US 3905094 A
Disclosed is a thermal display module having an array of heater elements, thermal drive matrix, character generator, and deposited conductive interconnections therebetween disposed upon an insulating substrate. The heater elements and drive matrix are initially formed as integral portions of a semiconductor wafer, the wafer being mounted to the substrate at one face by way of thickened or plated up contacts on the wafer interconnecting with a conductive pattern on the substrate, the heater elements extending to the opposite face in communication with thermally sensitive record material. Trapezoidal shaped heater elements are provided as a result of a unique sequence of processing steps.
Claims available in
Description (OCR text may contain errors)
United States Patent Ruggiero 1 Sept. 16, 1975 THERMAL DISPLAY MODULE Edward M. Ruggiero, Dallas, Tex.  Assignee: Displaytek Corporation, Dallas, Tex.
 Filed: Oct. 15, 1973  Ap l. No.: 406,365
Related US. Application Data  Continuation of Ser. No. 216.537, Jan. 10, 1972. abandoned, which is a division of Scr. No. 79,505, Oct. 9, 1970, Pat. No. 3,700,852.
Primary Examiner-W. Tupman Attorney, Agent, or Firm-Kenneth R. Glaser [5 7 ABSTRACT Disclosed is a thermal display module having an array of heater elements, thermal drive matrix, character generator, and deposited conductive interconnections therebetween disposed upon an insulatingsubstrate. The heater elements and drive matrix are initially formed as integral portions of a semiconductor wafer, the wafer being mounted to the substrate at one face by way of thickened or plated up contacts on the wafer interconnecting with a conductive pattern on the substrate, the heater elements extending to the opposite face in communication with themially sensitive record material. Trapezoidal shaped heater elements are provided as a result of a unique sequence of processing steps.
5 Claims, 9 Drawing Figures 57PN N+ PATENTEB SEP 1 6 I975 SHEET 5 0f HZJA AAAAAA A VRRQA THERMAL DlSPLAY MODULE The present application is a continuation of application Ser. No. 216,537, filed Jan. 10, 1972, now abandoned, and which is a division of application Ser. No. 79,505, filed Oct. 9, 1970, now U.S. Pat. No. 3,700,852.
The present invention relates tothermal displays, more particularly to thermal printing, and even more particularly to the process for fabricating, and the resulting product of, a thermal printing module of the type having an array of thermally isolated heater elements selectively energizable to produce desired information representation on thermally sensitive record material.
The search for new and better techniques for information display has extended the technology into a relatively new area of non-impact recording utilizing heat as the source of the printing or display energy. In accordance with this technique, a matrix of heating elements is selectively energized to produce a predetermined pattern of hot spots" in accordance with the desired information representation (letter, number, etc.). The specific letter or number, for example, is then reproduced on thermally sensitive record material in contact with this matrix. Thus, if a number 4 is desired to be displayed or printed, the heating element matrix will be energized in a pattern corresponding to this number, the number 4 thereby being defined or printed" upon the thermally sensitive record material.
The heart of the thermal display is the print module itself which contains the array of heating elements. In the last several years, various different designs of these print modules have been produced. One such design involves the selective deposition ofheat dissipative elements in the desired array upon a ceramic substrate. Another technique utilizes semiconductor technology to provide a plurality of thermally isolated discrete semiconductor portions or mesas providing the thermal printing elements upon an insulating substrate, various circuit components being formed within these mesas to effect the required temperature rise of the surrounding semiconductor material.
While the semiconductor approach generally offers advantages over the formerly recited approach, there has been a number of disadvantages associated with presently proposed semiconductor structures. First, external ohmic connections to either the components disposed within the semiconductor mesas or to the thermal drive matrix components formed in the same substrate as the printing elements normally require ball bonded leads which extend through openings in the substrate, and have generally proven unreliable. Second, the logic circuitry for actuating the thermaldrive matrix components has been external to, and separately disposed from, the print module, an extremely large number of individual wires necessarily extending from the external logic to the drive matrix of the module. This necessity has placed a limitation on the size as well as the overall reliability of the entire apparatus, particularly where the print module itself moves with respect to the thermally sensitive record material. Third, presently employed fabrication techniques for forming the printing elements or mesas themselves have prevented the maintaining of accurate and precise control over the size and shape of, and the spacing between, the printer elements, thus hampering the ability to achieve the necessary definition of the spot recorded on the thermally sensitive record material.
