Abstract:
Processes for forming waveguides ( 200 ) using multiple co-planar layers of LTCC substrates ( 212, 212   a,    212   b ) are described. Registration holes ( 222 ) on the substrates help align layering of the substrates. Arrays of circuit patterns are printed on each substrate, with each circuit being made up of conductor pattern ( 213 ) and/or via holes ( 224 ). Cavity alignment holes ( 226 ) formed around a periphery of each circuit allow alignment marks to be printed on the substrates for vision inspection. Similarly, circuit orientation holes ( 227 ) associated with each circuit allow orientation marks to be printed on the substrates to identify orientation of circuits in each finally formed waveguide. Substrate orientation holes ( 225 ) allow marks to be printed on one side of each substrate for alignment during screen printing. These in-process vision inspections and quality assurance tests allow product quality and process yields to improve.

Description:
RELATED APPLICATIONS 
     This patent application is a continuation-in-part of patent application Ser. No. 14/135,376 filed on Dec. 19, 2013 (granted as U.S. Pat. No. 9,168,731), which claims benefits from Malaysian patent application number PI 2012701229, filed Dec. 20, 2012, and the disclosure of which is incorporated entirely. 
    
    
     FIELD OF INVENTION 
     The present invention relates to processes for forming waveguides using low temperature co-fired ceramic (LTCC) substrates. In particular, these waveguides are formed by layering co-planar patterns on LTCC substrates. These processes produce improved quality and yield, and the waveguides obtained have enhanced performance and reliability. 
     BACKGROUND 
     Off-set screen printing form thick-film prints comprising ink, paste, or the like on the surface of a substrate using a printing plate (screen mesh); one such type of off-set screen printing is the so-called silk screen printing, which is used to form fine patterns at high production rates; hence, it is utilized in a wide variety of industrial fields. 
     In the field of electronic parts production, screen printing method has been employed from the points of both precision and mass-production. In this field, the demand for forming finer print patterns with high precision has steadily increased due to recent development of technology to miniaturize the sizes of electronic parts. 
     LTCC is now a popular technology for manufacturing high-frequency circuits and is used advantageously to print 3-D circuits within a ceramic block by enabling integration of passive elements such as resistors, inductors and capacitors with fine conductor patterns. This LTCC approach also allows a number of interfaces and the reduction of the overall substrate size. LTCC technology utilizes highly conductive metal and has a low dielectric constant, low surface roughness, low sintering temperature, and good thermal properties. 
     Standard screen printing technology has also been principally developed for hybrid circuit manufacturing. Hybrid circuits are electronic modules printed on ceramic substrates, a technology in between semiconductor integration and discrete realization on PCB technology, and they are commonly used when electronic modules have to meet high technical requirements. The advantages of screen printing technology are well known: versatility in the conception, miniaturization, and mass production at low cost. The thick film components are produced by screen printing of conductive, resistive, and dielectric layers in order to form passive components on an LTCC substrate. Fine line printing is used to achieve high-component density. Therefore, it is important to control each screen printing parameter to improve on the quality of components and the yield of the circuit. 
     In general, screen printing is the basic technology for thick-film micro-circuitry. Many variables will affect the screen printing process. For example, the setting of the screen printer is a manual operation, and the quality of screen printed thick-film strongly depends on the operator and the process variables. The parameter settings affect directly the desired thickness and uniformity of the pastes printed on the substrates. 
     U.S. Pat. No. 6,945,167, assigned to Matsushita Electric, discloses a screen printing apparatus and method. It discloses that the print parameter settings include a squeegee movement speed, a printing pressure, and plate release conditions. The squeegee movement speed is set at the first step, then a printing pressure for realizing a desired cream solder charging state is set at the second step, and then plate release conditions for realizing a desired cream solder transfer state is set at the third step. It does not mention firing, baking, or heating of ceramic printed board, and there is no mention of LTCC or green tape. 
     U.S. Pat. No. 4,817,524, assigned to Boeing, discloses a method for screen printing, drawing a contact edge of a squeegee on the screen in a feed stroke such that a layer of paste is deposited on the screen, and then drawing the squeegee over the screen in a print stroke with the contact edge in contact with the screen so that the paste is forced through the screen onto the substrate. It does not mention LTCC or ceramic printed board. 
