GB2298395A

GB2298395A - Ink jet recording head

Info

Publication number: GB2298395A
Application number: GB9603978A
Authority: GB
Inventors: Masao Mitani; Kenji Yamada; Katsunori Kawasumi; Kazuo Shimizu; Osamu Machida
Original assignee: Hitachi Koki Co Ltd
Current assignee: Koki Holdings Co Ltd
Priority date: 1995-03-03
Filing date: 1996-02-26
Publication date: 1996-09-04
Anticipated expiration: 2016-02-26
Also published as: GB9603978D0; GB2298395B; FR2731180A1; JP3573515B2; JPH08238771A; DE19604268C2; FR2731180B1; DE19604268A1

Description

2 - 1 INK JET RECORDING HEAD 2298395 The present invention relates to a

recording device for using thermal energy to eject ink droplets toward a recording medium.

Japanese Patent Application (KOKAI) No. SHO-48-9622 and No. SHO-54-51837 describe ink jet recording devices that apply pulses of heat to ink to rapidly vaporize a portion of the ink and eject an ink droplet from an orifice using the expansion of the vaporized ink.

As described on page 58 in the 12/28/1992 edition of Nikei Mechanical and in the August 1988 edition of Hewlett Packard-Journal, the simplest method for applying pulses of heat to ink is by energizing thermal resistors, otherwise known as heaters. The common configuration of these conventional heaters includes a thin-film resistor, a thin-film conductor, an anti-oxidation layer formed on these thin films, and an anti- cavitation layer formed on the antioxidation layer to prevent cavitation thereof.

Japanese Patent Application (KOKAI) No. HEI-06-71888 describes a protection-layerless heater formed from a thinfilm resistor and a thinfilm conductor. The thin-film resistor is made from an alloy of Cr-Si-SiO or of Ta-Si-SiO, 2 - and the thin-film conductor is made from nickel. The excellent pulsive operation property and the excellent antioxidation and anti-galvanization properties of these materials do away with the need for the anti- oxidation layer and the anti-cavitation layer, so that the protection- layerless heater has a much simpler configuration. Because the ink is in direct contact with the thin-film resistor, nucleation boiling can be achieved much more quickly. The protectionlayerless heater has a greatly improved thermal efficiency. A head mounted with the protection-layerless heaters has an improved thermal efficiency characteristic and is capable of ejecting ink droplets at a greater ejection frequency.

Because ink can be ejected using much less energy than required with conventional heaters, the protectionlayerless heaters can be formed near active elements on an LSI chip for driving the protect ion-layerle s s heaters without fear of heating and raising the temperature of the LSI device. As described in Japanese Patent Application Kokai No. HEI-6238901 and No. HEI-6-297714, because the protection-layeriess heaters can be formed on the same LSI chip, a monolithic LSI head having a simple configuration can be manufactured. A drop-on-demand ink jet print head with a plurality of ejection nozzles can be fabricated in a twodimensional, integrated, and high-density structure. This head can be used to form a full-color ink jet printer capable 3 of printing at high speeds.

The present inventors performed full-color printing using the integrated print heads described above filled with various water-based inks. The present inventors discovered that actual life of some heads was less than expected. After further investigation, the present inventors further discovered that the problem-free heads had been filled with ink having relatively high specific resistance and practically neutral pH. Problematic heads with life less than expected had been filled ink having low specific resistances of from 10 2 to 10 3 Qcm and pH of 8 to 9.

It is therefore an objective of the present invention to overcome the above-described problems and to provide a print head and heaters with the same heating and bubble generating properties of the protect ion-1 ayerle s s heater, but with long life even used with ink having low specific resistance and a non-neutral nature.

In order to attain the objective and other objectives, the present invention provides an ink ejection print head for ejecting ink droplets to print an image, the print head comprising: a silicon substrate; a partition wall provided on the silicon substrate for defining a plurality of individual ink channels; a plurality of heaters provided in the individual ink channels, each heater being made from a 4 - thin-film resistor and a thin-film conductor formed on the silicon substrate, a surface of each thin-film resistor having an electrically- insulation layer formed by hightemperature thermal oxidation of the thin- film resistor; and an ejection nozzle portion formed with a plurality of nozzles at positions in correspondence with the plurality of heaters.

