Scalable wide-array inkjet printhead and method for fabricating same

A scalable wide-array printhead is formed by mounting multiple thermal inkjet printheads to a carrier substrate. The printheads are mounted to one face and logic ICs and drive ICs are mounted to an opposite face. Interconnects are formed through the carrier substrate to electrically couple the printheads to the logic ICs and drive ICs. The carrier substrate is formed of silicon and etched to define ink refill slots. A solder bump mounting process is used to mount the printheads to the carrier substrate. Such process serves to align each of the printheads. The solder forms a fluidic boundary around a printhead ink slot.

BACKGROUND OF THE INVENTION 
This invention relates generally to inkjet printhead construction, and more 
particularly, to a wide-array inkjet printhead construction. 
There are known and available commercial printing devices such as computer 
printers, graphics plotters and facsimile machines which employ inkjet 
technology, such as an inkjet pen. An inkjet pen typically includes an ink 
reservoir and an array of inkjet printing elements, referred to as 
nozzles. The array of printing elements is formed on a printhead. Each 
printing element includes a nozzle chamber, a firing resistor and a nozzle 
opening. Ink is stored in an ink reservoir and passively loaded into 
respective firing chambers of the printhead via an ink refill channel and 
ink feed channels. Capillary action moves the ink from the reservoir 
through the refill channel and ink feed channels into the respective 
firing chambers. Conventionally, the printing elements are formed on a 
common substrate. 
For a given printing element to eject ink a drive signal is output to such 
element's firing resistor. Printer control circuitry generates control 
signals which in turn generate drive signals for respective firing 
resistors. An activated firing resistor heats the surrounding ink within 
the nozzle chamber causing an expanding vapor bubble to form. The bubble 
forces ink from the nozzle chamber out the nozzle opening. A nozzle plate 
adjacent to the barrier layer defines the nozzle openings. The geometry of 
the nozzle chamber, ink feed channel and nozzle opening defines how 
quickly a corresponding nozzle chamber is refilled after firing. To 
achieve high quality printing ink drops or dots are accurately placed at 
desired locations at designed resolutions. It is known to print at 
resolutions of 300 dots per inch and 600 dots per inch. Higher resolution 
also are being sought. There are scanning-type inkjet pens and 
non-scanning type inkjet pens. A scanning-type inkjet pen includes a 
printhead having approximately 100-200 printing elements. A non-scanning 
type inkjet pen includes a wide-array or page-wide-array printhead. A 
page-wide-array printhead includes more than 5,000 nozzles extending 
across a pagewidth. Such nozzles are controlled to print one or more lines 
at a time. 
In fabricating wide-array printheads the size of the printhead and the 
number of nozzles introduce more opportunity for error. Specifically, as 
the number of nozzles on a substrate increases it becomes more difficult 
to obtain a desired processing yield during fabrication. Further, it is 
more difficult to obtain properly sized substrates of the desired material 
properties as the desired size of the substrate increases. 
SUMMARY OF THE INVENTION 
According to the invention, a scalable wide-array printhead structure is 
formed by mounting multiple thermal inkjet printheads to a carrier 
substrate. Each printhead includes a plurality of printing elements. Each 
printing element includes a nozzle chamber, a firing resistor and a nozzle 
opening. In addition respective wiring lines couple each firing resistor 
to a contact on the printhead. By prescribing a different number of 
printheads to a carrier substrate for different embodiments, different 
sized wide-array printhead structure embodiments are achieved. Thus, one 
advantage of the mounting methodology is that a scalable printhead 
architecture is achieved. 
According to another aspect of the invention, a solder bump mounting 
process is used to mount the printheads to the carrier substrate. A 
benefit of such solder bumps is that they serve to align each of the 
printheads along the carrier substrate. Wetting pads for receiving solder 
are precisely placed on the carrier substrate and printheads using 
photolithographic or other accurate processes. Once the solder is placed 
and heated into a liquid solder reflow occurs. During solder reflow such 
pads tend to align with each other and thus align the printhead to the 
carrier substrate. 
In one embodiment the printheads include an underlying ink slot along a 
surface of the printhead facing the carrier substrate. According to 
another aspect of the invention, the solder for mounting a printhead forms 
a ring around such ink slot. The solder ring serves as a fluidic boundary 
for the ink slot. As a result an encapsulant is not required for fluidic 
isolation of the printhead ink slots. 
Contacts for the printheads typically are formed along the same surface as 
the nozzle openings. According to another aspect of the invention, a 
front-to-back interconnect is formed through the printhead to interconnect 
the printhead to the carrier substrate. According to an alternative aspect 
of the invention a wire bonded interconnect is formed from the printheads 
contacts to carrier substrate contacts. The wire bonded interconnects 
extend outside the printhead rather than through it. 
