Abstract:
A semiconductor substrate includes electronic circuitry and has a machined feature formed therein. The semiconductor substrate is formed by a process which includes providing the semiconductor substrate having the electronic circuitry formed therein, and performing a machining process on the substrate to form the machined feature therein. The machined feature includes a slot and the machining process forms cracks at ends of the slot that reduce a fracture strength of the substrate. Removing portions of the semiconductor substrate proximate the cracks such that end points of the cracks have a curved terminus as formed by the removed portions improves the fracture strength of the substrate.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
     This is a Divisional of U.S. patent application Ser. No. 09/532,105, filed on Mar. 21, 2000, now U.S. Pat. No. 6,560,871, which is assigned to the assignee of the present invention and incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to increasing the fracture strength of semiconductor substrates used in inkjet printheads and the like, and more generally, to increasing the fracture strength of semiconductor substrates, regardless of intended purpose, that are drilled or otherwise machined to form a hole or other feature therethrough or therein. 
     BACKGROUND OF THE INVENTION 
     Various inkjet printing arrangements are known in the art and include both thermally actuated printheads and mechanically actuated printheads. Thermal actuated printheads tend to use resistive elements or the like to achieve ink expulsion, while mechanically actuated printheads tend to use piezoelectric transducers or the like. 
     A representative thermal inkjet printhead has a plurality of thin film resistors provided on a semiconductor substrate. A nozzle plate and barrier layer are provided on the substrate and define the firing chambers about each of the resistors. Propagation of a current or a “fire signal” through a resistor causes ink in the corresponding firing chamber to be heated and expelled through the appropriate nozzle. 
     Ink is typically delivered to the firing chamber through a feed slot that is machined in the semiconductor substrate. The substrate usually has a rectangular shape, with the slot disposed longitudinally therein. Resistors are typically arranged in rows located on both sides of the slot and are preferably spaced an approximately equal distances from the slot so that the ink channel length at each resistor is approximately equal. The width of the print swath achieved by one pass of a printhead is approximately equal to the length of the resistor rows, which in turn is approximately equal to the length of the slot. 
     Feed slots are typically formed by sand drilling (also known as “sand slotting”). This method is preferred because it is a rapid, relatively simple and scalable (many substrates may be processed simultaneously) process. While sand slotting affords these apparent benefits, sand slotting is also disadvantageous in that it causes micro cracks in the semiconductor substrate that significantly reduce the substrates fracture strength, resulting in significant yield loss due to cracked die. Low fracture strength also limits substrate length which in turn adversely impacts print swath height and overall print speed. 
     As new printer systems are developed, a key performance parameter is print speed. One way of achieving higher print speed is to increase the width of the print swath of a printhead. One potential manner of increasing print swath width is to increase the length of the substrate and the feed slot therein. Due to micro cracks and other structural defects induced during sand slotting, however, substrates are rendered too fragile to be further extended. 
     A need thus exists for a machined semiconductor substrate that has increased fracture strength to better withstand the thermal and mechanical stresses induced in ink jet printhead manufacture and use. A need also exists for a printhead semiconductor substrate that has increased fracture strength and can therefore be elongated to achieve longer print swath width. A need further exists for a machined semiconductor substrate for any intended purpose that has increased fracture strength. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention is a semiconductor substrate and method of making the same having improved fracture strength. A semiconductor substrate is machined to define a feature therein. The machining process forms a micro-crack in the substrate that reduces the fracture strength of the substrate. The semiconductor substrate is processed to remove portions of the substrate proximate the micro-cracks to improve the fracture strength of the semiconductor substrate. 
     In one preferred embodiment, the portions of the semiconductor substrate proximate the micro-cracks are removed to increase the radius of curvature of portions of the crack using an etching process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an ink jet printer in accordance with the present invention. 
         FIG. 2  is a perspective view of one embodiment of an ink jet print cartridge in accordance with the present invention. 
         FIG. 3  is a cross-sectional view of the printhead of  FIG. 2  having a semiconductor substrate processed in accordance with the present invention. 
         FIG. 4  is a perspective, cut-away view of one end of the ink feed slot in a semiconductor substrate in accordance with the present invention. 
         FIG. 5  is a plan view of one end of the ink feed slot formed by sand slotting in a typical printhead substrate illustrating a micro-crack. 
         FIGS. 6A and 6B  is a greatly enlarged representation of the micro-crack of  FIG. 5  shown before,  FIG. 6A , and after,  FIG. 6   b , performing the method of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a perspective view of an ink jet printer in accordance with the present invention is shown. Printer  10  preferably includes a housing  12  having an openable cover  14  and printer status indicator lights  16 . A printhead (discussed in more detail below) is preferably located under cover  14 . A print media input/output (I/O) unit  18  provides suitable print media to the printhead(s). The print media I/O unit preferably includes paper input and output trays, guides, and appropriate sensors and transport mechanisms, etc. Printer  10  also includes a power supply, an ink supply and controller logic (not shown), amongst other related components. The power supply preferably provides regulated DC at appropriate voltage levels. 