It is therefore a primary object of the invention to provide an improved thermal display.
It is another object of the invention to provide an improved thermal display unit as the type employing a print module including an array of thermally isolated semiconductor heater elements wherein the unique electrical interconnection between, and mounting arrangement of, the various portions of the unit offer increased compactness and reliability.
It is a further object of the invention to provide an improved process for the fabrication of semiconductor devices having circuit components formed within discrete regions or physically separate portions of semiconductor material disposed upon a common substrate.
It is a still further object of the invention to provide an improved method of fabrication of a thermal print module.
It is an additional object of the invention to provide an improved method of forming a plurality of thermally isolated physically separate semiconductor heater portions of a thermal display wherein accurate and precise control is maintained over the size and shape of, and the spacing between, the heater portions.
in accordance with these and other objects, features, and improvements, the present invention is directed to a thermal display or printing module wherein the array of heating elements, drive matrix for selectively energizing the heating elements, and character generator for selectively actuating the components of the drive matrix are all disposed upon a single substrate. The components associated with the heating elements and drive matrix are initially disposed within a single wafer of semiconductor material, the electrical interconnections between these components extending on the wafer to enlarged and built up terminal portions for direct bonding to the metallization pattern on the substrate, thus eliminating the need for ball bonding through the substrate. The entire module is adapted for insertion or plugging into external housing.
Additionally, a unique sequence of fabrication steps are utilized in the formation of the thermally isolated heater elements, as well as the attachment of the semiconductor wafer in which the elements are formed to the supporting substrate. A preferential etching operation produces a plurality of trapezoidal shaped heater elements, which size, shape, and spacing are precisely controlled or determined by this etching operation.
The novel features believed to be characteristic of the invention are set forth in the appended claims. The invention, itself, however, as well as other objects and advantages thereof, may best be understood by reference to the following detailed description when read in conjunction with the accompanying drawings, in which:
FIG. 1 is a top perspective view of the thermal printing or display module of the present invention, shown in conjunction with thermally sensitive record material in communication therewith;
FIG. 2 is a block diagram schematic of the electrical interconnections between and showing the operation of the various subsystems of the module illustrated in FIG.
FIG. 3 is a circuit schematic of the components associated with the heater elements and the thermal drive networks, illustrating their interconnection;
FIG. 4 is a top pictorial view of the semiconductor wafer of the module illustrating the shape and design of the improved heater elements;
FIGS. 5-8 are sectional views taken along the line 88 of FIG. 4 illustrating the fabrication steps of the structure shown therein; and
FIG. 9 illustrates an interconnection pattern of the heater elements and thermal drive matrix with the metallization pattern on the supporting substrate of the module, in accordance with the invention.
The drawings are not necessarily to scale but in some instances have been exaggerated to emphasize the features of the invention.
Referring initially to FIG. 1, the thermal printing head or module 10 is illustrated incorporating the design of the present invention. Accordingly, the module includes a substrate 11 of electrically insulating material having an elngated major face 12, for example having a dimension of two inches by one inch, and edge portions 13 of minimal thickness. normally one sixteenth to one quarter inch,
Disposed upon the major face 12 is a semiconductor wafer 14 having a first area or region 15in which is disposed an array of heater elements 60, and a second area or region 16 in which is disposed a drive matrix network 28 for selectively energizing the elements 60. The area 16 is preferably disposed around the area 15, the area 16 shown in FIG. 1 as extending around three sides thereof.
As subsequently described in greater detail, the heater elements 60 are discrete portions of the semiconductor material of the wafer 14 substantially thermally isolated from one another along the top surface or face 17 of the wafer. Disposed respectively within each of the elements 60 are heat dissipative components formed at the interface of the bottom surface of the wafer 14 and substrate face 12, the driver network 28 disposed within the region 16 comprising a plurality of circuit components respectively electrically connected with these heat dissipative elements. The drivers 28 selectively energize and furnish power to the heater elements in accordance with the desired information display. Corresponding marks are thus produced on thermally sensitive record material 19 passing over the surface 12 in thermal contact with the selectively energized heating elements.
