Method of producing substrate for ink jet recording head, ink jet recording head and ink jet recording apparatus

A method for producing a substrate for an ink jet recording head comprises preparing a substrate with plural heat generating resistors for applying heat to the ink, plural wirings electrically connected thereto, and plural heat generating areas formed by the heat generating resistors exposed from the wirings, coating the heat generating resistors and the wirings on the substrate with a first insulating protective film, removing the first insulating protective film by wet etching in portions on the heat generating areas, and coating thus etched first insulating protective film with a second insulating protective film, wherein the etched portion of the first insulating protective film, in the longitudinal direction of the heat generating area, is positioned inside from the end of the heat generating area, by at least 1/2 of the thickness of the first and second insulating protective films covering the wirings. The protective film is thus made thinner on the heat generating areas, thus reducing the electric power consumption while maintaining sufficient durability.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an ink jet recording head, and more 
particularly to a method for producing a heat-generating substrate for an 
ink jet recording head adapted for effecting recording by ink discharge 
from a discharge opening by growth and contraction of a bubble generated 
in the ink by a discharge energy generating element, such recording head 
and a recording apparatus utilizing such recording head. 
2. Related Background Art 
The ink jet recording method described in the U.S. Pat. No. 4,723,129 or 
No. 4,740,796 is recently attracting particular attention as it is capable 
of image recording with a high definition and high image quality at a high 
speed and a high density, and is also suitable for color image recording 
and for compactization of the apparatus. In a representative configuration 
of the recording apparatus employing such method, there is provided a heat 
action area for applying heat to the recording liquid or the like 
(hereinafter called ink) in order to discharge the ink by thermal energy. 
More detailedly, corresponding to an ink flow path, there is provided an 
electro-thermal converting element including a pair of connecting 
electrodes and a heat-generating resistance layer connected between said 
electrodes and adapted to generate heat in the area between the 
electrodes, and the thermal energy generated from said heat-generating 
resistance layer is utilized for rapidly heating the ink on the heat 
action area to generate bubble whereby the ink is discharged by such 
bubble generation. 
Since such heat action area of the ink jet recording head is exposed to 
severe conditions including mechanical impact and erosion resulting from 
cavitation caused by repeated bubble generation and extinction in the ink 
and temperature ascent and descent of about 1000.degree. C. within an 
extremely short time of 0.1 to 10 microseconds, there is provided a 
protective film for protecting the heat-generating resistance layer from 
such harsh conditions. Such protective film is required to be excellent in 
heat resistance, liquid resistance, resistance to liquid permeation, 
stability against oxidation, electric insulation, breakage resistance and 
thermal conductivity, and is generally composed of an inorganic compound 
such as SiO or SiN. Also a single-layered protective film may not be 
sufficient for protecting the heat-generating resistance layer, and a 
metallic film of higher anticavitation property, composed for example of 
Ta, may be provided on the protective film. 
The above-explained configuration is employed not only on the 
heat-generating resistance layer but also on the wiring patterns for 
electric connection with the heat-generating resistance layer, in order to 
prevent corrosion of the wirings by the ink. 
FIG. 3 is a schematic plan view of a part of the substrate for a 
conventional ink jet recording head, and FIG. 4 is a partial 
cross-sectional view of said substrate along a chain line 4--4, in FIG. 3. 
Referring to FIGS. 3 and 4, a Si substrate 120 is provided thereon with a 
heat accumulating layer 106 composed of SiO.sub.2, formed for example by 
thermal oxidation. On said substrate 120 with the heat accumulating layer 
106, there are formed a heat-generating resistance layer 107 for applying 
thermal energy to the ink, and wirings 103, 104 for applying a voltage to 
said heat-generating resistance layer. A part of the heat-generating 
resistance layer 107, exposed from the wirings 103, 104 constitutes a 
heat-generating portion 102. On said heat-generating resistance layer and 
wirings, there are provided an insulating protective film 108 and an 
anticavltation Ta film 110. 
In the ink jet recording head, the heat-generating substrate constituting 
the heat action area is constructed as explained above, and the structure 
of the protective film mentioned above is an important factor determining 
the performance of the ink jet recording head, such as the electric power 
consumption and the service life thereof. 