     U.S. Pat. No. 5,699,733, assigned to the Industrial Technology Research Institute, discloses a process that requires firing at low temperature, i.e., 500-600° C. However, the process is directed to increase paste layer thickness by subsequent repeated layering up to 6 layers. There is no mention of 3-D circuit or interconnecting circuit layers as required in an LTCC process. 
     U.S. Pat. No. 5,448,948, assigned to Delco Electronics Corp, discloses a screen printing device for screen printing a thick-film paste through a screen so as to form a substantially void-free film on a surface of a microelectronics circuit. It is limited to squeegee design. 
     U.S. Pat. No. 4,604,298, assigned to Gulton Industries, Inc, is directed to the viscosity of conductive paste compound, a high-viscosity gold alloy, firing at 800-900° C. However, there is no mention of ceramics and no mention of 3-D circuitry or embedding of components. 
     U.S. Pat. No. 7,930,974, assigned to Mitsubishi Electric Corp, discloses vacuum suction holes for affixing a substrate to be printed. There is no mention of green tape or ceramic being made. Baking is disclosed for electrode material to form electrodes. 
     U.S. Pat. No. 7,908,964, assigned to Panasonic Corp, discloses specifically to the clearance gap between the screen mask and substrate. There is no mention of ceramic firing, baking, or application for LTCC green tape. 
     Chinese Patent No. 101188260, issued to Shanghai Univ., et al, discloses an LTCC process for fabricating a square or circular cavity as a base for a high-powered LED to be formed on the LTCC layer prior to screen printing. There is no mention of any process control parameters for screen printing. 
     Chinese Patent No. 101777413, issued to Shenzen Sunlord Electronics, discloses a process for forming an LTCC power inductor comprising ferrite magnetic core; it mentions the advantages of high-frequency ceramic material and thinner (finer or higher resolution) screen print lines besides other benefits such as less conductor loss, low dielectric constant, better coefficient of heat conductivity and better exothermic property. The process control parameters disclosed here are applicable for a very specific type of device, i.e., for an LTCC power inductor. 
     It can thus be seen that there exists a need for improving the processes for forming co-planar waveguides using LTCC substrates. 
     SUMMARY 
     The following presents a simplified summary to provide a basic understanding of the present invention. This summary is not an extensive overview of the invention, and is not intended to identify key features of the invention. Rather, it is to present some of the inventive concepts of this invention in a generalised form as a prelude to the detailed description that is to follow. 
     The present invention seeks to provide an improved method for manufacturing waveguides or antennae by stacking multiple layers of LTCC substrates. The LTCC are screen printed with circuit and via patterns. Alignment during stacking the LTCC substrates is crucial to obtaining good quality waveguides and high yield in the manufacturing process. In-process vision inspections and quality control also allow quality and yields of the processes to be attained and maintained. 
     In one embodiment, the present invention provides a process for forming a waveguide using multiple layers of low temperature co-fired ceramic (LTCC) substrates and screen printing. The process comprising: forming a screen printing mask to comprise registration openings that correspond to a predetermined number of spaced apart registration holes located on each sheet of LTCC substrate; placing the screen printing mask over each LTCC substrate and adjusting a printing table of an associated screen printing machine so that the registration openings on the printing mask align with the corresponding spaced apart registration holes on the LTCC substrate; printing circuit patterns by passing and depositing conductor paste through the screen printing mask onto each of the LTCC substrate, and allowing the printed conductor paste to dry in an oven; and stacking up multiple layers of the LTCC substrates with printed circuit patterns by building layers of the LTCC substrates on a stacking machine, wherein registration pins in the stacking machine align the registration holes on separate layers of the LTCC substrates with one another. 
     In-process inspection of depositing conductor paste under UV lighting or X-ray imaging allows continuous monitoring of quality and yield of the waveguide forming process. 
