Abstract
A process for forming inkjet nozzle devices on a frontside surface of a wafer substrate. The process includes the steps of: (i) providing the wafer substrate having a plurality of etched holes defined in the frontside surface, each etched hole being filled with first and second polymers such that the second polymer is coplanar with the frontside surface; (ii) forming the inkjet nozzle devices on the frontside surface using MEMS fabrication steps; and (iii) removing the first and second polymers via oxidative ashing, wherein first and second polymers are different.
Claims
1. A process for forming inkjet nozzle devices on a frontside surface of a wafer substrate, the process comprising the steps of: (i) providing the wafer substrate having a plurality of etched holes defined in the frontside surface, each etched hole being filled with first and second polymers such that the second polymer is coplanar with the frontside surface; (ii) forming the inkjet nozzle devices on the frontside surface using one or more MEMS fabrication steps; (iii) removing the first and second polymers via oxidative ashing, wherein the first and second polymers are different.
2. The process of claim 1, wherein each hole has a depth of at least 10 microns.
3. The process of claim 1, wherein each hole has an aspect ratio of >1:1.
4. The process of claim 1, wherein the first polymer is less viscous than the second polymer.
5. The process of claim 1, wherein the first polymer is a thermoplastic polymer.
6. The process of claim 1, wherein the second polymer is photoimageable.
7. The process of claim 1, wherein second polymer is superjacent the first polymer.
8. The process of claim 1, wherein each inkjet nozzle device comprises a nozzle chamber in fluid communication with at least one hole.
9. The process of claim 8, wherein a respective inlet for each nozzle chamber is defined by one of said holes.
10. The process of claim 1, further comprising the steps of: wafer thinning and backside etching of ink supply channels.
11. The process of claim 10, wherein each ink supply channel meets with one or more filled holes.
12. The process of claim 11, wherein each ink supply channel is relatively wider than said one or more holes.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:
(2) FIG. 1 is a schematic side view of a silicon substrate having a high aspect ratio hole etched in frontside surface;
(3) FIG. 2 shows the substrate shown in FIG. 1 after deposition of a thermoplastic first polymer;
(4) FIG. 3 shows the substrate shown in FIG. 2 after reflowing and curing of the first polymer;
(5) FIG. 4 shows the substrate shown in FIG. 3 after oxidative removal of the first polymer from the frontside surface;
(6) FIG. 5 shows the substrate shown in FIG. 4 after deposition of a photoimageable second polymer;
(7) FIG. 6 shows the substrate shown in FIG. 5 after exposure and development of the second polymer;
(8) FIG. 7 shows the substrate shown in FIG. 6 after chemical-mechanical planarization;
(9) FIG. 8 shows the substrate shown in FIG. 1 after deposition of a thermoplastic photoimageable third polymer;
(10) FIG. 9 shows the substrate shown in FIG. 8 after reflowing and curing of the third polymer;
(11) FIG. 10 shows the substrate shown in FIG. 9 after exposure and development of the third polymer;
(12) FIG. 11 shows the substrate shown in FIG. 10 after chemical-mechanical planarization;
(13) FIG. 12 shows the substrate shown in FIG. 1 after repeated deposition and reflow baking of the thermoplastic first polymer;
(14) FIG. 13 shows the substrate shown in FIG. 12 after deposition of the photoimageable second polymer;
(15) FIG. 14 shows the substrate shown in FIG. 13 after exposure and development of the second polymer;
(16) FIG. 15 shows the substrate shown in FIG. 14 after oxidative removal of the first polymer from the frontside surface;
(17) FIG. 16 shows the substrate shown in FIG. 15 after chemical-mechanical planarization;
(18) FIG. 17 is a schematic side view of a silicon substrate having a low aspect ratio hole etched in frontside surface;
(19) FIG. 18 shows the substrate shown in FIG. 17 after deposition of a conventional photoimageable polymer;
(20) FIG. 19 shows the substrate shown in FIG. 18 after exposure and development;
(21) FIG. 20 shows the substrate shown in FIG. 10 after chemical-mechanical planarization;
(22) FIG. 21 is a perspective view of inkjet nozzle devices each having a chamber inlet defined in a frontside surface of a silicon substrate; and
(23) FIG. 22 is a sectional side view of the inkjet nozzle device shown in FIG. 21.