However, in the conventional configuration of the protective film, the 
reduction in electric power consumption is a trade-off to the improvement 
in film reliability and service life. 
For example, the electric power required for bubble generation can be 
reduced as the film between the heat-generating resistance and the ink 
becomes thinner or has a higher thermal conductivity, since heat 
dissipation other than to the ink can be reduced. Stated differently, the 
efficiency of energy can be improved as the protective film becomes 
thinner. 
On the other hand, a thinner protective film is apt to form pinholes 
thereon or to be unable to sufficiently cover the stepped portion of the 
wiring, resulting in defective coverage on such stepped portion. Such 
defective coverage results in ink intrusion, thus leading to erosion of 
the wiring and the heat-generating resistance and deterioration in the 
reliability and in the service life. 
In consideration of the foregoing, the Japanese Patent Laid-open 
Application No. 62-103148 discloses a configuration of forming the 
protective film thinner only in a portion thereof involved in the bubble 
generation, thereby reducing the electric power consumption while 
improving the film reliability and the service life. 
However, in the above-mentioned patent, dry half etching is suggested for 
forming the thinner portion of the protective film, but the film thickness 
is difficult to control with such method because the film thickness is 
principally controlled by the etching time in this method. 
On the other hand, on the heat-generating portion in the ink jet recording 
head, the protective film is required to have a uniform thickness, since, 
if the protective film on the heat-generating portion is uneven in 
thickness, the center of bubble generation may be displaced from the 
center of the heat-generating resistor or the bubble generating 
characteristics may be altered to affect the ink discharge 
characteristics. 
As explained in the foregoing, the conventional configuration is apt to 
cause fluctuation in the thickness of the protective film on the heat 
generating portion in the recording head, so that uniform discharge 
characteristics are difficult to obtain among different discharge openings 
and there may result deterioration in the print quality. 
SUMMARY OF THE INVENTION 
The present invention has been attained in consideration of the prior art 
explained above, and an object thereof is to provide an ink jet recording 
head which enables easy control of the film thickness, thereby providing 
stable ink discharge performance. Another object of the present invention 
is to provide an ink jet recording head capable of reducing the electric 
power consumption for bubble generation, while improving the reliability 
and extending the service life. 
The above-mentioned objects can be attained, according to the present 
invention, by a method for producing a substrate for an ink jet recording 
head provided with at least two insulating protective films, comprising a 
step of preparing a substrate having thereon plural heat-generating 
resistors for applying heat to the ink, plural wiring electrically 
connected to said heat-generating resistors, and plural heat-generating 
portions composed of said heat-generating resistors exposed from said 
wirings; a step of coating said heat-generating resistors and said wirings 
on said substrate with a first insulating protective film; a step of 
eliminating said first insulating protective film with wet etching in 
areas on said heat-generating portions; and a step of applying a second 
insulating protective film on said first insulating protective film 
subjected to said etching, wherein the etched portion of said first 
insulating protective film in the longitudinal direction of said heat 
generating portion is provided inside the ends of the heat generating 
portion, by at least 1/2 of the thickness of said first and second 
insulating protective films covering said wiring. According to the present 
invention, the thickness of the thinner portion of the protective films 
can be securely controlled as the heat-generating resistor can be utilized 
as the etching stopper, so that there can be obtained an ink jet recording 
head with uniform ink discharge characteristics. Also there can be 
obtained an ink jet recording head with a reduced electric power 
consumption for bubble generation, with improved reliability and elongated 
service life.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Now the present invention will be clarified in detail in the following 
description. 
The present invention achieves different film thicknesses without half 
etching by employing a two-layered structure in the insulating protective 
film, thereby enabling secure film thickness control in the thinner 
portion of the film and eliminating the fluctuation in the thickness of 
the protective film on the heat-generating area. Also the insulating 
protective film of the present invention is free from, in the 
multi-layered structure thereof, interfacial peeling as sometimes 
encountered in the conventional configuration consisting of an inorganic 
film and an organic film, whereby the reduction in the electric power 
consumption can be securely achieved without deterioration in the 
reliability of the recording head. 