     In an aspect, the circuit pattern comprises an array of circuit patterns. The process further comprises forming cavity openings along peripheries of each circuit pattern in the array, so that cavity alignment marks printed on each substrate allow vision inspection to determine or monitor quality and yield of the waveguide forming process. 
     In another aspect, the process further comprises forming orientation openings in the printing mask to print a circuit orientation mark in each circuit pattern, so that orientation of the circuit pattern can be identified in a finally formed waveguide. 
     In another aspect, the process further comprises forming a substrate orientation opening in the printing mask to print a substrate orientation mark on each LTCC substrate. The substrate orientation mark allows alignment of the substrate in relation to an associated screen printing mask. 
     In another embodiment, the process further comprises disposing the stacked up LTCC substrates on a centre plate of the stacking machine, placing the stacked up LTCC substrates with the centre plate into a vacuum bag, vacuum sealing the vacuum bag and allowing the LTCC substrates to bond together in an oven. The sealed vacuum bag and contents are disposed in a laminating machine. The laminating machine is preheated for a predetermined preheat time, working pressure increased to substantially 21 MPa after the preheat time and holding the laminating pressure for a predetermined set time. The stacked up LTCC substrates are then moved a cutting machine and the circuit patterns are diced along the peripheries to produce individual blocks. The diced LTCC substrates are then fired in a vacuum furnace to fuse the substrates together and each block forms an individual waveguide. 
     In another embodiment, the present invention provides a system for manufacturing waveguides according to the processes of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This invention will be described by way of non-limiting embodiments of the present invention, with reference to the accompanying drawings, in which: 
         FIG. 1A  illustrates a process for manufacturing a waveguide using coplanar LTCC substrates according to an embodiment of the present invention, whilst  FIG. 1B  graphically depicts the process and  FIG. 1C  illustrates a section of the waveguide obtained by the process; 
         FIG. 2A  illustrates a screen mask for use in the process shown in  FIG. 1A , whilst  FIG. 2B  illustrates a mesh screen and  FIG. 2C  illustrates a printed LTCC substrate obtained by using the mesh screen shown in  FIG. 2B ; 
         FIG. 3A  illustrates a schematic of via hole filling station, whilst  FIGS. 3B and 3C  illustrate plates of a stacking machine for use with the process shown in  FIG. 1A ; and 
         FIGS. 4A and 4B  illustrate schematics of the screen printing machine for use with the process shown in  FIG. 1A , whilst  FIG. 4C  lists some parameters of the screen printing machine. 
     
    
    
     DETAILED DESCRIPTION 
     One or more specific and alternative embodiments of the present invention will now be described with reference to the attached drawings. It shall be apparent to one skilled in the art, however, that this invention may be practised without such specific details. Some of the details may not be described at length so as not to obscure the invention. For ease of reference, common reference numerals or series of numerals will be used throughout the figures when referring to the same or similar features common to the figures. 
     The present invention is exemplified in a process  100  of forming waveguides using coplanar LTCC substrates. Patterns are screen printed on the LTCC substrates; the printed LTCC substrates are then stacked up, cured in an oven, vacuum sealed and laminated, cut into individual blocks and fired in a vacuum furnace; the blocks of the LTCC substrates are used to configure giga-hertz waveguides or antennae  200  with enhanced performance and reliability.  FIG. 1A  shows the process  100  of fabricating waveguide using coplanar LTCC substrates whilst  FIG. 1B  shows the process flow in graphics.  FIG. 1C  shows a section of the waveguide  200  obtained by the process  100 . As shown in  FIG. 1A , details of the layered waveguide  200  are designed and simulated, in step  110 . For example, softwares such as, EMPIRE and MultiLib are used to design and simulate conductor layout patterns  213 . A CAD software, such as AutoCAD, is used to draw registration holes and via holes layout patterns  214 . In step  112 , the conductor patterns  213  are transferred to a screen mesh fabrication station, whilst the registration-via holes patterns  214  are transferred to a punching machine  220 . In the screen mesh fabrication station, the conductor pattern  213  of each layer of the layered waveguide  200  is transferred, in step  114 , to mask a photosensitive emulsion  8  that has been coated on the wire mesh  10  of a mesh screen  11  of a screen printing machine  230  for screen printing  130  the patterns on the LTCC substrates  212 . The photosensitive emulsion  8  is then exposed to UV light through each associated conductor pattern  213 . After washing, the UV cured emulsion forms a negative printing mask with pattern openings  9  that correspond to the conductor pattern  213 , via holes  224  pattern and other alignment/orientation holes  225 ,  226 ,  227  which are described in the next paragraph. For easy reference, the patterned printing masks are now denoted by numeral  209 . 