Spaced from the wafer 14, but disposed upon the same major face 12 ofthe substrate 11, is an integrated circuit module 18 comprising a plurality of electrically interconnected circuit components providing a character generator for selectively triggering or actuating the circuit components of the driver matrix 28. The module 18 may be an integrated circuit of the monolithic or hybrid type known in the art having its components appropriately interconnected to provide the desired logic network. In accordance with a preferred embodiment, however, the module 18 is an MOS (metal-oxidesemiconductor) array having its components formed at the interface of the module 18 with the substrate surface 12. As subsequently described, the character generator module 18 has a substantially smaller number of input terminals than output terminals, the output terminals being coupled to the thermal driver matrix 28.
The output terminals of the logic network of the character generator 18 are electrically connected to the input terminals of the driver matrix 28 by a plurality of leads provided by a metallization pattern 20 formed upon and adherent to the major surface 12. This pattern may be formed, for example, by the selective deposition of the metal directly upon the substrate in the desired configuration. As a specific feature, this metallization pattern, which may be of gold for example. extends to the bottom surface of the wafer 14 where it is uniquely bonded with metallization on the wafer to in terconnect the pattern with the driver components disposed within the area 16, as subsequently described in greater detail.
Another metallization pattern 21, also of gold for example, is formed upon and adherent to surface 12, and is electrically connected to the input terminals of the logic array of the character generator 18, this pattern 21 extending to the trailing edge 22 of the substrate 11 where it assumes a configuration of a plurality of expanded tabs 30-41, these tabs providing the input ter minals of the print head 10 for connection with exter nal circuitry (not shown) for appropriately actuating the logic network of generator 18.
An extremely thin layer 25 of glass, for example, is provided over the metallization patterns 20 and 21 (the broken line outline indicating where, for illustrative purposes only, a portion of the layer 25 has been removed in order to show the underlying metallization pattern) to protect these leads from the abrasive action of the movement of the record material 19 with respect to the surface 12. As a particular feature of the structure of the invention, the ends of the tabs 30-41 remain exposed, as illustrated, to enable the entire module 10 to be plugged into a housing containing the external circuitry, the exposed tab portions making direct electrical contact with electrical contacts of the external circuitry.
Referring now to FIGS. 2 and 3, the operation of the thermal print module can be briefly described. Disposed within each of the semiconductor heater elements is an active and heat dissipative circuit component pair, the active circuit component, when triggered, causing current to flow through the dissipation means, thereby resulting in the consequent rise in temperature of the surrounding element 60. When the heating elements 60 are arranged in a desired array (for example, the 5X7 array illustrated in FIG. 2), triggering of the active circuit components in selected elements 60 can thereby result in the selective heating of these thermally isolated elements in a defined pattern of information. For example, as illustrated in FIG. 2, selected elements 60' may be heated in the pattern of the letter Thus, when thermally sensitive record material, such as 19, is in thermal communication with this pattern, the letter A may be displayed or printed thereon.
As illustrated in FIG. 3, the active and heat dissipative component pair is provided by transistor 84 and collector resistor 83, respectively, which pair are disposed within each of the thermally isolated semiconductor portions 60. Desirably, the transistor 84 is formed by conventional diffusion techniques in the body of the semiconductor portion 60, a portion of this semiconductor material then providing the resistor 83 which is thus integrally joined through the semiconductor material to the transistor 84 collector.
Energization of the heating elements 60 is effected by the plurality of driver networks 28 all disposed within the area 16 of the wafer 14. Each of these networks, which are equal in number to the number of heating elements 60, comprise transistor 80 and emitter resistors 81 and 82. The outputs from these driver networks are respectively electrically connected to the inputs or base terminals of the transistors 84 by the electrical interconnections 85, so that'when a pulse is applied to the input terminal A of selected driver networks, the respectively interconnected heater component pair is-energized, and there is a resulting temperature rise in the corresponding heater element 60.