The first insulating protective film is-composed of a material with a high 
wet etching rate selected among the material ordinarily employed in the 
semiconductor process, and preferred examples of such material include PSG 
and SiO. 
Also the second insulating protective film is composed of a material 
showing few pinholes even at a small thickness and being excellent in 
insulating property, thermal conductivity and ink resistance, and 
preferred examples of such material include SiN and SiO. 
The thickness has to be about 1 .mu.m as in the conventional structure at 
least on the electrodes, but, in the heat-generating areas, can be at 
least 2000 .ANG., preferably at least 3000 .ANG. for securing the 
durability as in the conventional configuration. This is because TaN 
constituting the heat-generating resistance layer has a smoother surface 
in comparison with the Al electrode, so that pinhole formation can be 
suppressed even with a smaller film thickness. On the other hand, the 
effect of electric power reduction can no longer be observed if the film 
thickness on the heat generating area exceeds about 7000 .ANG.. 
Consequently the thickness of the film in the thinner portion thereof is 
preferably selected within a range from 2000 to 7000 .ANG.. 
In the following there will be explained embodiments of the present 
invention with reference to the attached drawings, but the present 
invention is not limited by such embodiments and can assume any form that 
can attain the objects of the present invention. 
Embodiment 1 
FIG. 1 is a plan view of a heat generating substrate, for generating 
bubbles in the ink, in an ink jet recording head, constituting an 
embodiment of the present invention, and FIG. 2 is a partial vertical 
cross-sectional view along a chain line 2--2 in FIG. 1. 
The heat-generating substrate of the present embodiment is prepared from Si 
substrate 120 or a Si substrate on which driving IC's are already formed. 
In case of the Si substrate, a heat accumulating SiO.sub.2 layer is formed 
by thermal oxidation, sputtering or CVD, under the heat-generating 
resistors. Also in case of the Si substrate bearing the driving IC's, a 
heat accumulating SiO.sub.2 layer is formed with a thickness of 2.9 .mu.m 
in the manufacturing process. Said layer is indicated by 106 in FIG. 1. 
Then a TaN layer 107 serving as the heat generating resistor is formed by 
reactive sputtering with a thickness of ca. 1000 .ANG., and Al layers 103, 
104 serving as the wirings are formed by sputtering with a thickness of 
6000 .ANG.. 
Subsequently wiring patterns shown in FIG. 1 are formed by a 
photolithographic process, and Al and TaN are etched consecutively by 
reactive etching. 
Then the photolithographic process is used again and Al is removed by wet 
etching, in order to expose the heat generating portion as indicated by 
102 in FIGS. 1 and 2. Such removed portion constitutes the heat generating 
resistor. Ends of the wiring patterns are formed as bonding pads in case 
of the Si substrate, but are connected to the lower electrodes through 
contact holes in case of the substrate bearing IC's thereon. 
Then, on the Si substrate, a PSG layer serving as the first insulating 
protective film is formed by plasma CVD with a thickness of 7000 .ANG.. 
Subsequently a window pattern is formed, by a photolithographic process, 
inside the heat Generating area 105 shown in FIGS. 1 and 2 by at least 0.5 
.mu.m as represented by 108a shown in FIG. 2 so as to avoid the influence 
of step difference in the electrodes, and wet etching is conducted with 
buffered fluoric acid for 1 to 5 minutes until the PSG layer is etched 
off. The buffered fluoric acid has an etching rate of 2000-10000 
.ANG./min. for the PSG layer. As long as the range of the window formed by 
the photolithographic process is on the heat generating resistor, the wet 
etching does not require particular control in time because the heat 
generating resistor consisting of TaN serves as an etching stopper, but, 
in consideration of the step coverage, the distance from the end of the 
electrode to the window in the longitudinal direction thereof is 
preferably at least 1/2 of the thickness of the protective film provided 
thereon. In the present embodiment, the window is positioned at a distance 
of 0.5 .mu.m from the end face of the Al electrode as explained before. 
