Abstract:
An apparatus for manufacturing an integrated circuit having a thick film metal layer includes an applicator configured to selectively apply a paste on a heat-conducting substrate. The paste includes particles of a first metal constituent of particles having sizes substantially within a narrow predetermined range about a predetermined size. The apparatus further includes a radio frequency (RF) generator to selectively inductively coupling RF energy into the paste. The first metal particles of the predetermined size are inductively couplable with the RF energy, and the frequency of the RF energy corresponds to a coupling frequency of the first metal particles of the predetermined size so that the inductive heating of the first metal particles is substantially maximized.

Description:
This application claims the benefit or priority of and describes relationships between the following applications: wherein this application is a continuation of U.S. patent application Ser. No. 13/258,105, filed Sep. 21, 2011, which is the National Stage of International Application No. PCT/IB2010/051295, filed Mar. 24, 2010, which claims the priority of foreign application EP09156417.9 filed Mar. 27, 2009, all of which are incorporated herein in whole by reference. 
    
    
     The present invention relates to an apparatus and method for manufacturing an integrated circuit having a thick film metal layer. 
     Present integrated circuits, such as solar cells for a solar panel, are currently manufactured by a process wherein a metal paste is applied to a substrate, and the entire assembly is heated in order to fuse, melt or sinter the metal particles in the paste and thereby create the desired circuitry. A sufficient amount of energy is applied to heat both the metal paste and the substrate up to the melting/sintering temperature of the metal. Beneficial interaction between the substrate and conducting material occurs at these high temperatures, but prolonged interaction decreases performance due to substrate damage and/or substrate property changes. 
     The limits in heating and cooling rate are normally governed by the heat power transfer to the total substrate with the metal paste. Conventionally, the total heat capacity of a substrate and metal needs to be considered to calculate a temperature increase for a given power input per second, and the heat capacity of, for example, a silicon substrate, is much larger than the heat capacity of the metal paste, e.g. a silver paste, arranged on the substrate. This heat capacity ratio ensures that much more power is needed, using the conventional methods, to get the same temperature increase over the same time period as if the metal were heated alone. 
     DE 100 41 889 A discloses a procedure for thermally changing the electrical properties of a semi-conducting coating material. 
     DE 102006005026A discloses an electrically conductive coating of sintered particles on a glass substrate. The sintered particles disclosed are nano-particles of ITO. The glass and ITO are heated capacitively in a resonant cavity using microwaves between 300 MHz and 30 GHz. 
     US Patent Publication 2005/0087226 discloses an electrode-arranging method for thin films on non-flat substrates. The method uses inductive heating in the range of several kHz to 1 MHz, which requires highly conductive, pre-sintered materials. The substrate and electrode material are both heated to the eutectic temperature of the substrate and electrode material. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide an apparatus and method for rapid heating and cooling of a metal paste on a heat-conducting substrate for an electronic device having improved electrical performance and significant energy savings. 
     Another object of the invention is to provide a passive method of rapid cooling of a metal paste on a substrate, wherein the cooling rate is faster than with the active cooling methods used in the case of conventional infrared heating. 
     A further object of the invention is to provide a method for selectively coupling radio frequency (RF) energy with metal particles in a metal paste on a substrate, wherein only the heat capacity of the metal and the small energy losses to the surrounding substrate need to be considered, wherein significantly less power must be delivered to heat the metal very quickly to a desired temperature as compared to conventional methods. 
     A subsequent object of the invention is to provide a method for manufacturing an integrated circuit having a metal layer wherein the substrate itself is not heated to the melting/sintering temperature of the metal, wherein only the metal layer needs to be cooled. 
     In a first aspect of the present invention, an apparatus for manufacturing an integrated circuit having a thick film metal layer is proposed, including an application means for applying a layer of metal paste on a heat-conducting substrate, the metal paste having metal particles of a predetermined size, and an RF generator for selectively inductively coupling RF energy into the metal paste, wherein the metal particles of the predetermined size are selectively inductively couplable with the RF energy, the predetermined size of the metal particles corresponding to a coupling frequency of the RF energy, for heating the metal particles. 