     The ink supply may be formed integrally with printhead  10  or formed separately. The ink supply may be separately replaceable from the printhead or replaceable with the printhead. Ink level detection logic (not shown) is preferably provided with the ink supply to indicate an ink volume level. Suitable ink supply arrangements are known in the art. 
     Printer  10  preferably receives print data from a host machine which may be a computer, facsimile machine, Internet terminal, camera, plotter or other device that is capable of propagating print data to printer  10 . 
     The printhead is preferably provided on a moveable carriage (also located under cover  14 ) that may move transversely along guide rods as is known. It should be recognized, however, that the printhead could be stationary and, for example, formed as wide as a sheet (or section of a sheet) of print media, such as paper. 
     Referring to  FIG. 2 , one embodiment of an ink jet print cartridge in accordance with the present invention is shown. Print cartridge  20  includes a housing  21  that is configured to provide a printhead region  22  and a reservoir region  26 . In the embodiment of  FIG. 2 , print cartridge  20  is a tri-color print cartridge having three ink feed slots and corresponding arrays of nozzles, preferably for cyan, magenta and yellow. The reservoir region  26  in the case of a color print cartridge  20 , preferably includes individual ink reservoirs for each different color of ink. It should be recognized that print cartridge  20  may alternatively be configured for use with an “off-axis” ink supply that is physically detached from the printhead and in fluid communication therewith. 
     Each printhead  40  preferably includes a substrate  50  on which one or more ink feed slots  60  is machined (see  FIGS. 3–5 ). Ink is delivered (from an on-axis or off-axis source) through the feed slot  60  to ink expulsion elements  52  formed proximate the slot. The ink expulsion elements (e.g., resistor, piezo-electric transducer, etc.) are preferably provided in two rows, and are located on opposite sides of the feed slot (see  FIGS. 3 and 4 ). Nozzles  44 , are aligned with the corresponding ink expulsion elements  52  and are formed in a nozzle plate  46 . A plurality of electrical interconnects  28  are coupled to the substrate  50  by conductive drive lines (not shown). The electrical interconnects  28  engage a corresponding electrical interconnect which is located in the printer carriage (discussed above), thereby allowing printer  10  to selectively control the ejection of ink droplets as a cartridge traverses across the print media. 
     Referring to  FIG. 3 , a cross-sectional view of the printhead  40  of  FIG. 2  having a semiconductor substrate processed in accordance with the present invention is shown. Ink enters chamber  62  from a reservoir within region  26  or a feed conduit from an off-axis source as discussed above. Components  64  represent portions of housing  21  of print cartridge  20  (or of a suitable conduit) that are preferably joined to substrate  50  by thermally cured structural adhesive  66 . Ink in chamber  62  flows through feed slot  60  to firing chamber  54  formed adjacent ink expulsion elements  52 . 
     Contact pads  56  propagate fire or drive signals from interconnects  28  via signal traces  58  to the ink expulsion elements  52 . In a preferred embodiment, the ink expulsion elements are thin film resistors of a type known in the art, though it should be recognized that the ink expulsion elements may alternatively be piezo-electric transducers, etc. 
     The substrate  50  is preferably formed of a semiconductor material such as silicon. Barrier layer  42  is formed on the substrate in such a manner as to define firing chambers  54  (see  FIG. 4 ), and a nozzle plate  46  is mounted on the barrier layer such that nozzles  44  and their associated ink expulsion resistors  52  are appropriately aligned. 
     Referring to  FIG. 4 , a perspective, cut-away view of one end of the ink feed slot  60  that is formed using a machining technique such as sand drilling is shown. This figure depicts the details of the inkfeed slot  60 , firing chambers  54 , resistors  52 , barrier structure  42 , and orifice plate  46 . 
     The formation of the feed slot  60  in substrate  50  has been described in many publications including U.S. Pat. No. 4,680,859 to Johnson, entitled “Thermal Inkjet Print Head Method of Manufacture” and assigned to the present assignee. During the slot  60  formation process, the nozzle of the slotting tool is brought into close proximity with the back of the substrate  50  and high pressure abrasive particles strike the substrate  50 . Due to the random nature of the abrasive striking the substrate  50 , the size and shape of the slot  60  is difficult to control. In addition, the point at which the slot  60  “breaks through” the front of the substrate  50  also varies and depending upon this location micro-cracks can be formed in the substrate  50 . Tests have shown that if the slot  60  breaks through the center of the substrate  50  that stress cracks form at the ends of the ink feed slot  60 . These micro-cracks act as fracture initiation sites and can cause the die to fracture when it is placed under the mechanical and thermal stress such as during the manufacturing process. 