     In addition to the registration-via holes pattern  214 , the CAD software is also used to draw additional substrate orientation holes  225 , cavity alignment holes  226  and circuit orientation holes  227 . As will be described, layout of these substrate orientation holes  225 , cavity alignment holes  226  and circuit orientation holes  227  are also sent to the screen mesh fabrication station and these alignment/orientation holes form parts of the patterned printing masks  209 . As will be appreciated, the substrate orientation holes  225  are located in each of the patterned printing mask  209  corresponding to a fourth corner of the substrate  212  that is without the registration hole  222 ; for illustration, two substrate orientation holes  225  are shown in the figures. As will also be appreciated, the cavity alignment holes  226  are formed as rectangular openings along a periphery surrounding each unit of circuit making up an array of the conductor patterns  213 . These substrate orientation holes  225 , cavity alignment holes  226  and circuit orientation holes  227  are shown as part of the patterned printing mask  209  in  FIG. 2A . 
     Now referring back to  FIGS. 1A and 1B , rolls of green LTCC tapes  204  are fed, in step  116 , into a blanking machine  210 . In one embodiment, the blanking machine  210  includes a platen with blade cutters embedded on the platen. After blanking, the tapes  204  are cut, in step  118 , for example, into substrates  212  of A4 size (210 mm by 297 mm). A Mylar backing sheet  217  is then attached, in step  119 , onto each of the cut substrate  212  by means of a pressure sensitive adhesive (PSA). The substrate  212  with Mylar backing  217  is then transferred to various machines or stations for further processing. The Mylar backing sheet  217  serves as a carrier and prevents the soft, green substrate  212  from being stretched and distorted during mechanical handling and transportation to the punching machine  220 , via filling station, ovens, printing machine  230  and stacking machine  240 . In another embodiment, the substrate  212  with Mylar backing  217  is additionally attached onto a carrier plate  218 . The substrate with Mylar backing on the carrier plate  218  is disposed on the printing table  13  of the screen printing machine  230 , as shown in  FIG. 4B . The LTCC substrates  212  are not limited to A4 size, but it depends on the size of the printing table  13  of the screen printing machine  230  to be used. 
     In step  120 , each of the substrate  212  with the Mylar backing  217  is transferred to the punching machine  220 , such as KEKO pin-punch model PAM-4S. According to the registration holes-via holes pattern  214 , each substrate  212  is punched with three registration holes  222 , with each registration hole being located at each of three corners of the substrate  212 . The significance of the registration holes will be appreciated when a substrate stacking process is described. Some of the substrates  212  are also punched with via holes  224  for forming electrical inter-connections between layers of the substrates. In one embodiment, the registration holes  222  are substantially 3 mm diameter, whilst the via holes  224  are 200 microns diameter. 
