METHOD OF MAKING A RESISTANCE HEATER

Abstract: A unique inverse processed film resistance heater struc­ ture is disclosed. A conventional passivation wear layer (110) is deposited directly on a first substrate (120), followed by the deposition and patterning of resistance (140) and conductive (150) layers, and covered by an isolation layer (180) and a thick support layer (190). The thick support layer is then bonded to a second substrate (310) and the first substrate (120) is re­ moved so that a uniform, flat passivation layer is exposed. The result is a film resistor which has a reduced failure rate as com­ pared to the prior art because it is covered by a passivation wear layer with fewer pin-holes and reduced stress.

Description: INVERSE PROCESSED RESISTANCE HEATER This invention is concerned with resistance heaters and method of manufacture thereof. Thermal ink jet resistors and direct writing thermal print heads have conventionally been fabricated by means of standard thick and thin film resistor deposition techniques. In one example of this technique as shown in Figure 1 a thin layer of resistor material 10, such as 500 angstroms thickness of tantalum/aluminium alloy is deposited on an isolation layer 15 such as silicon dioxide overlaying a silicon substrate 20. The isolation layer 15 provides the necessary electrical and thermal insulation between the resistive layer 10 and the silicon subs#trate 20. A conductive layer 30 such as 1 micron of aluminium is deposited on top of the resistance layer 10, and the conductive layer 30 and resistance layer 10 are patterned forming a resistor 40 connected by conductors 50. Finally, a passivation wear layer 60, for example 2-3 microns of silicon dioxide or silicon carbide, is deposited over the entire structure. The resistors 40 are then used to heat the ink or thermal paper which is just above the passivation layer 60. In such film resistor devices, failures often occur in regions where there is a step height change in the surface profile, such as in a region 70 in Figure 1, which results from patterning the resistance layer 10 and conductive layer 30. Stress in the passivation wear layer 60 is highest in the step regions 70, and the occurrence of pin-holes is greatest along these steps. It is possible to reduce the stress and pin-holes in the passivation layer 60 by making the passivation layer 60 thicker, but this is usually undesirable since it increases the thermal isolation of the resistor 40 from the ink or paper, thereby reducing heat transfer from the resistor 40 to the ink or paper and causing higher resistor temperatures which can induce further failures. The present invention provides a method of fabricating a resistance heater on a substrate comprising, in order, the steps of depositing a first layer onto the substrate, and depositing a resistor connected to a plurality of conductors onto said first layer; characterized in that the first layer comprises a first passivation layer and in that the method further comprises the steps of depositing a support layer over the resistor and plurality of conductors; and removing the substrate thereby exposing the first passivation layer. In carrying out a method as set forth in the last preceding paragraph, it is preferred that the first layer comprises a second passivation layer and that the method further comprises the step of depositing the second passivation layer after depositing the first passivation layer and before depositing the resistor and the plurality of conductors. A method as set forth in either one of the last two immediately preceding paragraphs may further comprise the step of depositing an isolation layer after depositing the resistor and the plurality of conductors and before depositing the support layer. A method as set forth in any one of the last three immediately preceding paragraphs may further comprise the step of bonding a second substrate to the support layer, after depositing the support layer and before removing said substrate. Alternatively, the second substrate may be bonded to the support layer after said substrate has been removed. In carrying out a method as set forth in any one of the last four immediately preceding paragraphs, it is preferred that the step of depositing a resistor connected to a plurality of conductors onto said first layer comprises the steps of depositing a layer of resistive or conductive material, depositing a layer of conductive or resistive material thereon, and patterning the materials to form the resistor and the plurality of conductors. The present invention further provides a resistance heater comprising a uniformly thick layer, and a film resistor connected to a plurality of conductors, covered by the uniformly thick layer; and characterized by a support layer covering the film resistor and the plurality of conductors. In a heater as set forth in the last preceding paragraph, it is preferred that said uniformly thick layer is a passivation layer and is substantially flat. In a heater as set forth in either one of the last two immediately preceding paragraphs, it is preferred that a substrate is coupled to the support layer. In a heater as set forth in any one of the last three immediately preceding paragraphs, it is preferred that the support layer comprises a conductive layer and in that an insulating isolation layer is provided between the conductive layer and the film resistor connected to the plurality of conductors. In a heater as set forth in the last preceding paragraph it is preferred that the insulating isolation layer is 2-3 