       FIG. 5  depicts a plan view of a representative printhead substrate  50  with a typical mirco-crack  74  that has formed in the silicon substrate as a result of the slot  60  formation process. As can be seen, micro-crack  74  and similar cracks form at an end of the elongated feed slot  60  and tend to follow the crystalline grain boundaries that are parallel to an axis of elongation of the ink feed slot  60 . As the substrate  50  is stressed either thermally or mechanically, the crack  74  readily increases until substrate  50  fractures. Once the substrate  50  is fractured the electrical traces  58  and active components on the substrate  50  are broken resulting in printhead  40  failure. 
     There has been a great deal of effort spent on making the slotting process more repeatable and controllable, and in eliminating these micro-cracks  74 . Up to the present invention, however, there has not been a complete solution to the die fracture problem. 
     Accordingly, to improve fracture strength, substrate  50  is preferably etched after machining to remove portions of the semiconductor material where the micro-cracks are formed. This etching process changes the nature of the semiconductor material such that the line of the micro-crack is modified. One modification to the mirco-crack performed by the etch process is to alter the terminus of the end point of the crack. 
     Referring to  FIGS. 6A and 6B , there is shown a greatly enlarged representation of the micro-crack  74  shown in  FIG. 5 .  FIG. 6A  is a representative of the micro-crack  74  before processing using the etching process of the present invention. The micro-crack  74  has a terminus  75  which tends to be a point in the substrate  50  where mechanical stress applied to the substrate  50  becomes focused or concentrated. In contrast,  FIG. 6B  is a representation of the micro-crack  74  of  FIG. 6A  after processing using the etching process of the present invention. The terminus  76  of the micro-crack  74  is modified by the etch process to have an increased radius of curvature. The stress concentration within the substrate  50  is proportional to 1/(radius of curvature). Thus increasing the radius of curvature using the etch process of the present invention reduces the stress concentration and the likelihood of further cracking. The etching process of the present invention not only increases the radius of curvature at the terminus  76 , but also throughout the micro-crack  74 . The etching process of the present invention tends to increase the critical radius or radius of curvature of the micro-cracks  74  and this in turn reduces the stress concentration in the substrate  50 . 
     Substrate  50  and printhead  40  in which the substrate is utilized is preferably made as follows. Printhead circuitry for a plurality of printhead substrates is formed on a wafer. Standard thin film techniques are preferably utilized to form the printhead substrate conductive patterns. Following this fabrication, the wafer is cleaned and prepared for barrier layer mounting. The barrier layer is typically formed by a polymer lamination process. 
     After barrier layer  42  formation, the ink feed slot is sand drilled as described above for each of the plurality of printhead substrate  50  on the wafer. This sand drill process tends to form small cracks  74  that tend to reduce the fracture strength of the substrate  50 . 
     The preferred etch process is then performed to improve the fracture strength of the substrate  50 . This process is performed by rinsing the wafer preferably in a BOE bath for 3.5 minutes at 20.9° C. to remove naturally grown SiO 2  ( 72  of  FIG. 4 ). After a deionized water rinse, the wafer is etched in 5% wt. TMAH for 7 minutes at 84.9° C. This etch is followed by another deionized water rinse and the mounting of individual orifice plates ( 46  of  FIGS. 2–3 ) on the barrier layer  42  material. The wafer is then singulated to produce a plurality of printhead substrates  50  that each exhibit increased fracture strength. A flex circuit having interconnects  28  may be connected to each substrate to produce a printhead sub-assembly. The printhead head assembly is then attached to the printhead housing or structure  64  with a thermally cured adhesive  66  to completes the “dry portion” of the printhead assembly process. 
     By processing substrate  50  as discussed above or in a related manner, a printhead  40  is produced that has increased fracture strength. The increased fracture strength in turn results in less substrate cracking during manufacture thereby increasing production yield and product life. In addition, increased fracture strength allows larger printheads  40  having longer ink feed slots  60  allowing larger print swaths to be printed. The ability to print larger print swaths enables greater printing speed and greater through put for the printing system  10 . 
     In an alternate embodiment, the etch is performed after the wafer is singulated with a diamond saw. The edges of the die are typically chipped and cracked due to the shear loads induced by the cutting of the rotary blade. These chipped areas typically include cracks which under thermal and mechanical loads can propagate into the die. Performing an etch on these die has been show to remove these local cracks resulting in a more fracture resistant and manufacturable substrate. 
     Another benefit related to the use of TMAH and like substances is that TMAH is an anisotropic etch (i.e., etches more rapidly in certain crystalline orientations than in others) and as such tends to form pyramidal shaped recesses in the monocrystalline silicon material  50 . This characteristic pattern provides a way of easily determining whether a die has been etched. 
     With respect to alternatives for TMAH, it should be recognized that Si material may also be removed with KOH or other similarly acting chemical echants. 
     Although etching is a preferred manner for removal of crack containing material, other techniques also fall within the spirit and scope of the present invention including re-heating or re-melting techniques and laser annealing.