According to another aspect, the present invention provides an ink ejection printer for printing an image with ejected ink, the printer comprising: a print head including a silicon substrate, a partition wall provided on the silicon substrate for defining a plurality of individual ink channels, a plurality of heaters provided in the individual ink channels, each heater being made from a thin-film resistor and a thinfilm conductor formed on the silicon substrate, a surface of each thinfilm resistor having an electricallyinsulation layer formed by hightemperature thermal oxidation of the thin-film resistor, and an ejection nozzle portion formed with a plurality of nozzles at positions in correspondence with the plurality of heaters; support means for supporting an image recording medium at a position confronting the plurality of nozzles of the print head; and motion means for attaining a relative motion between the print head and the support means in a direction orthogonal to a direction along which the plurality of nozzles are aligned.

According to a further aspect, the present invention provides a method of fabricating an ink ejection head, the method comprising the steps of: providing a plurality of heaters to one surface of a silicon substrate, each heater including a thin-film resistor and a thin-film conductor; subjecting each thin-film resistor to a thermal oxidation process, thereby forming an electricaIly-insulation layer on an exposed surface of the tl-iin-film res-stors; partition wall on the surface of the silicon substrate, the partition wall being formed with a plurality of ink channels in correspondence with the plurality of heaters; and forming an orifice plate on the surface of the silicon substrate, the orifice plate being formed with a plurality of nozzles.

During the thermal oxidation process, the plurality of thin-film resistors may be pulsingly energized in an oxidizing atmosphere. The thermal oxidation process may include the steps of: monitoring the resistance values of the respective thin-film resistors while the thin-film resistors are pulsingly energized; and adjusting the pulsingly energization applied to the respective thin-film resistors based on the monitored results, thereby controlling the resistance values of all the thin-film resistors to be substantially uniform.

Preferred embodiments of this invention will now be described with reference to the accompanying drawings, in which:- 6 - Fig print head invention; Fig. 2 (a) is a cross-sectional view of the ink jet print head of the first embodiment taken along a line IIA IIAI of Fig. 2(b); Fig. 2 (b) is a sectional view of the ink jet print head of the first embodiment taken along a line IIB - IIB' of Fig. 2(a); Fig. bubbles and embodiment; Fig. bubbles and tive example; Fig. 4 is a graph showing how the resistance of a TaSi-SiO alloy thin- film resistor changes in the oxidizing atmosphere under temperature of 500 IC: and Fig. 5 is an enlarged sectional view of an ink jet print head according to a second embodiment.

1 is an enlarged sectional view of an ink jet of a first embodiment according to the present 3 (a) illustrates observation results showing how ink droplets move in the nozzle of the present 3(b) illustrates observation results showing how ink droplets move in the nozzle of the compara- An ink jet print head according to preferred embodi ments of the present invention will be described while 7 referring to the accompanying drawings wherein like parts and components are designated by the same reference numerals to avoid duplicating description.

A first embodiment will be described below with reference to Figs. 1 through 4.

In an ink jet print head of the present embodiment, as shown in Figs. 2 (a) and 2 (b), a partition wall 8 is provided over a silicon substrate 1 for forming a plurality of individual ink channels 9 and a common ink channel 10. A nozzle plate 11 is further provided over the partition wall 8. The nozzle plate 11 is formed with a plurality of ink ejection nozzles 12 juxtaposed along a line. The nozzles 12 are in fluid communication with corresponding individual ink channels 9. The common ink channel 10 connects the ink channels 9 to one another. A thin film resistor 3 is formed at the end of each ink channel 9 in confrontation with the nozzle 12. Two thin film conductors 4 and 5 are connected to each heater 3. The thin film conductor 5 serves as a common electrode for all the resistors 3. The thin film conductor 4 serves as an individual electrode for the corresponding resistor 3.

The partition wall 8, which forms the ink channels 10 and 9, covers all of the individual conductors 4 and further covers part of the heaters 3. The partition wall 8 has a thickness of less than 30 tm. In other words, the ink 8 channel 9 is formed to a height of less than 30 im. The nozzle plate 11 has a thickness of less than 80 Lm. Accordingly, the nozzle 12 of a straight cylindrical shape has a depth of less than 80 Lm. The heater 3 is formed into a square shape. The nozzle 12 and the heater 3 are shaped and aligned so that the inner perimeter of the nozzle 12 at the end thereof nearest the heater 3, when projected onto the heater, does not extend beyond the perimeter of the heater 3 by more than 5 gm.