microns thick and comprises silicon dioxide, and the conductive layer is 10-1000 microns thick and comprises a metal. In a heater as set forth in the last preceding paragraph but three or in any one of the last three immediately preceding paragraphs as appended thereto, it is preferred that said uniformly thick passivation layer comprises a first uniformly thick sublayer, and a second uniformly thick sublayer between said first sublayer and the film resistor connected to the plurality of conductors. In a heater as set forth in the last preceding paragraph, it is preferred that said first uniformly thick sublayer is 1-2 microns thick and comprises silicon carbide, and the second uniformly thick sublayer is less than 0.5 microns thick and comprises silicon dioxide. Height changes in the passivation wear layer between the film resistor and the ink in a thermal ink jet printer or the thermal paper in a direct writing print head can be eliminated by fabricating the device in reverse order as compared to conventional film resistors and then etching away the underlying substrate. The result is an inverse fabricated resistor with reduced failures due to stress or pin-holes in the passivation layer. A passivation film such as 1-2 microns of silicon dioxide or silicon carbide is deposited directly on a first substrate such as silicon or glass to form a flat, smooth passivation wear layer. This is followed by deposition and subsequent patterning of resistance and conductive layers, for example made of 500 angstroms of tantalum/aluminium and 1 micron of aluminium respectively. A thermal isolation layer such as 2-3 microns of silicon dioxide is then deposited over the resistor and conductor pattern, followed by a thick layer (10-1000 microns) of a metal such as nickel or copper, which serves as both a heat sink and support layer. The thick metal layer may then be bonded to a support bearing substrate and the first substrate is removed for example by etching. The result is a film resistor overlain with a uniform, thin passivation wear layer which can be used to produce localized heating as needed in a thermal ink jet printer or in a contact thermal printing head with increased reliability over the prior art. There now follows a description, which is to be read with reference to Figures 2 and 3 of the accompanying drawings, of a heater according to the invention and a method by which it can be fabricated; it is to be clearly understood that the heater and method have been selected for description to illustrate the invention by way of example and not by way of limitation. In Figures 2 and 3: Figure 2 shows a preferred embodiment of an intermediate thermal heater structure according to the present invention; and Figure 3 shows a preferred embodiment of the final thermal heater structure according to the present invention. A first passivation layer 110 for example of 1-2 microns of silicon carbide is deposited on a first substrate 120 such as a 0.5mm thick silicon wafer. The first substrate 120 can also be made of glass or other etchable materials which are smooth and flat. A second passivation layer 130,for example 0.2-0.5 microns of silicon dioxide, is then deposited on top of the first passivation layer 110. In alternative embodiments, the first passivation layer 110 and second passivation layer 130 may be made of other suitable passivation materials or combined as a single passivation layer made from silicon carbide, silicon dioxide or other suitable passivation materials that are well known in the art. In either case, the result is a passivation layer which is flat and smooth with very few pin-holes. A resistive layer 140, such as 500 angstroms of tantalum/aluminium, and a conductive layer 150, such as 1.0 micron of aluminium, are deposited on the passivation layers 110 and 130 and then patterned to form resistors 160 and conductors 170. In Figure 2 the conductive layer 150 is on top of the resistive layer 140, but the order of these layers can also be reversed. An isolation layer 180 such as 2-3 microns of silicon dioxide is then deposited on the patterned resistors 160 and conductors 170. Then a support layer 190 of a film such as 100-200 microns of nickel or copper is deposited on the isolation layer 180. The support layer 190 can be fabricated for example by sputtering or evaporating a thin coat of metal film followed by electroplating of the necessary relatively thick support layer 190. The support layer 190 forms a good heat sink and support layer during subsequent processing and use. The isolation layer 180 thus serves to provide thermal and electrical insulation between the resistor 160 and the support layer 190. As shown in Figure 3, the support layer 190 of the intermediate structure of Figure 2 is then bonded to a second substrate 310. Finally, the first substrate 120 of Figure 2 is removed by an appropriate process such as etching to reveal the resistor 160 completely covered by the uniform and flat passivation layers 110 and 130. In alternative embodiments, the isolation layer 180 and support layer 190 can be made sufficiently thick so as to eliminate the need of the second substrate 310, or the first substrate 120 may be removed before the application of the second substrate 310. As would be apparent to one skilled in the art, the previously described invention is not only suitable for the production of resistors in thermal ink jet printers and direct writing thermal print heads, but also has various other uses for power film resistors which are subjected to high temperatures and high mechanical stress.