     In a further aspect, a method for manufacturing an integrated circuit having a (sintered/molten) thick-film metal layer is proposed, including applying a layer of metal paste on a heat-conducting substrate, wherein the metal paste includes metal particles of a predetermined size, and selectively inductively coupling RF energy into the metal paste from an RF generator, wherein the predetermined size of the metal particles corresponds to a coupling frequency of the RF energy, and the metal particles of the predetermined size are selectively inductively couplable with the RF energy, for heating the metal particles. 
     The present invention provides an apparatus and a method for manufacturing an integrated circuit having a (sintered/molten) thick-film metal paste on a substrate. The method is accomplished in a fast and energy-efficient manner, due to selective inductive coupling of the metal particles in a metal paste on the substrate so that most of the inductive energy goes into heating the metal particles directly. The substrate may receive some energy from the metal particles, but is not coupled with the RF energy, or is coupled in a very limited manner, which results in a fast heating process which requires little energy, especially as compared to conventional processes. 
     The conventional processes require that the electrode material is first sintered before any heating step, whether inductive or capacitive, in order to create a highly conductive layer. At the low frequencies disclosed in the conventional art for inductive heating, the effect of the induction heating is very limited, and not very fast, in particular due to the requirement for heating both the electrode material and the substrate together. Further, in the case of conventional processes using capacitive heating with microwaves, a resonance cavity is required, due to the nature of capacitive heating and microwaves. 
     According to the present invention, pre-sintering of the electrode material is unnecessary because of the higher frequency which couples into the individual silver particles. The highly-conductive layer is not required at all. The efficiency of coupling is also greater. Because of this greater efficiency, the speed of heating is significantly higher than conventional technologies, including inductive heating done at lower frequencies. 
     A wide range of frequencies is known for use in capacitive heating processes, which are used to heat non-conducting materials as well as conducting materials. Inductive heating, for conducting materials, is a common technology at frequencies of several 10&#39;s or 100&#39;s of kHz. However, induction at a range of 2-200 MHz, or, in particular, about 27 MHz is not known because of a number of complications (such as electric breakdown due to the high voltage required, and controlling the heating process) that need to be dealt with, as described below. 
     At low frequencies, e.g. 10-1000 kHz, it is common to use a coil that is as large as the product to be heated. This is for a non-localized heating scenario. Occasionally, a smaller coil is chosen to increase the field strength that can be applied for more localized heating. Of course, in order to be effective, the conventional arts teach that the application of this low-frequency energy requires that the patterns to be heated, e.g. electrode material, is highly conductive. This requires that the electrode material is pre-sintered prior to inductive heating. 
     In the intermediate frequency range, e.g. 2-200 MHz, in particular 27 MHz, between the conventional inductive frequencies and microwaves, both a local and a non-local heating approach can be used. However, the use of a localized field, and a small coil, has strong advantages: better temperature field homogeneity can be achieved more easily, and the field strength can be much higher. Further, the coupling of the RF field to the individual metal particles in the metal paste means that pre-sintering is unnecessary. The present disclosure uses the RF field to sinter the electrode materials as they are selectively heated. 
     Low-frequency inductive heating, as provided in the conventional art, sets strict demands on the geometry of the electrode-material patterns on the substrate in order to control the temperature field and provide homogeneity. The presently disclosed heating arrangement is much less sensitive to the geometry of the patterns to be heated. 
     In contrast to the conventional art, the present disclosure uses a localized field from an RF generator that may be generated by a small coil. This arrangement has two important advantages: 
     Increased homogeneity of the heating process; and 
     Very high field strengths are possible without the use of extremely high voltages. 
     According to an embodiment, the substrate, due to its greater mass, has a higher heat capacity than the metal in the metal paste. This makes it possible for the substrate to act as a heat sink to provide rapid cooling of the metal layer. 
     According to a second embodiment, the substrate may include silicon, gallium-arsenic compounds, germanium, indium-tellurium compounds, and copper-indium-gallium-sulfur compounds. These materials provide the advantage of a large inductive heating efficiency difference (and heat capacity) difference between themselves and many of the metals used in integrated circuit manufacturing. 
     According to another embodiment, the substrate may act as a heat sink for rapidly cooling the selectively-coupled metal. The advantage of selectively coupling the metal on the substrate means that when a metal is selectively coupled, the substrate will remain relatively cool throughout the coupling period. Thus, a large capacity to absorb the temperature and energy differences between the substrate and the metal is available, wherein the metal may be cooled very rapidly. 