According to another aspect of this invention, the carrier substrate does 
not contain integral devices of active electronic circuits. With regard to 
electrical features, the carrier substrate merely includes interconnects 
for electrically coupling the contacts of the respective printheads to 
logic circuitry and drive circuitry. Because the carrier substrate is not 
used for active electronic circuits the silicon used can be a lower grade, 
less expensive silicon. Such silicon is chosen to have a crystalline 
orientation useful for obtaining a desired ink refill slot profile. 
According to another aspect of the invention, logic circuitry and drive 
circuitry integrated circuit chips are mounted to the carrier substrate 
and interfaced to the printheads to control the printing elements of the 
multiple printheads. The carrier substrate with the logic ICs, drive ICs 
and printheads is referred to herein as the wide-array printhead. 
According to another aspect of the invention, the printheads are mounted to 
one face of the carrier substrate and the logic ICs and drive ICs are 
mounted to an opposite face of the carrier substrate. Interconnects are 
formed through the carrier substrate to electrically couple contacts of 
the printheads to contacts of the logic ICs and drive ICs. A front-to-back 
metallization process is used to form the interconnects through the 
carrier substrate. 
According to a preferred embodiment, the carrier substrate is formed by the 
same material as the printhead substrate, (e.g., silicon). An advantage of 
using the same material for the carrier substrate and printheads is that 
coefficient of thermal expansion mismatches are avoided. In another 
embodiment the carrier substrate is formed of a multilayered ceramic 
material having a coefficient of thermal expansion matched to silicon. The 
material for the substrate is etched to form an ink refill slot for ink to 
flow from a reservoir to the printheads through the carrier substrate. 
These and other aspects and advantages of the invention will be better 
understood by reference to the following detailed description taken in 
conjunction with the accompanying drawings.

DESCRIPTION OF SPECIFIC EMBODIMENTS 
Overview 
FIG. 1 shows a wide-array inkjet pen 10 according to an embodiment of this 
invention. The pen 10 includes a wide-array printhead 12 and a pen body 
14. The pen body 14 serves as a housing to which the printhead 12 is 
attached. The pen body 14 defines an internal chamber 16 which serves as a 
local ink reservoir. In various embodiments the reservoir is a replaceable 
or refillable reservoir. In one embodiment the reservoir is coupled to an 
external reservoir which supplies the local reservoir. In another 
embodiment the reservoir is non-refillable. 
Referring to FIGS. 1 and 2, the printhead 12 includes a plurality of 
thermal inkjet printhead dies 18 mounted to a carrier substrate 20. The 
printheads dies 18 are aligned in one or more rows 26 on a first surface 
28 of the carrier substrate 20. Each one of the printheads dies 18 
includes a plurality of rows 22 of inkjet printing elements 24, also 
referred to as nozzles (see FIG. 4). In the embodiment of FIGS. 1, 2 and 4 
the printheads dies 18 are aligned end to end with the respective rows of 
each printhead die also being aligned. 
The carrier substrate 20 is made of silicon or a multilayer ceramic 
material, such as used in forming hybrid multichip modules. The substrate 
20 preferably has a coefficient of thermal expansion matching that of 
silicon, is machinable to allow formation of an ink slot, is able to 
receive solder and interconnect layers, and is able to receive mounting of 
integrated circuits. 
Each printhead die 18 includes an array of printing elements 24. Referring 
to FIG. 5, each printing element 24 includes a nozzle chamber 36 having a 
nozzle opening 38. A firing resistor 40 is located within the nozzle 
chamber 36. Referring to FIG. 6 wiring lines 46 electrically couple the 
firing resistor 38 to a drive signal and ground. Referring again to FIG. 
5, each printhead die 18 also includes a refill slot 42. Ink flows from 
the internal reservoir within chamber 16 through one or more carrier 
substrate refill channels 32 to the refill slots 42 of the printhead die 
18. Ink flows through each printhead refill slot 42 into the printhead 
nozzle chambers 36 via ink feed channels 44. 
In one embodiment one or more of the printhead dies 18 is a fully 
integrated thermal inkjet printhead formed by a silicon die 52, a thin 
film structure 54 and an orifice layer 56. In an exemplary embodiment, the 
silicon die 52 is approximately 675 microns thick. Glass or a stable 
polymer are used in place of the silicon in alternative embodiments. The 
thin film structure 54 is formed by one or more passivation or insulation 
layers of silicon dioxide, silicon carbide, silicon nitride, tantalum, 
poly silicon glass, or another suitable material. The thin film structure 
also includes a conductive layer for defining the firing resistor 40 and 
the wiring lines 46. The conductive layer is formed by aluminum, gold, 
tantalum, tantalum-aluminum or other metal or metal alloy. In an exemplary 
embodiment the thin film structure 54 is approximately 3 microns thick. 