DETAILED DESCRIPTION OF THE INVENTION
First Embodiment
(24) Referring to FIG. 1, there is shown a substrate 1 having a high aspect ratio hole 2 defined in a frontside surface 3 thereof. The substrate is a CMOS silicon wafer having an upper CMOS layer 5 disposed on a bulk silicon substrate 4. The CMOS layer 4 typically comprises one more metal layers interposed between interlayer dielectric (ILD) layers. The hole 2 may be defined by any suitable anisotropic DRIE process (e.g. Bosch etch as described in U.S. Pat. No. 5,501,893). The hole 2 may have any desired shape in cross-section, the shape being defined by a photoresist mask during the etching process.
(25) FIG. 2 shows the substrate 1 after spin-coating a reflowable thermoplastic polymer 7 onto the frontside surface 3 followed by soft-baking. The thermoplastic polymer 7 is non-photoimageable and may be of any suitable type known to those skilled in the art. For example, the thermoplastic polymer 7 may be an adhesive, such as a polyimide adhesive. A specific example of a suitable thermoplastic polymer 7 is HD-3007 Adhesive, available from HD MicroSystems.
(26) Soft-baking after deposition of the thermoplastic polymer 7 removes solvent to provide a tack-free film. Since the thermoplastic polymer 7 has a relatively low viscosity (e.g. <1500 Cps), any air or solvent bubbles present in the polymer can readily escape during soft-baking. Still referring to FIG. 2, it can be seen that the thermoplastic polymer 7 is readily deposited inside the high aspect ratio hole 2 during spin-coating due to it relatively low viscosity.
(27) Referring now to FIG. 3, there is shown the substrate 1 after reflow-baking at a relatively higher temperature than soft-baking. This reflow-baking step raises the thermoplastic polymer 7 to a temperature above its glass transition temperature, allowing the polymer to reflow and fill the hole 2 more completely. For example, reflow-baking may be performed at about 300 C., while soft-baking may be performed at about 90 C.
(28) Depending on the depth and aspect ratio of the hole 2, as well as the type of thermoplastic polymer 7 employed, the steps described in connection with FIGS. 2 and 3 may be repeated one or more times until the hole is filled to a level just below the frontside surface, as shown in FIG. 3. The hole 2 may be >60% filled, >70% filled, >80% or >90% after all spin-coating and reflowing steps have been completed.
(29) After the hole 2 has been partially-filled to a desired level, the thermoplastic polymer 7 is then cured at a relatively higher temperature than the reflow baking temperature in order to cross-link and harden the polymer. The resultant plug of thermoplastic polymer 7 shown in FIG. 3 is substantially free of any air or solvent bubbles. Moreover, the reflow step(s) ensure the thermoplastic polymer 7 uniformly contacts sidewalls of the hole 2 to provide a robust foundation for subsequent MEMS processing.
(30) Turning now to FIG. 4, the substrate 1 is shown after removal of a predetermined thickness of the thermoplastic polymer 7 via a controlled oxidative removal process (ashing). Typically, the controlled oxidative removal process comprises a timed exposure to an oxygen-based plasma in a conventional ashing oven. A planar thickness of polymer removed by the ashing process is proportional to the period of ashing. As shown in FIG. 4, the ashing process removes a thickness of the thermoplastic polymer 7, such that removal is complete from the frontside surface 3 in regions outside the periphery of the hole 2. However, the hole 2 remains partially-filled with the thermoplastic polymer 7 by virtue of the additional thickness of polymer in the hole.
(31) Next, as shown in FIG. 5, a conventional photoimageable (non-thermoplastic) polymer 9 is deposited onto the frontside surface 3 of the substrate 1 by spin-coating followed by soft-baking. The photoimageable polymer 9 is spin-coated to a thickness of about 8 microns so as to overfill the hole 2. The photoimageable polymer 9 may be of any suitable type known to those skilled in the art. For example, the photoimageable polymer 9 may be a polyimide or a conventional photoresist. A specific example of a suitable photoimageable polymer 9 is HD-8820 Aqueous Positive Polyimide, available from HD Micro Systems.
(32) Referring to the FIG. 6, the photoimageable polymer 9 is then exposed and developed, by conventional methods known to those skilled in the art, so as to remove substantially all of the polymer 9 from regions outside a periphery of the hole 2. The resultant substrate 1 has an overfilled hole 2 having an 8 micron cap of the photoimageable polymer 9.