     According to a subsequent embodiment, the metal paste may comprise metal particles of silver, aluminum, copper, stainless steel and other conducting metals appropriate for use in integrated circuits. A wide variety of metals may be used to advantage depending on the intended application of the integrated circuit and the heat capacity of the substrate. 
     According to another embodiment, the metal particles in the metal paste are selectively coupled at a very high frequency in the range of 2 to 200 MHz. Selective coupling of the metals means that the substrate is not affected thermally in a direct manner and remains relatively cool with respect to the metal particles. The selected frequency range is higher and more effective in coupling than typical frequencies used for induction, and are significantly below the microwave frequencies, which require resonance cavities and extensive shielding. This frequency range provides sufficient energy to selectively couple metal particles in the metal paste, without over-penetrating the substrate materials or requiring extensive shielding arrangements, such as with microwaves. Some simple shielding may be required only to prevent disturbances in electronic appliances. 
     According to another embodiment, the metal particles of the metal paste are micro-particles, which are couplable with RF energy of about 27 MHz. This provides the advantage that a predetermined, uniform size of metal particles may be used with a narrowly-selected frequency band to provide controllable coupling without numerous undesirable considerations, e.g. over-penetration of the RF energy or excessive heating of the substrate. A properly sized particle complements nicely with a particular frequency band to provide a controlled method for manufacture. The micro-particles are larger than nano-sized particles, which would require microwave RF energy for coupling. Microwave RF energy would require a shielding arrangement to protect the surroundings. 
     According to a further embodiment, a substrate table may be provided to move the substrate having the metal paste under the RF generator at a predetermined rate, wherein the RF energy is distributed in a predetermined manner. This provides the advantage of uniform and controlled distribution of the RF energy, for selective heating or non-heating of the metal paste on the substrate. The RF energy may be modulated relative to the position of the substrate to enhance temperature homogeneity. It was observed that the heating under constant RF power and constant substrate speed was not homogeneous enough in the substrate movement direction, in some situations. The effect of heat conduction into the substrate caused a gradient in temperature increase when RF power was constant over the whole substrate travel length. This is caused by cooling in the first part, as a large, relatively cool substrate is available to act as heat sink, while in the last part, only a small piece of the substrate is relatively cool and the heat sink capacity of this piece is smaller. In one embodiment, modification of the RF power delivery is done by electrical or mechanical adjustment of the RF power output, controlled by signals indicating the substrate position. By adjusting the RF power to the coil while the substrate passes beneath it, the power loading can be “programmed” for each substrate position individually, enabling increased homogeneity or intended non-homogeneity. 
     According to a subsequent embodiment, the apparatus may include a substrate heater for pre-heating the substrate, wherein the conduction properties of the substrate are changeable with a temperature change. This provides the advantage of changing or manipulating the physical properties of the substrate, e.g. conduction properties, with the temperature increase. An advantage with this step is that a more conductive substrate, e.g. heated silicon, even when slightly heated, provides a more efficient heat sink arrangement for cooling the metal paste. In addition, the pre-heating step increases process stability. Silicon is non-conductive at low temperatures, and therefore does not heat up in the applied inductive field. At higher temperatures, however, it becomes conductive and starts to couple RF energy. When the silicon substrate is not pre-heated, the sudden change in the absorbing properties of the silicon can render the process uncontrollable, causing very inhomogeneous heating. In contrast, if the substrate is pre-heated to &gt;400° C., the absorbing properties change very gradually, and can be accounted for in the process settings. The amount of energy absorbed per unit material is much less for silicon than for silver, even at this temperature, so the selective-heating principle is still fully applicable. This effect may also be significant if the substrate is a different semi-conductive material, for instance GaAs, Ge, InTe, CuInGaS or any other material with a band gap property. 
     According to a subsequent embodiment, the layer of metal paste is arranged with a predetermined three-dimensional geometry, wherein the temperature of the inductively-coupled metal particles is manipulable by the geometry. When the RF energy is carefully modulated, wherein the size of the metal particles in the metal paste is complementary with the RF energy frequency, a specific controllable penetration depth may be accomplished for the RF energy, and different temperatures may be achieved for different portions of the metal paste layer, depending on the predetermined three-dimensional geometry of the metal paste. 