Then an SiN layer 108b constituting the second insulating protective film 
is formed by plasma CVD with a thickness of 3000 .ANG., so as to cover 
thus pattern PSG layer. Since the PSG layer and the SiN layer are both 
formed at 300.degree. C. or higher, both layers show extremely strong 
mutual adhesion, thus scarcely resulting in interfacial peeling which is 
sometimes encountered in the conventional two-layered structure consisting 
of an inorganic film and an organic film. In this manner there can be 
formed an insulating protective film having a thickness of 3000 .ANG. in 
the heat generating areas and a thickness of 10000 .ANG. in other parts. 
Then, on said inorganic insulating film, Ta is deposited by sputtering as 
an anticavttation and ink resistant film 110 shown in FIG. 2, with a 
thickness of ca. 2500 .ANG.. Finally Ta, PSG and SiN are 
photolithographically removed by reactive etching to form wire bonding 
pads, whereby a heat-generating substrate 101 in FIG. 2, for bubble 
formation in the ink, for use in the ink jet recording head, is completed. 
Said substrate in the ink jet recording head, is completed. Said substrate 
is used in the known manner for preparing an ink jet recording head. 
The ink jet recording head thus prepared was subjected to ink discharge 
with a frequency of 3 kHz, with a voltage of ca. 23 V and a pulse duration 
of 7 .mu.s corresponding to 1.3 times of the bubble forming energy. The 
breakage by the destruction of the heat generating resistors was not 
observed until 3.times.10.sup.8 pulses, so that the durability was 
comparable to that of the ordinary protective film with a thickness of 1 
.mu.m. Also the electric power consumption required for bubble formation 
was about 30% less in case of the protective film of 3000 .ANG. on the 
heat generating resistors, in comparison with the ordinary protective film 
of 1 .mu.m. 
Embodiment 2 
In the Embodiment 1, the window pattern of the first insulating protective 
film is formed inside the heat generating resistor, so that the exposed 
width thereof is determined by said window pattern. However, particularly 
in case the heat generating resistors are arranged with a high density, it 
may become impossible to secure enough accuracy for the window pattening, 
so that the widths of the heat generating resistors become uneven. Such 
uneven widths of the heat generating resistors lead to uneven discharge 
characteristics at the ink discharge openings, thereby deteriorating the 
print quality. In consideration of such drawback, the window pattern in 
the first insulating protective film in this embodiment is made larger 
than the heat generating resistor in the direction of array thereof, 
whereby the width of the heat generating area is always defined by the 
width of the heat generating resistor. Consequently there can be obtained 
an ink jet recording head with uniform ink discharge characteristics even 
in case the heat generating resistors are arranged with a high density. 
In the following there will be explained the method of producing the 
insulating protective films of the present embodiment. Other parts can be 
same as those in the Embodiment 1. 
After a heat accumulating layer, heat generating resistors and electrodes 
are prepared on a Si substrate as in the Embodiment 1, a PSG layer as the 
first insulating protective film is formed on said substrate by plasma CVD 
with a thickness of 7000 .ANG.. Then window are photolithographically 
formed on said first insulating protective film. In the present 
embodiment, the window pattern is formed, as shown in FIG. 5, inside by 
0.5 .mu.m from the end face of the electrodes in the longitudinal 
direction and outside by 4 .mu.m at each side of the heat generating 
resistor in the direction of array thereof. Such window pattern, made 
larger than the width of the heat generating resistor in the direction of 
array thereof, allows to obtain uniform widths of the heat generating 
areas, but such window pattern results in etching of a part of the heat 
accumulating layer. Consequently the etching ratio of the heat 
accumulating layer and the first insulating protective layer is selected 
as 1:4, so that the etch depth of the heat accumulating layer, even if it 
is etched, remains at 500 to 1500 .ANG. and the step coverage of the 
protective films in this area is not significantly deteriorated. The 
window patterning is achieved by wet etching with buffered fluoric acid 
for 1 to 5 minutes until the PSG layer is etched off, and the buffered 
fluoric acid is so selected to have etching rates of 2000 to 10000 
.ANG./min. for the PSG layer and 500 to 2500 .ANG./min. for the heat 
accumulating SiO.sub.2 layer. 