(33) Following final curing of the photoimageable polymer 9, the frontside surface 3 of the substrate 1 is then subjected to chemical-mechanical planarization (CMP) so as to remove the cap of photoimageable polymer 9 and provide a planar frontside surface, as shown in FIG. 7. Advantageously, the amount of photoimageable polymer 9 that is required to be removed by CMP is relatively small due to the previous exposure and development steps described in connection with FIG. 6. Hence, the CMP process has acceptable process times (e.g. 5 minutes or less), good stopping selectivity and minimal gumming of CMP pads, which reduces the cost of consumables.
(34) In the resultant substrate 1, shown in FIG. 7, the hole 2 is plugged with the thermoplastic polymer 7 and the photoimageable polymer 9. This polymer plug is robust and substantially free of any solvent or air bubbles. Furthermore, an upper surface 11 of the plug is coplanar with the frontside surface 3 by virtue of the final planarizing process. The plugged hole therefore provides an ideal foundation for subsequent frontside MEMS processing steps, such as fabrication of inkjet nozzle structures.
Second Embodiment
(35) A second embodiment of the present invention will now be described with reference to FIGS. 8 to 11. Referring firstly to FIG. 8 the hole 2 is filled with a polymer 13 having both thermoplastic and photoimageable properties. An example of the thermoplastic photoimageable polymer 13 is Level M10 coating, available from Brewer Science. The thermoplastic photoimageable polymer 13 has a relatively low viscosity which is comparable to the thermoplastic polymer 7 described hereinabove. The polymer 13 is therefore able to fill the hole 2 in a single spin-coating followed by soft-baking to removal solvent. The low viscosity and thermoplastic reflow properties of the polymer 13 enable any solvent or air bubbles to escape during soft-baking and reflow baking.
(36) FIG. 9 shows the polymer 13 after reflow-baking at a relatively higher temperature than soft-baking. This reflow-baking step raises the polymer 13 to a temperature above its glass transition temperature, allowing the polymer to reflow and ensure the hole 2 is overfilled.
(37) Referring to the FIG. 10, the thermoplastic photoimageable polymer 13 is then exposed and developed by conventional methods known to those skilled in the art, so as to remove substantially all of the polymer 13 from regions outside a periphery of the hole 2. The resultant substrate 1 has an overfilled hole 2 with a cap of the polymer 13.
(38) Following final curing (e.g. UV curing) of the thermoplastic photoimageable polymer 13, the frontside surface 3 of the substrate 1 is then subjected to chemical-mechanical planarization (CMP) so as to remove the cap of polymer 13 and provide a planar frontside surface, as shown in FIG. 11. Advantageously, the amount of polymer 13 that is required to be removed by CMP is relatively small due to the previous exposure and development steps described in connection with FIG. 10. Hence, the CMP process has acceptable process times (e.g. 5 minutes or less), good stopping selectivity and minimal gumming of CMP pads, which reduces the cost of consumables.
(39) In the resultant substrate 1, shown in FIG. 11, the hole 2 is plugged with the thermoplastic photoimageable polymer 13. This polymer plug is robust and substantially free of any solvent or air bubbles. Furthermore, an upper surface 15 of the plug is coplanar with the frontside surface 3 by virtue of the final planarizing process. The plugged hole therefore provides an ideal foundation for subsequent frontside MEMS processing steps, such as fabrication of inkjet nozzle structures.
Third Embodiment
(40) Referring to FIGS. 12 to 16, there is shown a third embodiment of the present invention employing the first polymer 7 and the second polymer 9, as described above in connection with the first embodiment. FIG. 12 shows the substrate 1 after spin-coating of the thermoplastic first polymer 7 and reflow baking. By contrast with the first embodiment, the hole 2 is overfilled with the polymer 7, typically using two or more cycles of spin-coating and reflow baking. After reflow baking, the substrate 1 may be exposed to an oxidative plasma to remove the polymer 7 from the frontside surface 3. However, this step is optional and FIG. 12 shows an alternative process where there is no ashing step after each cycle of spin-coating and reflow baking.