     Also integrated circuits with multiple (2 or more) layers of thick film metal layers, with non-conductive layers in between, can be heated using the invention with RF heating. Connections with the 2 layers using vias are also possible. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. In the following drawings 
         FIG. 1  illustrates a top view of a substrate for an integrated circuit having a pattern of a conductive metal paste thereon, in accordance with an embodiment of the invention. 
         FIG. 2  illustrates a side sectional view of a substrate for an integrated circuit having an applied pattern of a metal conductive paste under an RF coil, in accordance with an embodiment of the invention. 
         FIG. 3  illustrates a low-frequency induction-heating arrangement, in accordance with conventional methods. 
         FIG. 4  illustrates a medium-frequency induction-heating arrangement, in accordance with an embodiment of the invention. 
         FIG. 5  illustrates a temperature-change curve for a substrate, in accordance with an embodiment of the invention. 
         FIG. 6  illustrates a perspective view of an apparatus for manufacturing an integrated circuit having an applied metal conductive paste, in accordance with an embodiment of the invention. 
         FIGS. 7A-7D  illustrate top and side views of elements for a multi-layer integrated circuit having an applied metal paste, in accordance with an embodiment of the invention. 
         FIG. 8  illustrates the coupling frequencies and corresponding penetration depth for different metals, in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates an arrangement of an integrated circuit  10  having a substrate  12  and an applied metal paste  14  arranged in a particular pattern. The thick film metal layer including the metal paste  14  is applied to the heat-conducting substrate  12  via an applicator  25 , as shown in  FIG. 6 . The metal paste  14  includes metal particles of a predetermined size. As shown in  FIG. 2 , an RF generator  16 , such as a coil, is used to selectively inductively couple RF energy ( 18 ) into the metal particles of the metal paste  14 , in order to heat the metal particles. 
     The present invention provides an apparatus and a method for manufacturing an integrated circuit  10  having a thick-film metal layer including the metal paste  14  on a substrate  12 . The method is accomplished in a fast and energy-efficient manner, due to the selective coupling of the metal particles in a metal paste  14  on the substrate  12  so that most of the inductive energy  18  goes into heating the metal particles directly. The substrate  12  may receive some thermal energy via heat conduction from the metal particles, but is not coupled, or is coupled only to a very limited degree in case of a pre-heated silicon substrate, with the RF energy, which results in a fast heating process which requires very little energy, especially as compared to conventional processes. 
     The conventional processes require that the electrode material is first sintered before any heating step, whether inductive or capacitive, in order to create a highly conductive layer. At the low frequencies disclosed in the conventional art for inductive heating, the effect of the induction heating is very limited, and not very fast, perhaps due to the requirement for heating both the electrode material and the substrate together. Further, in the case of conventional processes using capacitive heating with microwaves, a resonance cavity is required, due to the nature of capacitive heating and microwaves.  FIG. 3  illustrates the application of a low-frequency field, e.g. 100 kHz, to a layer of a sintered electrode material S having a thickness d. Due to the sintering process, the sintered electrode material S will exhibit an induction current I. The induction current I through the electrode material S causes heating of the electrode material S. Heating is the by-product of pre-sintering the electrode material S, the application of the low-frequency field and the induction of a current I in the electrode material S. This process takes too much effort and the same results can be achieved more easily via a different process. 
     According to the present invention, and as illustrated in  FIG. 4 , pre-sintering of the electrode material is unnecessary because of the higher frequency, e.g. 2-200 MHz, field which couples into the individual metal, e.g. silver, particles  24  having a particle size Q. The particle size Q is typically much smaller than the thickness d of the electrode material of  FIG. 3 . As a result of the coupling of individual particles Q, the highly-conductive electrode layer S is not required at all. The efficiency of higher-frequency coupling of  FIG. 4  is also greater than conventional processes. Because of this greater efficiency, the speed of heating is significantly higher than conventional technologies, including inductive heating done at lower frequencies. 