     Next, in step  124 , the substrates  212  with via holes  224  are transferred one at a time to a via filling station. As shown in  FIG. 3A , at the via filling station, an electric conductor paste  6  (shown in  FIG. 4A ) is applied on each substrate  212  and a squeeze  4  is used to manually fill the via holes  224  under UV lighting. The via holes  224  are filled up by carefully and uniformly stroking the squeeze to ensure that the via holes  224  are filled up completely with the electric conductor paste  6 . Under UV lighting, quality assurance and determination of complete via filling is made easy. During via filling, care is noted on the squeeze pressure, squeeze speed and squeeze angle to the substrate. In one embodiment, the electrically conductive paste  6  is a ferro silver via fill paste CN  33 - 407  supplied by Ferro Corporation. In another embodiment, an X-ray box  228  is disposed below the substrate  212  and additional quality assurance and determination of complete via filling can be performed before the process  100  proceeds further. The substrates with filled via holes are then transferred to an oven where the electric conductor paste  6  is dried at a temperature of about 70 degree C. for about 5-10 minutes. To exemplify the present invention, six of such substrates  212  with filled via holes  224  are used to fabricate a waveguide  200 . These six substrates  212  with conductor filled via holes  224  are then sandwiched between a top substrate  212   a  and a bottom substrate  212   b , as shown by a sectional view of the waveguide in  FIG. 1C . For ease of description, the top and bottom substrates  212   a ,  212   b  are assumed to be screen printed  130  with the same conductor patterns  213 . In another embodiment, some or all of the six substrates  212  with conductor filled via holes  224  are also screen printed  130 , for example, with other conductor patterns according to circuit design of the waveguides to be manufactured. The screen printing process is now described: 
       FIG. 4A  is a schematic of the screen printing machine  230  for screen printing  130  the conductor patterns  213  onto the top substrate  212   a , bottom substrates  212   b  and substrates  212  with conductor filled via holes. As is well known, the LTCC technology is used to produce multilayer circuits. In other embodiments, sheets of LTCC substrates are screen printed  130  with conductive, dielectric, and/or resistive pastes; these sheets are then stacked up, laminated together and fired in one step. This saves time and money and produces fine circuit lines and line spacings. With low firing temperature of about 850 degree C., it is possible to use low resistive materials, such as silver and gold, instead of molybdenum and tungsten. 
     In one embodiment, the screen printing machine  230  is a KEKO screen printer model P-200Avf. The above screen printing machine  230  comprises: a printing table  13  having suction holes (not shown in the figures) to hold the LTCC substrate  212 ,  212   a ,  212   b  via the Mylar backing  217 ; at least one print station includes a support suitable for receiving a mesh screen  11 , and a translation means for moving the support. The mesh screen  11  comprises a frame  5 , a wire mesh  10 , and a squeegee head  2  that is moveable along the support. The wire mesh  10  is made of fine woven stainless steel wires. The mesh screen  11  with pattern openings  9  (obtained in step  114 ) is positioned on the printing table  13  in the screen printing machine  230 , so that the centre of the mesh screen  11  falls within X-Y adjustment limits to the centre of the printing table  13 ; these X-Y adjustment limits are inherent to the printing machine  230 . Conductor paste  6  is placed on the mesh screen  11  and one of the substrate  212  is placed directly under the mesh screen  11 . A squeegee  4   a  is moved across the mesh screen  11  at a predetermined angle  3 , speed  15 , pressure, and a snap-off distance  7  between the mesh screen  11  and the LTCC substrate. Preferably, the squeegee  4   a  does not spread the conductor paste  6  to the registration holes  222 . After each of the LTCC substrate  212 ,  212   a ,  212   b  with a Mylar backing  217  is loaded on the printing table  13 , vision alignment is done to ensure that the substrate  212 ,  212   a ,  212   b  is aligned with the patterned mask  209 , ie. by aligning openings on the patterned mask corresponding to the registration holes  222  with the registration holes  222  formed through the thickness of the substrates  212 ,  212   a ,  212   b . This alignment process is crucial for manufacturing this multi-layered waveguide  200 . The screen printing process is then carried out after all the parameters are set and vision alignment is completed. In addition, after screen printing, the prints on the substrate  212 ,  212   a ,  212   b  corresponding to the cavity alignment holes  226  can be vision inspected to ensure that the printed circuit patterns are located within tolerances from the registration holes  222 ; this would ensure that the circuit patterns on stacked-up substrates  212 ,  212   a ,  212   b  are within predetermined vertical alignment limits; the circuits in the array that fail the vision alignment inspection may be recorded and identified as rejects; the above vision alignment inspection would ensure that good product quality and process yields are maintained. 
     After the conductor patterns  213 , including the prints corresponding to the substrate orientation holes  225 , cavity alignment holes  226  and circuit orientation holes  227 , are formed on the substrates  212 ,  212   a ,  212   b , each of the printed substrate is moved into an oven, and the printed conductor paste  6  is allowed to dry at about 70 degree C. for about 10 minutes. 