(41) Referring to FIG. 13, the photoimageable second polymer 9 is then spin-coated on the substrate 1 over the thermoplastic polymer 7. Subsequent masked exposure and development of the second polymer 9 removes the second polymer from regions outside a periphery of the hole 2. Accordingly, as shown in FIG. 14, a relatively thick polymeric layer, comprised of the first polymer and second polymer 9, is disposed over the hole 2; and a relatively thin polymeric layer, comprised of the first polymer 7, is disposed over the remainder of the frontside surface 3 in regions outside a periphery of the hole 2.
(42) Referring to FIG. 15, the substrate 1 is then exposed to a controlled oxidative plasma (ashing) so as to remove a predetermined thickness of polymeric material. The first polymer 7 is removed completely from regions outside a periphery of the hole 2 to reveal the frontside surface 3. However, since a relatively thick polymeric layer was disposed over the hole 2 prior to ashing, a polymeric cap 17 remains over the hole after the ashing step, as shown in FIG. 15.
(43) Finally, as shown in FIG. 16, the frontside surface is subjected to chemical-mechanical planarization (CMP) to remove the polymeric cap 17, stopping on the frontside surface 3. The process according to the third embodiment advantageously provides a plug of the first polymer 7 filling the hole 2. Moreover, an upper surface 19 of the plug of the first polymer 7 is coplanar with the frontside surface 3.
(44) The process according to the third embodiment is potentially advantageous compared to the first embodiment by avoiding any of the second polymer 9 in the final plugged hole. Therefore, any solvent or air bubbles present in the second polymer 9, which may grow at an interface between the first and second polymers, are avoided in the final plugged hole.
Fourth Embodiment
(45) The fourth embodiment described herein is suitable for filling relatively shallow and/or low aspect ratio holes (e.g. holes having an aspect ratio of <1:1 and/or holes have a depth of less than 10 microns or less than 5 microns). FIG. 17 shows the silicon substrate 1 having a low aspect ratio hole 21 defined in a frontside surface 3 thereof.
(46) FIG. 18 shows the substrate 1 after spin-coating a conventional photoimageable polymer 23 onto the frontside surface 3 followed by soft-baking. The photoimageable polymer 23 may be of any suitable type known to those skilled in the art, such as polyimide or photoresist.
(47) The hole 17 is intentionally overfilled with the polymer 23 and then the polymer is subsequently removed from regions outside the periphery of the hole by conventional exposure and development steps. FIG. 19 shows the substrate 1 after exposure and development of the polymer 23; the hole 17 is plugged with the polymer and has a cap of polymeric material protruding from the frontside surface 3.
(48) Following final curing of the photoimageable polymer 23, the frontside surface 3 of the substrate 1 is then subjected to chemical-mechanical planarization (CMP) so as to remove the cap of polymer 23 and provide a planar frontside surface, as shown in FIG. 20. Advantageously, the amount of polymer 23 that is required to be removed by CMP is relatively small due to the previous exposure and development steps described in connection with FIG. 19. Hence, the CMP process has acceptable process times (e.g. 5 minutes or less), good stopping selectivity and minimal gumming of CMP pads, which reduces the cost of consumables.
(49) Moreover, the plug of polymer 23 has a uniform upper surface 25, which is coplanar with the frontside surface 3. The plugged hole therefore provides a good foundation for subsequent frontside MEMS processing steps.
(50) Although the process described above in connection with the fourth embodiment employs a single hole-filling step, it will be appreciated by those skilled in the art that the hole may be filled in multiple stages, similar to the process described in U.S. Pat. No. 7,923,379. However, in contrast with the process described in U.S. Pat. No. 7,923,379, the process according to the third embodiment overfills the hole for subsequent planarization (see FIGS. 18 and 19).
(51) MEMS Inkjet Nozzle Devices
(52) By way of completeness, there will now be described an inkjet nozzle device fabricated by leveraging the hole-filling process described above.
(53) Referring to FIGS. 21 and 22, there is shown an inkjet nozzle device 10 comprising a main chamber 12 having a floor 14, a roof 16 and a perimeter wall 18 extending between the floor and the roof. FIG. 21 shows a CMOS layer 20, which may comprise a plurality of metal layers interspersed with interlayer dielectric (ILD) layers.
(54) In FIG. 21 the roof 16 is shown as a transparent layer so as to reveal details of each nozzle device 10. Typically, the roof 16 is comprised of a material, such as silicon dioxide or silicon nitride.