Claims: CLAIMS 1. A method of fabricating a resistance heater on a substrate (120), comprising, in order, the steps of: depositing a first layer onto the substrate; and depositing a resistor (140) connected to a plurality of conductors (150) onto said first layer; characterized in that the first layer comprises a first passivation layer (110); and in that the method further comprises the steps of depositing a support layer (190) over the resistor and plurality of conductors; and removing the substrate (120) thereby exposing the first passivation layer (110). 2. A method according to claim 1 characterized in that the first layer further comprises a second passivation layer (130) and in that the method further comprises the step of depositing the second passivation layer after depositing the first passivation layer and before depositing the resistor and the plurality of conductors. 3. A method according to either one of claims 1 and 2 characterized by the step of depositing an isolation layer (180) after depositing the resistor and the plurality of conductors and before depositing the support layer. 4. A method according to any one of the preceding claims characterized by the step of bonding a second substrate (310) to the support layer, after depositing the support layer and before removing said substrate (120). 5. A method according to any one of claims 1 to 3 characterized by the step of bonding a second substrate (310) to the support layer after said substrate has been removed. 6. A method according to any one of the preceding claims characterized in that the step of depositing a resistor connected to a plurality of conductors onto said first layer comprises the steps of depositing a layer of resistive or conductive material, depositing a layer of conductive or resistive material thereon, and patterning the materials to form the resistor and the plurality of conductors. 7. A resistance heater comprising: a uniformly thick layer, and a film resistor (140) connected to a plurality of conductors (150), covered by the uniformly thick layer; and characterized by a support layer (190) covering the film resistor and the plurality of conductors. 8. A resistance heater according to claim 11 characterized in that said uniformly thick layer is a passivation layer (110) and is substantially flat. 9. A resistance heater according to either one of claims 7 and 8 further characterized by a substrate (310) coupled to the support layer. 10. A resistance heater according to any one of claims 7 to 9 characterized in that the support layer comprises a conductive layer and in that an insulating isolation layer (180) is provided between the conductive layer and the film resistor connected to the plurality of conductors. 11. A resistance heater according to claim 10 characterized in that the insulating isolation layer is 2-3 microns thick and comprises silicon dioxide, and the conductive layer is 10-1000 microns thick and comprises a metal. 12. A resistance heater according to claim 8 or any one of claims 9 to 11 as appended to claim 8 characterized in that said uniformly thick passivation layer comprises: a first uniformly thick sublayer (110); and a second uniformly thick sublayer (130) between said first sublayer and the film resistor connected to the plurality of conductors. 13. A resistance heater according to claim 12 characterized in that said first uniformly thick sublayer (110) is 1-2 microns thick and comprises silicon carbide, and the second uniformly thick sublayer (130) is less than 0.5 microns thick and comprises silicon dioxide.

Inventors: LLOYD, WILLIAM JAMES
Applicants: HEWLETT-PACKARD COMPANY
Application year: 1983
Main classification: H01L49
IPC codes: B41J2, H05B3, H01C7