The input pulses to the terminals A of the driver network are provided by the character generator 18. The generator 18,.the circuitry of which may be any generally known in the art, is adapted to receive a coded digital word at the input terminals B through B provided by expanded contact tabs 31 through 37 and thereby provide the desired output signals to the terminals A of the driver network. In the illustrated example, the generator 18 can receive a 7-bit digital word. Thus, assuming that the letter A is to be displayed or printed, the coded digital work corresponding to the letter -A" for example 0000001) is applied toterminals B B As a consequence, the generator 18 circuitry produces output signals ,which.are applied to input terminals A of the specific driver. networks 28 electrically connected to the transistors 84 which aredisposed within the heater elements 60, thus resulting in the selected heating of these elements 60 to define the letter A.
Expanded tabs 30, 38 and 41 provide the terminal connectionsto ground, supply voltage V,,,, to generator l8, and supply voltage -V to .drive transistors 81 and heater transistors 84, respectively. Tabs 39 and 40 provide terminal connections to external temperature compensating resistors R and R (not shown).
With reference to the cross-sectional-views of FIGS. 5-8 as well as FIGS. 1, 4 and 10, the fabrication of the thermal printing head or module l0-is now described, particularly the formation of the array of heating elements 60 and the thermal drive networks 28 within the wafer 14.
Accordingly, single crystal N-type semiconductor material, such as silicon, may be used asthe starting material for the wafer 14. A portion of this wafer is illustrated in FIG. 5. Utilizing conventional maskingand diffusion techniques, the heater transistor-resistor pairs (T -R and the driver network components (transistor T resistor R8]- resistor R are formed within the substrate 14, the oxide layer 50 being formed during the diffusion operations and having the stepped configuration illustrated. P-type resistors R,, and R (not shown) are desirably formed simultaneously-with the diffusion of the base regions of the transistors '11, and T801 while heat dissipation resistors R, are actually providedby an extension of, and are an integral portion of, the collector regions of the transistors T provided by the starting N-type material.
The electrical interconnection of the various'circuit components is then effected by providing openings in the oxide layer 50 where contact is necessary, and the deposition and selective removal of a metal film to provide the desired interconnection pattern, as illustrated. This metal over oxide contact pattern includes theleads 85 which interconnect one side of the resistors R with the base of heater transistor T thus providing the interconnection between a drive network 28 and a heater network, as show in FIG. 3. while .contact pattern including the lead 85 is specifically illustrated as, and may be formed of. a single metal layer, as gold, it
may be desirable to utilize a multi-laycr structure, for example platinum, titanium, platinum, and gold, the gold providing the top layer thereof. Low resistance N+ regions may be diffused at the location of contact to collector resistors R all as known in the artv As the next step in the fabrication operation. select portions of the interconnection pattern are built up or thickened, these thickened portions being subsequently connected with the deposited leads (20, 30, 39, 40, 41) on the substrate 11. Accordingly, by utilizing known deposition, masking, and plating techniques, thickened contact portions 51 are formed along the periphery of the wafer 14 on, for example, the base lead of the drive transistor T as illustrated in cross sectionalview FIG. 5. These plated up portions 51 which preferably are of gold, for example, would also be formed on the emitter leads to the heater transistors T collector lead to transistor T etc.
The structure of FIG. 5 is then turned upside down (the orientation of the components then being as shown in FIG. 6) and temporarily mounted to a carrier (not shown) by means of a soluble adhesive. The substrate 14 may then be lapped to a thickness of, say 34 mils. A thin layer 54 of masking material, for example silicon dioxide, is formed on the wafer 14 on the side opposite the diffusions and metallization. Utilizing known photographic masking and selective etching techniques, select portions of the silicon dioxide layers 54, 50, and of the N-type silicon substrate are removed around the periphery 58 of the wafer 14 to expose the lead pattern at the situs of the plated up portions 51. These removed portions are illustrated by the dotted lines in FIG. 6 for one such situs, the view being taken along only the section line 88, it being understood that this removal occurs to expose all plated up portions around the periphery 58 of the wafer.