(55) The main chamber 12 of the nozzle device 10 comprises a firing chamber 22 and an antechamber 24. The firing chamber 22 comprises a nozzle aperture 26 defined in the roof 16 and an actuator in the form of a resistive heater element 28 bonded to the floor 14. The antechamber 24 comprises a main chamber inlet 30 (floor inlet 30) defined in the floor 14. The main chamber inlet 30 meets and partially overlaps with an endwall 18B of the antechamber 24. This arrangement optimizes the capillarity of the antechamber 24, thereby encouraging priming and optimizing chamber refill rates.
(56) A baffle plate 32 partitions the main chamber 12 to define the firing chamber 22 and the antechamber 24. The baffle plate 32 extends between the floor 14 and the roof 16.
(57) The antechamber 24 fluidically communicates with the firing chamber 22 via a pair of firing chamber entrances 34 which flank the baffle plate 32 on either side thereof. Each firing chamber entrance 34 is defined by a gap extending between a respective side edge of the baffle plate 32 and the perimeter wall 18.
(58) The nozzle aperture 26 is elongate and takes the form of an ellipse having a major axis aligned with a central longitudinal axis of the heater element.
(59) The heater element 28 is connected at each end thereof to respective electrodes 36 exposed through the floor 14 of the main chamber 12 by one or more vias 37. Typically, the electrodes 36 are defined by an upper metal layer of the CMOS layer 20. The heater element 28 may be comprised of, for example, titanium-aluminium alloy, titanium aluminium nitride etc. In one embodiment, the heater 28 may be coated with one or more protective layers, as known in the art.
(60) The vias 37 may be filled with any suitable conductive material (e.g. copper, tungsten etc.) to provide electrical connection between the heater element 28 and the electrodes 36. A suitable process for forming electrode connections from the heater element 28 to the electrodes 36 is described in U.S. Pat. No. 8,453,329, the contents of which are incorporated herein by reference.
(61) Part of each electrode 36 may be positioned directly beneath an end wall 18A and baffle plate 32 respectively. This arrangement advantageously improves the overall symmetry of the device 10, as well as minimizing the risk of the heater element 28 delaminating from the floor 14.
(62) As shown most clearly in FIG. 21, the main chamber 12 is defined in a blanket layer of material 40 deposited onto the floor 14 and etched by a suitable etching process (e.g. plasma etching, wet etching etc.). The baffle plate 32 and the perimeter wall 18 are defined simultaneously by this etching process, which simplifies the overall MEMS fabrication process. Hence, the baffle plate 32 and perimeter wall 18 are comprised of the same material, which may be any suitable etchable ceramic or polymer material suitable for use in printheads. Typically, the material is silicon dioxide or silicon nitride.
(63) A printhead 100 may be comprised of a plurality of inkjet nozzle devices 10. The partial cutaway view of the printhead 100 in FIG. 21 shows only two inkjet nozzle devices 10 for clarity. The printhead 100 is defined by a silicon substrate 102 having the passivated CMOS layer 20 and a MEMS layer containing the inkjet nozzle devices 10. As shown in FIG. 21, each main chamber inlet 30 meets with an ink supply channel 104 defined in a backside of the printhead 100. The ink supply channel 104 is generally much wider than the main chamber inlets 30 and provides a bulk supply of ink for hydrating each main chamber 12 in fluid communication therewith. Each ink supply channel 104 extends parallel with one or more rows of nozzle devices 10 disposed at a frontside of the printhead 100. Typically, each ink supply channel 104 supplies ink to a pair of nozzle rows (only one row shown in FIG. 21 for clarity), in accordance with the arrangement shown in FIG. 21B of U.S. Pat. No. 7,441,865.
(64) The printhead 100 may be fabricated by building the MEMS layer containing inkjet nozzle devices 10 on a wafer substrate having the plugged hole shown in FIG. 7. The planarized frontside surface 3 of the substrate facilitates frontside MEMS fabrication processes. After frontside MEMS fabrication steps are completed, the wafer is thinned from a backside and the ink supply channels 104 are etched from the backside to meet with the plugged frontside holes. Finally, the polymer plug (e.g. polymers 7 and 9) is removed from the frontside hole 2 by oxidative ashing to define the main chamber inlets 30.
(65) It will, of course, be appreciated that the present invention has been described by way of example only and that modifications of detail may be made within the scope of the invention, which is defined in the accompanying claims.
