[0001] This invention is concerned with resistance heaters and method of manufacture thereof.
[0002] 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 substrate 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] Alternatively, the second substrate may be bonded to the support layer after said
substrate has been removed.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.
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.