Referring now to FIG. 7, utilizing conventional masking and etching techniques, select portions 55 are now removed from the oxide layer 54 to expose the underlying semiconductor material of the wafer 14 between the area 16 in which the drive matrix components (T R and R,,,,) are formed and the area 15 in which the components (T R83) of the heater elements are formed, as well as between adjacent transistor-resistor pairs (T and R,,;,).
The resulting structure is then removed from the carrier and mounted to the insulating substrate 11 so that the thickened or plated up portions 51 contact and are aligned with the metallization pattern on the face 12 of the substrate. Accordingly, as illustrated in FIG. 7, the plated up beam 51 connectedto the base of drive transistor T contacts the external lead 20 from the character generator 18, thus providing the input terminal A to the drive network 28. In similar manner, other thickened or plated up portions 51 around the periphery 58 of the wafer 14 will contact other external leads 20, 30, 39, 40, and 41 to provide additional input terminals A, ground connection to transistors T resistor R and R connections, and supply voltage V,,, connection to resistor R respectively.
An adhesive material 57 is next inserted within the void between the wafer 14 and the electically insulating substrate 11, as well as over the situs of the plated up leads 51. The material thus increases the adhesion of the wafer 14 to the insulating substrate 11 while also providing protection for the interconnections and encapsulation of the components.
The exposed semiconductor material beneath the oxide openings 55 is then selectively removed, leaving the raised thermally isolated discrete semiconductor portions 60 with the transistor-resistor pairs T -R formed therein, and the networks 28 (including transistors T and resistors R, and R f rmed within the surrounding portion 16 of wafer 14, as illustrated in FIG. 8.
The etching of the semiconductor material between the openings 55 may be effected by any etchant which selectively removes the silicon while leaving the oxide layer 54 substantially unaffected. In accordance with a specific feature of the invention, however, the removal of the semiconductor material beneath the openings 55 is carried out by an etchant, for example a dilute solution of sodium hydroxide, which preferentially etches the silicon semiconductor material along the 100 plane. The oxide mask 54 is then removed in a conventional manner.
As a consequence of these fabrication steps, the resulting structure illustrated in FIG. 4 (and partial sectional view FIG. 8) is produced. The array of printing elements 60, which are substantially thermally isolated from one another, and the matrix of driver networks 28, disposed within area 16, are integrally joined by the unremoved oxide material on the semiconductor wafer 15. The area 16 desirably surrounds the area 15 in which the heater element array is disposed, thus contributing to the overall compactness of the structure. In the specific embodiment illustrated, the area 16 surrounds the array of heater elements on three sides, although it may be desirable to completely surround the area 15 on all four sides under certain circumstances.
The various circuit components of the heater element array and the driver matrix, as well as the conductive interconnections therebetween, are formed at the interface of the wafer 14 and the supporting substrate 11, and are thus positioned away from the surface over which the thermally sensitive record material passes. The interconnection pattern on the back face of wafer 14 (one example being that illustrated in FIG. 9) selectively interconnects the various circuit components within the wafer and, as previously described, have enlarged or thickened bonding pads 51 which are bonded directly to the metallization pattern on the substrate 11. These bonding pads 51, which are disposed around the periphery of the wafer I4, not only provide the conductive coupling between the external leads 20, 30, 39, 40, and 41 and the conductive interconnect pattern on the wafer 14, but provide the support or mounting of the wafer itself upon the substrate 11.
As mentioned, the thermal printing elements 60 are substantially thermally isolated from one another along the top surface of the wafer 14, as well as through the supporting substrate. In this regard, it is desirable that the adhesive material 57 which joins the elements 60 to the substrate 11 be of sufficiently low thermal conductivity to avoid thermal cross talk between adjacent elements. Examples of such material includes epoxy, silicone, and various types of glass.
As a consequence of the previously described preferential etching of the semiconductor material along the 100 plane to provide the physically separated wafer portions 60, several advantages may be uniquely achieved thereby. First, the resulting configuration of each of the elements 60 is trapezoidal (as shown in FIG. 4), the top surface of each of the trapezoids having essentially square corners, thus providing excellent definition of the printing spot imprinted upon the record material 19. In addition, the preferential etch enables accurate and precise control over the size and spacing between the heater elements.