     The substrate  12  may have a higher heat capacity than the metal paste  14 . Accordingly, the substrate  12  may be a heat sink to provide for rapidly cooling the selectively inductively-coupled metal paste  14 . Rapid cooling complements the selective coupling, and selective heating, wherein relatively little energy is used to heat the substrate and a significant amount of energy is saved as compared to the conventional art. 
     The substrate  12  may be made from a number of materials including silicon, gallium-arsenic compounds, germanium, indium-tellurium compounds, copper-indium-gallium-sulfur compounds and other compounds or materials having heat capacities and conductive properties similar to the foregoing materials. A larger number of materials may be used with the disclosed method to manufacture integrated circuits and thick-film metal layers in a very energy-efficient manner. 
     The metal paste  14  may include a variety of metals, including silver, aluminum, copper and stainless steel, or other metals capable of being processed according to the disclosed method. A wide variety of metals having different properties may be used according to the disclosed method for producing integrated circuits or thick-film layers in an energy-efficient manner. 
     The metal paste  14  may be selectively coupled with the RF energy  18  at a very high frequency. The complementary RF frequency and metal particle size provide more controllability. According to another embodiment, the frequency of the RF energy  18  is around 27 megahertz. This particular frequency range provides the advantage of sufficient penetration of the metal paste  14  via coupling while avoiding the necessity for RF shielding, as would be the case for microwave energies. 
     The metal particles of the metal paste  14  are sized so as to be responsive to the RF energy  18 . Appropriate selection of the RF energy  18  with a complimentary particle size results in efficient, selective heating of the metal particles in the metal paste  14 , without excessive heating, and associated energy waste, in the substrate  12 . In a further embodiment, the metal particles are micro-particles, e.g. 5-50 μm, which are responsive to RF energy  18  in the range of 2-200 MHz, in particular about 27 MHz. However, the particles may be even larger than 50 μm, depending on the particular metal selected. The micro-particles may be about 12 μm diameter. The micro-particles are far larger than the nano-particles that would require the use of microwave energy-type frequencies and the corresponding shielding requirements. Thus, the combination of the micro-particles with a 27 megahertz frequency is both efficient and easily controlled. 
     As illustrated in  FIG. 6 , the substrate  12  having the metal paste  14  may be moved beneath the RF coil  16  at a predetermined rate. This arrangement provides for even distribution of the RF energy  18  from the RF coil  16  into the metal paste  14 . 
     The substrate  12  may be preheated to effect a change in the conduction properties of the substrate  12 . Certain materials exhibit significant changes in their conductive properties with changes in their temperature, e.g. silicon, as illustrated in  FIG. 5 .  FIG. 5  illustrates that the temperature change ΔT of the substrate  12  due to passing through the RF field depends on the initial temperature T 0  of the substrate  12 . For the example of a silicon substrate, the characteristic temperature K is about 400° C. That is, the temperature at which the conductive properties have changed to the point where they can be used effectively to advantage is about 400° C. Below about 400° C., the characteristic, e.g. conductive, properties are not very great. With respect to silicon, at about 400° C., the RF energy coupled into the silicon equals the heat loss via conduction/convection. Thus, above this characteristic or “critical” temperature, the RF field can induce a significant temperature increase, while below this temperature it cannot. 
     Thus by raising the temperature of the substrate  12  to a point above the characteristic temperature K, the increase of- and stability of-conduction of the substrate  12  results in a more efficient heat sink arrangement of the substrate  12  with respect to the selectively-coupled, e.g. selectively-heated, metal paste  14 . That is, when the substrate materials are appropriately selected and matched to the metal paste layer  14 , the heated substrate  12  is better and more constant at absorbing the thermal energy of the metal paste  14  than when the substrate  12  is cool. 
       FIG. 6  illustrates an apparatus for manufacturing an integrated circuit with a thick-film metal layer having a substrate table  20  to support the substrate  12 . The substrate table  20  may be used to move the substrate  12  under the RF generator  16  at a predetermined rate, for the application of RF energy in a predetermined manner. In addition, the apparatus may include a substrate heater  22  for pre-heating the substrate  12  in a manner to take advantage of the change in conductive properties of the substrate  12  above its characteristic temperature K. By modulating the RF power to the RF generator  16  as the substrate  12  passes beneath it, the power loading P, or programmable power wave form, can be determined and applied for each position of the substrate  12 . This may be used to achieve improved homogeneity of non-homogeneity of RF application, and resultant temperature, as desired. 
