EP0113950B1

EP0113950B1 - Method of making a resistance heater

Info

Publication number: EP0113950B1
Application number: EP19830304617
Authority: EP
Inventors: William James Lloyd
Original assignee: Hewlett Packard Co
Current assignee: HP Inc
Priority date: 1982-11-24
Filing date: 1983-08-10
Publication date: 1989-03-29
Also published as: JPH0340911B2; JPS59155067A; EP0113950A3; EP0113950A2; DE3379526D1

Description

This invention is concerned with a method of fabricating a resistance heater comprised of a support and a plurality of superposed layers disposed at one side of said support.
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 known from US-A-4 169 032 and shown in Figure 1 a thin layer of resistor material 10, such as 50 nm 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 substrate 20. A conductive layer 30 such as 1 ₁₁m 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 11m 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 occurence 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 of 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 in such a way that the discussed deficiencies are overcome.
This method is characterized by providing a substrate having a plane surface, depositing on said surface of said substrate successively a uniformly thick passivation layer, a resistive layer, a conductive layer which is patterned to form resistors and conductors, and a support layer, and then removing said substrate to expose the plane surface of said uniformly thick passivation layer.
Thus, according to the invention, 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 are 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.
In carrying out the method of the invention, 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 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, and/or 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 allows the fabrication of a resistance heater comprising a uniformly thick passivation layer which is substantially flat, a film resistor connected to a plurality of conductors, covered by the uniformly thick passivation layer, and a support layer covering the film resistor and the plurality of conductors. The support layer may be produced to comprise a conductive layer, and an insulating isolation layer may be provided between the conductive layer and the film resistor connected to the plurality of conductors.
In a heater fabricated according to the invention, it is preferred that the insulating isolation layer is 2-3 11m thick and comprises silicon dioxide, and the conductive layer is 10-1000 pm thick and comprises a metal.
In a preferred embodiment of the invention, a passivation film such as 1-2 µm 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 50 nm of tantalum/aluminium and 1 pm of aluminium respectively. A thermal isolation layer such as 2-3 pm of silicon dioxide is then deposited over the resistor and conductor pattern, followed by a thick layer (10-1000 11m) 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 produced according to the invention and the method by which it is 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 an intermediate thermal heater structure and
Figure 3 shows the final thermal heater structure both produced according to a preferred embodiment of the present invention.

A first passivation layer 110 for example of 1-2 11m of silicon carbide is deposited on a first substrate 120 such as a 0.5 mm 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 11m 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 50 nm oftantalum/ aluminium, and a conductive layer 150, such as 1.0 11m 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 µmof 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 ^pm 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

1. A method of fabricating a resistance heater comprised of a support (190) and a plurality of superposed layers (180, 170, 140, 130, 110) disposed at one side of said support, characterized by, providing a substrate (120) having a plane surface, depositing on said surface of said substrate successively a uniformly thick passivation layer (110), a resistive layer (140), a conductive layer (150) which is patterned to form resistors (160) and conductors (170), and a support layer (190), and then removing said substrate to expose the plane surface of said uniformly thick passivation layer.

2. A method according to claim 1, characterized by providing a further substrate (310) and bonding the resistance heater with its support (190) to said further substrate.

3. A method according to claim 2, characterized by bonding the resistance heater to said further substrate (310) before said substrate (120) adjacent said uniformly thick passivation layer (110) is removed.

4. A method according to any one of claims 1 to 3, characterized in that said support layer comprises a conductive support layer (190) and in that an insulating isolation layer (180) is provided between said conductive support layer and said patterned conductive layer (150) disposed adjacent said resistive layer (140).

5. A method according to claim 4, characterized in that said isolation layer (180) is 2 to 3 um thick and comprises silicon dioxide, and the conductive support layer (190) is 10 to 1000 µm thick and comprises a metal.

6. A method according to claim 4 or 5, 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 said resistive layer (140).

7. A method according to claim 6, characterized in that said first uniformly thick sublayer (110) is 1 to 2 pm thick and comprises silicon carbide, and said second uniformly thick sublayer (130) is less than 0.5 µm thick and comprises silicon dioxide.