As a final step in the fabrication of the thermal print module 10, the character generator 18 is secured to the face 12 of substrate 11, the input and output terminals theeof respectively bonded with the metallization patterns 21 and 20 on face 12. The substrate 12, in accordance with a specific feature of the module design, serves not only as a support or carrier for the character generator, thermal drive matrix, printing elements, and metal interconnection patterns therebetween, but also as a heat sink for the isolated printing elements after each printing operation. To this end, the substrate 12 is desirably formed ofa material which is not only electrically insulating, but also is a good thermal conductor. An example of one such material is alumina.
The structure of the thermal module 10 of the present inventin thus offers considerable advantages over prior structures. First, the consolidation of the heater elements, drive matrix, character generator, and interconnections therebetween on a single substrate not only offers advantages due to the compactness of the overall structure, but reduces the number of leads that must extend to the module from an external source. This is particularly advantageous since the module 10 normally moves with respect to the record 19, and any external leads extending from or to the module must necessarily be moved also. In particular, the incorporation of the character generator 18 with the module 10 and its interconnection with the wafer 14 by way of metallization adherent to the substrate surface eliminates the requirement of a large number of separate lead wires to be attached to the module.
Secondly, by fabricating the module 10 to the size and shape described herein, and by utilization of the extending tab portions 3041, the entire module may be plugged into external housing with all the attendant advantages thereof.
Various modifications may be made to the specific embodiment described herein. For example, the module 10 has been illustrated in conjunction with thermally sensitive paper 19, the cooperation between these elements thereby utilized in thermal printer apparatus. Alternatively, the record material 19 may be formed of a material which is permanently disposed over the array of heater elements, portions of this material adapted to change state when subjected to heat. Thus, the resulting apparatus would be in the nature of a display rather than a hard copy" printer.
Furthermore, the 5 7 array of heater elements is given herein as an example since any number and shape of the array may be chosen depending upon the character of the information desired to be printed or displayed on the cooperating thermally sensitive record material.
Various other modifications of the disclosed embodiment, as well as other embodiments of the invention, may become apparent to persons skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims.
What is claimed is:
I. A method of fabricating a thermal printing device of the type including a first array of heat dissipative elements, selected ones of which, when energized, define a form of information representation and a second group of circuit components for respectively energizing said heat dissipative elements. said method comprising:
a. forming said first array of heat dissipative elements within a first confined area, and at one major face, of a body of semiconductor material and said second group of circuit components within a second area, and at the same major face, of the same said body of semiconductor material so that said second area is spaced from, but at least partially surrounds, said first confined area;
b. selectively forming thin film conductive means at said one major face to provide an electrical interconnection pattern between selected ones of said heat dissipative elements and selected ones of said second group of circuit components, said electrical interconnection pattern being of a first defined thickness,
c. forming conductive bonding pad portions of a thickness substantially greater than said first defined thickness, said conductive bonding pad portions being formed over said thin film conductive means solely at locations at the outer periphery of said second area,
(1. mounting said semiconductor body to a supporting insulating substrate having a metallization pattern disposed thereon so that said thin film conductive means at said one major face is adjacent to, but raised and separated from, said supporting substrate with said conductive bonding pad portions in physical contact with end portions of said metallization pattern, said substantially thicker bonding pad portions thereby providing the sole support for said semiconductor body; and
e. selectively removing semiconductor material from the other major face of said semiconductor body within said first confined area as well as between said first confined area and said second area, thereby to substantially thermally isolate said heat dissipative elements from one another as well as from said second group of circuit components along said other major face.
2. The method as claimed in claim 1 further including the step of providing protective wearresistant material over said bonding pad portions and said metallization pattern at least at the location of the contact of said end portions with said bonding pad portions.
3. The method as claimed in claim 1 wherein said removal of said semiconductor material is by etching,
4. The method as defined in claim 3 wherein said etching is effected along the plane of the semiconductor material.
5. The method as defined in claim 3 wherein said heat dissipative elements are formed simultaneous with the formation of said circuit components.