Subsequently an SiN layer, constituting the second insulating protective 
film, is formed by plasma CVD with a thickness of 3000 .ANG., so as to 
cover thus patterned PSG layer. Since the PSG layer and the SiN layer are 
both formed at a high temperature exceeding 300.degree. C., these two 
layers show extremely strong mutual adhesion and are substantially free 
from interfacial peeling, which is sometimes encountered in the 
two-layered structure consisting of an inorganic film and an organic film. 
In this manner there is obtained an inorganic insulating film having 
thicknesses of 3000 .ANG. in the heat generating areas and 10000 .ANG. in 
other parts. 
An ink jet recording head, utilizing thus obtained substrate of the present 
embodiment, did not show breakage by the destruction of the heat 
generating resistors up to 3.times.10.sup.8 pulses in an ink discharge 
durability test under same conditions as those in the Embodiment 1. Also 
the electric power consumption required for bubble generation was reduced 
by 30% in case the protective film of 3000 .ANG. was formed on the heat 
generating area, in comparison with the case with the ordinary protective 
film of 1 .mu.m. 
In the foregoing description, each heat generating area has an independent 
etched area in the first insulating protective film, but, in case the heat 
generating areas are arranged with a high density and a common wiring is 
formed in a lower layer, the etched portion of the heat generating area 
may be connected to that of another heat generating area adjacent in the 
direction of array of the heat generating areas, and the effect of the 
present invention can still be attained. 
In the following there will be explained the ink jet recording head and the 
ink jet recording apparatus in which the substrate of the present 
invention is applicable. 
FIG. 7 is a schematic view of such ink jet recording head, composed of 
electrothermal converters 1103, wirings 1104 and liquid path walls 1105 
formed on a substrate 1102 through semiconductor process steps such as 
etching, evaporation and sputtering, and a top plate 1106. 
Recording liquid 1112 is supplied, from an unrepresented liquid reservoir, 
through a liquid supply pipe 1107 to a common liquid chamber 1108 of the 
recording head 1101. 
1109 indicates a liquid supply pipe connector. The liquid 1112 supplied 
into the common liquid chamber 1108 is further supplied to the liquid 
paths 1110 by capillary action, and is stably maintained, by meniscus 
formation, at the surface of discharge openings (orifice surface) at the 
ends of the liquid paths. 
The energization of the electrothermal converter 1103 causes rapid heating 
of the liquid present on the face of said electrothermal converter, 
thereby generating a bubble in the liquid path, and the liquid is 
discharged from the discharge opening 1111 by the expansion and 
construction of said bubble to form a liquid droplet. 
FIG. 8 is a schematic perspective view of an ink jet recording apparatus in 
which the present invention is applicable, wherein a carriage HC engaging 
with a spiral groove 5005 of a lead screw 5004, rotated according to the 
forward or reverse rotation of a driving motor 5013 through transmission 
gears 5011, 5009, is provided with a pin (not shown) and is reciprocated 
as indicated by arrows. A paper support plate 5002 is provided to press a 
recording sheet toward a platen 5000 over the moving direction of the 
carriage. Photocouplers 5007, 5008 constitute home position detecting 
means, for detecting the presence of a carriage lever 5006 in the position 
of said photocouplers and switching the rotating direction of the motor 
5013. A support member 5016 is provided for supporting a cap member 5022 
for capping the front face of the recording head, and suction means 5015 
sucks the interior of said cap member, thereby effecting suction recovery 
of the recording head through a cap aperture 5023. A cleaning blade 5017 
and a member 5019 for advancing or retracting said blade are supported by 
a support plate 5018 of the main body. The cleaning blade is not limited 
to the illustrated form but can assume any known form. A lever 5012 for 
initiating the suction of the suction recovery operation is moved by a cam 
5020 engaging with the carriage, and is controlled by the driving force of 
the driving motor through known transmeans such as a clutch. 
These operations of capping, cleaning and suction recovery are conducted at 
respective positions by the function of the lead screw 5004 when the 
carriage is brought to the area at the home position side, and they are 
all applicable to the present embodiment if each desired operation is 
conducted at the known timing. Configurations explained above are 
excellent singly or in combination and constitute preferred embodiments 
for the present invention. The above-explained apparatus is further 
provided with drive signal supply means for driving the elements for 
generating the ink discharge pressure.