     The layer of metal paste  14  may be arranged on the substrate  12  with a predetermined three-dimensional geometry, as shown in  FIG. 2 , wherein the temperature of the coupled metal paste  14  may be manipulated as a result of the geometry. The RF energy necessary for coupling with the metal paste  14  is calculable so that it is not excessive with respect to over-penetration of the metal paste  14 , and will penetrate the metal paste  14  to a desired depth. Various heating and cooling arrangements of the metal paste  14  in particular areas of the substrate  12  may be created through various geometries, e.g. thicknesses, widths and lengths, of the applied metal paste  14  on the substrate  12  to achieve the desired conductive results. 
     In one example, the invention relies on selective coupling of the RF energy  18  into a metal paste  14  containing silver. In this case, only the heat capacity of the silver and the small energy loss to the surrounding substrate  12  needs to be considered for delivering RF energy, which results in a requirement for significantly less power to heat the silver very quickly to a desired temperature, via coupling, than would be required in conventional processes. However, once heated, the silver would need to be cooled quickly as well to prevent detrimental effects to the combined electrical properties of substrate and silver conductor. This problem is solved due to the selective coupling of the metal paste  14  via RF energy  18 , in that the substrate  12  is not heated to high temperatures, so only the silver needs to be cooled. 
     The limits in heating and cooling rate are normally governed by the heat power transfer to the total substrate, including the metal paste  14 . Conventionally, the total heat capacity of the substrate  12  and metal paste  14 , e.g. silver, needs to be considered to calculate a required temperature increase for a given power input per second. The heat capacity of the substrate  12  in this example, e.g. silicon, is much larger than silver&#39;s heat capacity, which is present atop the substrate  12 . This disparate ratio ensures that much more power is needed to heat the metal paste when conventional methods are used to achieve the same temperature increase in the same amount of time. 
     Conversely, roughly the same calculation can be applied in determining the necessary cooling power using conventional methods. Much more power is required to cool the substrate  12 , e.g. silicon, when using conventional methods than if the metal paste  14 , e.g. silver, is selectively coupled using RF power  18 . As described above, the benefit in this case is that the substrate  12  itself acts as a heat sink to cool the metal paste  14  many degrees for every few degrees temperature increase in the substrate  12 . This means that, effectively, the metal paste  14  on the substrate  12  can be cooled virtually instantly to the substrate&#39;s temperature. 
     These small dimensions of the silver particles Q, e.g. micro-particles, in the metal paste  14  may be selectively coupled only with very high RF frequencies in the range of 1-50 MHz, or in particular, about 27 MHz. The penetration depth of the RF energy  18  is governed by a formula:
 
δ=503√(ρ/ f*μ   r .)
 
The values used here are μ r =1, ρ=16*10 −−9  Ωm, and f=27*10 6  Hz, wherein
 
     δ=penetration depth (m); 
     ρ=electric conductivity (Ohm m); 
     f=RF frequency (Hz); and 
     μ r =relative magnetic permeability. 
     An estimation based on this formula indicates that the penetration depth δ at 27 MHz is 12 micrometers, which is the same order of magnitude of the size of the silver particles in the metal paste  14  in this example. The relationship between the particle size and the RF frequency necessary for coupling is established such that particles/objects with a typical size that is much larger than the penetration depth will be heated. Generally, the particle size should be greater than six times the penetration depth of the RF field for optimal coupling. However, it is found that RF coupling will work for a particle size equal to penetration depth of the RF field. If the particle size becomes much smaller, the efficiency is reduced. Thus, there is an approximate lower limit to particle size, e.g. the penetration depth of the RF field. 
       FIG. 8  illustrates the relationship between penetration depth δ and the RF frequency f for particles of silver, aluminum and nickel. The lower limit in particle size that can be heated inductively with a given frequency f is determined by the penetration depth, δ=503√(ρ/f*μ r ), where ρ and μ r  are material properties. For given particle sizes, the frequency to be used is subject to a minimum, given by the same equation. 