The orifice layer 56 has a thickness of approximately 7 to 30 microns. The 
nozzle opening 38 has a diameter of approximately 10-50 microns. In an 
exemplary embodiment the firing resistor 40 is approximately square with a 
length on each side of approximately 10-30 microns. The base surface of 
the nozzle chamber 36 supporting the firing resistor 40 has a diameter 
approximately twice the length of the resistor 40. In one embodiment a 
54.7.degree. etch defines the wall angles for the opening 38 and the 
refill slot 42. Although exemplary dimensions and angles are given such 
dimensions and angles may vary for alternative embodiments. 
In an alternative embodiment one or more of the printhead dies 18 is formed 
by a substrate within which are formed firing resistors and wiring lines. 
A barrier layer overlays the substrate at the firing resistors. The 
barrier layer has openings which define nozzle chambers. An orifice plate 
or flex circuit overlays the barrier layer and includes the nozzle 
openings. An ink refill slot is formed in the substrate by a drilling 
process. 
Upon activation of a given firing resistor 40, ink within the surrounding 
nozzle chamber 36 is ejected through the nozzle opening 38 onto a media 
sheet. Referring to FIGS. 2-4 logic circuits 29 select which firing 
resistors 40 are active at a given time. Drive circuits 30 supply a given 
drive signal to a given firing resistor 38 to heat the given firing 
resistor 38. In one embodiment the logic circuits 29 and drive circuits 30 
are mounted to the carrier substrate 20. In an alternative embodiment the 
logic circuitry and drive circuitry are located off the wide-array 
printhead structure 12. Referring to FIGS. 2 and 3, the logic circuits 29 
and drive circuits 30 are mounted to a second surface 33 of the substrate 
20, opposite the first surface 28 in an exemplary embodiment. In another 
exemplary embodiment (see FIG. 4) the logic circuits 29 and drive circuits 
30 are mounted to the same surface 28 as the printhead dies 18. Referring 
to FIG. 3, the carrier substrate 20 includes interconnects 50 fabricated 
or applied to the substrate 20. The printhead dies 18 are mounted to the 
carrier substrate into electrical contact with respective interconnects 
50. In a preferred embodiment there is an interconnect 50 for each 
electrical contact of each printhead die 18. The printhead die 18 includes 
a plurality of contacts for coupling the printing element wiring lines 46 
to respective drive signals. The interconnects 50 extends to the drive 
circuits 30 which source the drive signals. In one embodiment a daughter 
substrate 52 is mounted to the carrier substrate. The logic circuits 29 
and drive circuits 30 are mounted to such daughter substrate. The daughter 
substrate interconnects the logic circuits 29 and drive circuits 30 to 
each other, and interconnects the drive circuits 30 to the carrier 
substrate interconnects 50. In an alternative embodiment the logic 
circuits 29 and drive circuits 30 are mounted directly to the carrier 
substrate 20. 
During operation, the wide-array printhead 12 receives printer control 
signals from off the substrate 20. Such signals are received onto the 
substrate 20 via a connector 34. The logic circuits 29 and drive circuits 
30 are coupled directly or indirectly to such connector 34. The printhead 
die 18 are coupled to the drive circuits 30. 
Method of Mounting the Printheads 
Each printhead die has a first surface 58 and a second surface 60, opposite 
the first surface 58. The nozzle openings 38 occur in the first surface 
58. Ink refill slots 42 occur in the second surface 60. The silicon die 52 
has one or more dielectric layers 62 (e.g., nitride or carbide layers) at 
the second surface 60. During fabrication of the printhead die 18 an 
interconnect metal 66 and a wetting metal 68 are deposited onto the second 
surface 60 at prescribed locations. The interconnect metal is deposited 
onto the dielectric layer(s) 62, and the wetting metal is applied onto the 
interconnect metal. In one embodiment photolithographic processes are used 
to define a precise location, size and shape of the wetting metal 68. Such 
processes enable accurate placement of the wetting metal to within 1 
micron. 