     Referring back to  FIGS. 1A and 1B , the process  100  has now reached step  150 . In step  150 , the LTCC substrates  212 ,  212   a ,  212   b  with printed conductor lines and conductor filled vias are stacked up using a stacking machine  240 , as shown schematically in  FIGS. 3B and 3C . The stacking machine  240  is made up of a base plate  241 , a centre plate  242  and a top plate  243 . The base plate  241  has three registration pins  245 , which are shaped and spaced apart according to the layout of the registration holes  222 . Corresponding to the registration pins  245  are three holes formed in each of the centre and top plates to receive the registration pins  245 . Preferably, the top plate  243  is relatively thicker and heavier than the centre plate  242 . Preferably, the base plate  241  is also relatively thick. To begin the stacking step  150 , the centre plate  242  is guided by the registration pins  245  to rest on the base plate  241 . The bottom LTCC substrate  212   b  is then placed onto the centre plate  242  by aligning the registration holes  222  through the registration pins  245  with the Mylar backing  217  facing up. The Mylar backing  217  is then carefully peeled off, and the next LTCC substrates  212  are sequentially mounted one on top of another until all the eight layers of LTCC substrates are aligned and stacked-up. The top plate  243  is then added on top of the upper LTCC substrate  212   a  by guiding the top plate  243  through the registration pins  245 . The top plate  243 , being relatively heavier than the centre plate, provides a light pressure on the stacked-up substrates  212 ,  212   a ,  212   b . The entire stacking machine  240  is then moved into an oven, which is set to about 70 degree C. for about 5-10 minutes to bond the stacked-up substrates together. After the entire stacking machine  240  is retrieved from the oven, the top plate  243  is removed. The centre plate  242  together with the stacked-up substrates are then separated from the base plate  241 . An additional quality inspection step may be performed by testing the interlayer electric conductivity, for example, by using a resistance meter or a multi-meter. The circuits in the array which fail the electric conductivity tests are recorded and identified as rejects. 
     After the interlayer electric conductivity inspection, the centre plate  242  and the stacked-up substrates are placed into a vacuum bag  247 , which is then evacuated and sealed. The sealed stacked-up substrates together with the centre plate  242  are then moved into an isostatic laminating machine  250 . The laminating machine  250  is set to a temperature of about 70 degree C. and a pressure of about 21 MPa. In use, the laminating machine  250  may be set at a preheat temperature for about 10 minutes followed by setting the working laminating temperature and pressure for about 10 minutes. This completes the laminating step  170  shown in  FIG. 1A . 
     After the stacked-up substrates are laminated, the component layers (as seen in  FIG. 1C ) are bonded together but the LTCC substrates remain soft enough for cutting or dicing, in step  180 , into individual blocks with each block containing circuits for configuring into an individual waveguide  200 . In one embodiment, a hot blade cutting machine  260  is used for dicing the LTCC substrates into individual blocks. The diced blocks of LTCC substrates are then placed, in step  190 , in a vacuum furnace  270  for firing the ceramic material. In one embodiment, the vacuum furnace  270  uses quartz heaters to fire up the furnace to about 850 degree C. After furnace firing, the layered LTCC substrates become fused together. 
     In the present invention, the screen printing parameters follow substantially those described in the parent application Ser. No. 14/135,376.  FIG. 4C  lists some parameters of the screen printing machine  230 . However, in the present invention, the conductor line and spacing widths are allowed to increase to 150 microns. 
     While specific embodiments have been described and illustrated, it is understood that many changes, modifications, variations and combinations thereof could be made to the present invention without departing from the scope of the invention. For example, the stacked-up substrates have been described by building upwards from the bottom substrate  212   b ; the effects of this invention are not changed by layering the stacked-up substrates from the top substrate  212   a . In another example, three registration holes  222  are formed on each LTCC substrate; it is possible that only two registration holes are provided and used in the present invention, preferably with the registration holes being spaced apart. Also, the LTCC substrates are not limited in shape and size as described above. The above LTCC manufacturing process is also not limited to forming conductor lines and vias, and may include other circuit elements.