     For instance: 
     15 μm silver particles require a frequency of (more than) 18 MHz; 
     15 μm aluminum particles require a frequency of (more than) 30 MHz; 
     15 μm nickel particles require a frequency of (more than) 0.8 MHz; 
     10 μm silver particles require a frequency of (more than) 40 MHz; and 
     5 μm silver particles require a frequency of (more than) 160 MHz. 
     Individual excitation of the particles  24  via the RF field heats the metal particles  24  and sinters them together. The efficiency of RF penetration may be increased because of the additional electrical connections in the x and y direction, permitting higher currents to flow. A further insight is that the geometry of the metal paste  14  deposited on the substrate  12  can be used in modifying temperature distribution.  FIG. 2  illustrates a side sectional view of a substrate  12  for an integrated circuit having an applied pattern of a metal paste  14  under an RF coil  16 . When more metal paste  14  is present, e.g. when it is thicker, see element A of  FIG. 2 , the temperature of the metal paste  14  is reduced. Conversely, element B of  FIG. 2  illustrates a thinner layer of the metal paste  14  which would heat more quickly in response to RF coupling. Similar effects may be created with distributions of the layers of the metal paste  14  in the x- and y-axes. 
     This principle can be used in specific types of solar cells, for example. This principle may also be used in other applications, wherein a metal layer may be used to shield sensitive parts of the structure from the RF field. Alternatively, a multi-layer integrated circuit structure may be used, as illustrated in  FIG. 7 . A further insight is that the dimensions in the z axis, e.g. thickness, of the metal paste  14  are determining for the feasibility of the method, and that pre-sintering of the metal particles of the metal paste  14  increases the efficiency of the method. 
     A further insight is that the homogeneity of power transfer from the RF energy  18  into the metal particles of the metal paste  14 , e.g. silver, is greatly enhanced by moving the substrate  12  beneath the RF coil  16  at a predetermined rate. The substrate may be moved at a steady or variable rate depending on the desired application of RF energy  18  to each part of the substrate  12  with metal paste  14 . 
     A further insight is that the homogeneity of power transfer from the RF energy  18  into the metal particles of the metal paste  14 , e.g. silver, is greatly enhanced by modifying the RF energy  18  dependent on the exact substrate position so that each part of metal paste  14  on substrate  12  reaches the same temperature, or a unique temperature desired for that part of the substrate  12 . 
     A multi-layered integrated circuit is illustrated in  FIGS. 7A-7D .  FIG. 7A  illustrates a side view of a multi-layer integrated circuit arrangement  30  having two metal layers  32 ,  34  arranged on the substrate  12 , as shown in  FIGS. 7B and 7C . The first metal layer  32  and second metal layer  34  may include a metal paste  14 , and are shown separated by an insulating layer  36 . According to the present invention, the entire multi-layer integrated circuit arrangement  30  may be subjected to an inductive RF field for simultaneously coupling both the first and second metal layers  32 ,  34 . According to the present teachings, the particle sizes for the metal paste  12  and the RF field strength and frequency should be selected to ensure sufficient penetration depth. 
       FIG. 7D  illustrates an alternative embodiment of a multi-layer integrated circuit arrangement  46  wherein the substrate  12  has two metal layers on each side. One side of the substrate  12  includes the first and second metal layers  32 ,  34 , separated by the insulating layer  36 . The opposite side of the substrate  12  includes third and fourth metal layers  38 ,  40 , separated by an insulating layer  42 . The substrate  12  may include a via  44  filled with metal particles. According to the present invention, the entire multi-layer integrated circuit arrangement  46  may be subjected to an inductive RF field for simultaneously coupling all of the metal layers  32 ,  34 ,  38 ,  40 . The RF field may be provided from two sides of the substrate  12 , simultaneously, so as to effectively inductively couple all of the metal layers  32 ,  34 ,  38 ,  40 . The RF field couples less effectively into the via  44  due to the orthogonal orientation relative to the RF field, so temperature reached in the via is lower than within the 4 metal layers. 
     In this manner, the device and method disclosed provide an effective and inexpensive way in manufacture integrated circuits having a substrate within applied metal paste. 
     While the invention has been illustrated and described in detail in the drawings and forgoing description, such illustration and description are considered to be illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. 
     In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude the plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited and neutrally different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.