The carrier substrate 20 also includes a first surface 70 and a second 
surface 72 opposite the first surface 70. The printhead die 18 is mounted 
to the carrier substrate 20 with the printhead second surface 60 facing 
the carrier substrate 20 as shown in FIG. 5. The spacing between the 
printhead die 18 and carrier substrate 20 is exaggerated for purposes of 
illustration. Like the printhead dies 18, a dielectric layer 75 (e.g., 
nitride layer) is applied to the surfaces 70, 72, and an interconnect 
metal 74 and wetting metal 76 (also referred to herein as metal pads or 
wetting pads) are deposited onto the nitride layer 72 at prescribed 
locations. In one embodiment photolithographic processes are used to 
define a precise location, size and shape of the wetting metal 68. Such 
processes enable accurate placement of the wetting metal to within 1 
microns. In preferred embodiments the wetting metals 76 are on the 
substrate 20 are formed in locations corresponding to the wetting metals 
66 of the printheads. Specifically, there is a one to one correspondence 
between the wetting metal locations on the carrier substrate 20 and the 
printhead dies 18. 
Solder bumps are deposited onto the wetting metal of either the printhead 
die 18 or carrier substrate 20. To mount a printhead die 18, the printhead 
die 18 is pressed to the carrier substrate so that the wetting metals of 
each line up. The wetting metals 68, 76 are separated by the solder bumps 
78. The solder is then heated liquefying the solder. The solder then flows 
along the wetting pads 68, 76 and pulls the printhead die 18 into precise 
alignment with the carrier substrate 20. More specifically the solder 78 
pulls the printhead wetting pad 68 into precise alignment with the 
corresponding carrier substrate metal pad 76. It has been demonstrated 
that solder reflow forces align the respective wetting metals 68, 76 to 
within 1 micron. Thus, it is by precisely locating the wetting metals 68, 
76 using the photolithographic and other deposition processes, that the 
printhead dies 18 are able to be precisely placed and aligned on the 
carrier substrate 20 to within desired tolerances. 
According to an aspect of the invention, the solder also forms a fluid 
barrier. As described above the printheads include one or more refill 
slots 42 and the carrier substrate includes one or more refill channels 
32. Each refill slot 42 is to be in fluidic communication with a refill 
channel 32. As shown in FIG. 5 the refill slot 42 is aligned to the refill 
channel 32. To prevent ink from leaking at the interface between the 
printhead die 18 and the carrier substrate 20, a seal is to be formed. In 
one embodiment the solder 78 is corrosive resistant and serves as the 
seal. Specifically the wetting metal 68, 76 are deposited around the 
respective openings of the refill slot 42 and refill channel 32. Thus, 
when solder is applied to mount the printhead die 18 to the substrate 20, 
the solder defines a seal or fluidic barrier which prevents ink from 
leaking at the interface. In alternative embodiments an underfill process 
is performed in which an adhesive or a sealant is used to form a fluidic 
barrier. 
Interconnect Method Coupling Printhead and Carrier Substrate 
As described above, the printing elements 24 with wiring lines 46 are 
formed toward the first surface 58 of the printhead. Because the carrier 
substrate is adjacent to the second surface 60 of the printhead die 18, an 
electrical interconnect is to extend from the first surface 58 to the 
second surface 60 of the printhead die 18. FIG. 5 shows an embodiment in 
which an interconnect 80 extends from the thin film structure 54 adjacent 
the first surface 58 through the silicon die 52 toward the second surface 
60. An electrical connection extends from a wiring line 46 through a via 
101 to a conductive trace 107 to via 99 and interconnect 80 (as shown in 
FIG. 8). The interconnect 80 connects to an interconnect metal layer 82 
and a wetting metal layer 84 at the second surface 60. Solder 78 then 
completes the electrical connection to an interconnect 90 at the carrier 
substrate. A wetting metal layer 86 and an interconnect metal 88 are 
located on the carrier substrate between the solder 78 and the 
interconnect 90. In the embodiment shown the interconnect 90 extends 
through the carrier substrate to an interface with a drive circuit 30. In 
another embodiment the interconnect 90 extends along a first surface 70 of 
the carrier substrate to an interface with a drive circuit 30. For drive 
circuits 30 mounted to the second surface 72 of the substrate 20, a solder 
connection also is established, although an alternative electrical 
coupling scheme may be used. 
To form the interconnect 80 extending through the printhead 18 a trench 92 
is etched in the underside (e.g., second surface 60) of the die 52 for one 
or more interconnects 80. In one embodiment a tetramethyl ammonium 
hydroxide etch is performed. A hard mask covers portions of the die 52 
undersurface not to be etched. The hard mask is then removed by wet 
etching. A plasma carbide or nitride layer 62 and an Au/Ni/Au layer 96 are 
deposited on the undersurface as shown in FIG. 7. A photosensitive 
polyamide layer or an electroplating photoresist 98 is applied over a 
portion of the Au/Ni/Au layer 96 to define where the metal is to remain 
for the interconnect 80. The Au/Ni/Au layer 96 then is wet etched and the 
polyamide or photoresist 98 removed to define the interconnect 80. To 
protect the Au/Ni/Au during etching of the refill slot 42, a plasma oxide 
(not shown) then is deposited. The plasma oxide and the carbide or nitride 
layer 62 then are patterned to define a window to etch the refill slot 42. 
