in FIGS. 10 a  to  10   b,  using non-focussed laser beam  92   b . Wherever laser beam  92   b  is not blocked by mask  95 , heated volumes  93  are generated in the chip, initiating multiple depositions  94 . Consequently, multiple depositions  94  remain on chip  91  in FIG. 10 d,  after the laser beam has been discontinued, the reaction gas has been pumped off and vacuum  90   b  prevails. 
     It is important for cost-effective mass production to employ equipment suitable for batch processing of individual chips. One such apparatus is illustrated in FIGS. 11,  12 , and  13 . The purpose of this apparatus is to precisely position and hold a multitude of chips so that the pattern of the rerouting can be generated. In FIG. 11, one chip  110  (out of a multitude of approximately 100) is shown on a support  111 , held by the pulling force of reduced air pressure (“vacuum”) supplied through numerous holes  111   a  opened in support  111 . This positioning is best supported by a thin thermoplastic adhesive. Loading and unloading of the chips is preferrably performed by a robot. The support  111  is connected to a rod  112 , held by bearings  113  in frame  114 . Rod  112  can rotate around its axis for at least 90°. When rod  112  is positioned so that one edge side  110   a  of chip  110  faces upward, as shown in FIG. 12, the height set tool  115  is gently lowered, until it touches the chip and comes to rest on pins  116 . All other chips which may be adjacent to chip  110  will be aligned simultaneously. As a result, all chips on support  111  will orient their respective edge sides in one plane (with a precision of approximately plus/minus 20 μm). The laser for chemical vapor deposition will be focussed on this plane. This focus plane has been give the reference designator  117  in FIGS. 13 a  and  13   b.    
     The laser (Argon Ion or Nd:YAG) operates at high precision (focus considerably better than 25 μm) so that fine feature sizes of the rerouting metal and insulator layers can be produced, approaching the feature size of the interconnecting metallization in the semiconductor circuit. As a result, the conductors for rerouting can be generated in fine feature size even on the edge sides of the chips. Exploiting the fact that the laser is computer controlled, different widths of the metal lines can be obtained for different portions of a conductor, and different line thicknesses can be generated by different heating times. These capabilities enable various line geometries, but sill equal electrical sheet resistance, in the rerouting pattern, allowing for instance the fabrication of wider electrical ground or power supply lines before they branch off to a multitude of finer line widths for connecting to the circuit contact pads. 
     By rotating rod  112  by 90°, the circuit surface of the chips  110  get into the focal plane  117  of the laser, as indicated in FIG. 13 b.  This enables the laser again to fabricate the fine feature sizes of the rerouting metal pattern, this time on the circuit surface of the chips. It is an important characteristic of the apparatus used in this invention that it generates the same length referenced “L” in FIG. 13 a for the distance from the axis of rod  112  to the surface  110   a  of the edge side of chips  110 , as it will generate the length referenced “L” in FIG. 13 b  for the distance from the axis of rod  112  to the surface  110   b  of the circuit surface of chips.  110 . 
     In a variation of the equipment arrangement shown in FIG. 8, the deposition and gas handling systems may be modified such that the gas handling system  86   a,    86   b,  and  86   c  becomes a vaporization system containing the condensed phase (i.e., solid or liquid) metalorganic precursor reagents. For this purpose, this system has to be designed to maintain the metalorganic species at temperatures ranging from 110° C. down to liquid nitrogen temperatures. At 77° K, the precursor compounds of interest have negligible vapor pressures, so the entire system may be pumped down by vacuum pump  86   d  to a base background pressure of 1 μTorr or less. After the entire system is pumped down, the vacuum cut-off valve  86   c  separating the deposition chamber  82  and the vaporization system  86   a  and  86   b  is closed, and the vaporization chamber is warmed to a few degrees above the melting point of the precursor. At room temperature, the metalorganic precursors typically have elevated vapor pressures ranging from 9 to more than 150 Torr. As a result, the deposition chamber  82  and the vaporization chamber  86   a  and  86   b  contain elevated metalorganic precursor reagents.