The refill slot 42 and the feed channels 44 then are etched. Referring to 
FIG. 8 at a next step one or more vias 99 are cut through passivation 
layers 100, 102, 104 and a carbide layer 106 of the thin film structure 54 
and the carbide or nitride layer 62. The vias 99 extend from the 
interconnect 80 to the in-process upper surface. A via 101 also is cut to 
expose a portion of a wiring line 46. Metal then is deposited in the vias 
99, 101. Next, a conductive trace 107 (see FIG. 8) is conventionally 
deposited, photolithographically patterned, and etched onto a layer of the 
thin film structure 54 to electrically couple the wiring line 46 and the 
interconnect 80. The second dielectric layer 64 (e.g., nitride layer) then 
is deposited (see FIG. 5). A polyamide or electroplating photoresist 
process then is performed to mask the layer 64 and form an opening in the 
layer 64 to expose a portion of the interconnect 80 (see FIG. 5). The 
interconnect metal 82 and wetting metal 84 then are deposited onto the 
exposed portion of the interconnect 80 and patterned and etched in manner 
similar to that used for other films on the second surface. The 
interconnect 80 as fabricated extends from a wiring line 46, through the 
carrier substrate 20, along a trench 92 to an interconnect metal 82 and 
wetting metal 84 at a second surface 60 of the printhead die 18. 
Thereafter the thin film structure is completed and the orifice layer 56 
is applied. 
Method of Fabricating Through-Interconnects and Refill Slot in Carrier 
Substrate 
Referring again to FIG. 5, the carrier substrate 20 includes an 
interconnect 90 extending from one surface of the substrate to the 
opposite surface of the substrate. In one embodiment the interconnect 90 
is formed as described above for the printhead die by etching a trench and 
depositing the interconnect metal. In an alternative embodiment a straight 
etch is performed to define a through-opening 110 in the substrate 20. An 
electroplating method then is performed to fill the etched through-opening 
110 with metal. The metal defines the interconnect 90. Referring to FIG. 
9, to plate the through-opening 110, the substrate 20 is dipped into a 
plating solution 112. A bias signal 114 is applied to an electroplate 116 
to which the substrate 20 is attached. The electroplate 116 is formed so 
that a bias current does not flow in the region of the ink refill channel 
32 of the substrate. More specifically, a metal layer 115 forms a contact 
between the substrate 20 and electroplate 116 at desired locations. Thus, 
the refill channel 32 is not electroplated. In addition, only a small gap 
118 occurs between the substrate 20 and the electroplate. This prevents 
electroplating the undersurface 72 of the substrate 20 while dipped in the 
plating solution 112. 
Alternative Interconnect Method Coupling Printhead and Carrier Substrate 
Rather than form an interconnect extending through the die 52 of the 
printhead die 18, in an alternative embodiment a wire bond is formed 
external to the printhead. Referring to FIG. 10, a printhead die 18' is 
shown with like parts given like numbers. Respective wiring lines 46 for 
each printing element 24 extend to respective contacts 120. The contact 
120 is located on the same side of the printhead die 18' as the nozzle 
openings 38. A wire 122 is bonded to a contact 120 on the printhead die 
18' and a contact 130 on the substrate 20. The contact 130 is located on a 
surface 70 of the substrate 20. The wire 122 extends outside of the 
printhead 18' between the printhead die 18' and substrate 20. The wire 122 
is affixed to the contacts 120, 130 An encapsulant is applied around the 
wire 122 to seal the wire and protect it from breaking away from the 
printhead die 18' or substrate 20. The substrate 20 includes a refill 
channel 32 through which ink flows toward the printhead die 18. Although 
such channel is shown as a straight etched channel the walls of the 
channel alternatively are etched at an angle, (e.g. 54.7.degree.). 
Meritorious and Advantageous Effects 
One advantage of the invention is that a scalable printhead architecture is 
achieved wherein different numbers of printhead dies are attached to a 
carrier substrate to define the size of the printhead. 
Although a preferred embodiment of the invention has been illustrated and 
described, various alternatives, modifications and equivalents may be 
used. Therefore, the foregoing description should not be taken as limiting 
the scope of the inventions which are defined by the appended claims.