In a representative example, each nozzle 12 has a SO gm diameter. The ink channels 9 are formed to a height of 25 Lm. Each heater 3 is formed into a square shape with width of 50 gm.

The partition wall 8 is preferably made from a heatresistant resin such as polyimide which has a thermal breakdown starting point of 400 OC or more. The nozzle plate 11 may be made from the same material with the partition wall 8.

As shown in Figs. 2(a) and 2(b), a drive LSI device 2 is formed on the silicon substrate 1. The drive LSI device 2 is constructed from a shift register circuit and a plurality of drive circuits. Each conductor 4 is connected to a corresponding drive circuit by passing through a throughhole 6. This configuration allows sequential drive of the resistors 3 by an external signal supplied to the device 2.

9 The heater 3 and the conductors 4 and 5 will be described below in greater detail with reference to Fig. 1. Fig. 1 is a cross-sectional magnified view showing the area around one of the ink ejection nozzles 12 shown in Figs. 2 (a) and 2 (b).

The heaters 3 and the conductors 4 and 5 are provided over an approximately 1 to 2 micrometer thick SiO 2 insulation layer 17 provided over the silicon substrate 1. This SiO 2 layer 17 is for insulating the silicon wafer 1 from heat generated by the heater 3. Each heater 3 is formed to an approximately 0.2 micrometers thickness from Ta-Si-SiO alloy, for example, which is very stable for pulsive operation up to the temperature of about 400 IC. The conductors 4 and 5 are formed from 1 gm thick nickel (Ni) thin-film conductors.

The upper surface of the Ta-Si-SiO alloy thin-film heater 3 is thermally oxidized into an oxidized layer 31. This oxidized film 31 has an electrically-insulat ion property and has a good anti-galvanization property against electrolytic ink filled in the ink channel 9. The oxidized film 31 prevents the nonoxidized inner portion of the heater 3 from coming directly into contact with electrolytic ink filled in the ink channel 9. Accordingly, the life of each Ta-Si-SiO alloy thin-film heater 3 will not be shortened by galvanization. Because the oxidized portion 31 is extremely thin, heat is transferred to the ink equally as well as with the - 10 case where the heater 3 is not provided with the oxidized portion 31.

The oxidized film 31 will be described below in greater detail hereinafter.

Ta-Si-SiO alloy thin-film resistor has a certain thermal oxidation property. According to this thermal oxidation property, the resistance of the Ta-Si-SiO alloy thin-film resistor gradually increases when the resistor is placed in an air atmosphere under high temperature more than thin-film atmosphere when the temperature increases to reach the range of 450 OC and 500 OC, the Ta-Si-SiO alloy thin-film resistor begins being oxidized at its surface. When the Ta-Si-SiO alloy thin-film resistor is heated in an oxidizing gas, such as air and oxygen, under 500 'C for ten minutes, the Ta-Si-SiO alloy thin-film resistor will be oxidized at its surface to a depth of in the range of 100 to 200 A. In other words, the Ta-SiSiO alloy thin-film resistor is formed with an electricallyinsulating layer of a thickness in the range of 100 to 200 A. The Ta-Si-SiO alloy thin-film resistor thus covered with the insulation layer will be stable unless the film is further heated under temperature of more than 500 'C. When the TaSi-SiO alloy thin-film resistor covered with the insulation 500 OC. More specifically, the Ta-Si-SiO alloy resistor is stable even when heated in an oxygen at temperature of less then 400 OC. However layer is employed in the print head, the resistor will be heated to a temperature in the range of 300 to 350 IC or less when applied with pulses to jet ink droplets. Accordingly, the film will stably perform the ink jet printing operation.

The present inventors performed the following measurements.

The resistance of about 400 A thick Ta-Si-SiO alloy thin-film resistors (referred to as resistor hereinafter) was measured while the resistcrs were placed in an air atmosphere at 500 degrees centigrade. Fig. 4 shows changes observed during these measurements in terms of Ro/R ratio, wherein Ro represents the original resistance of the resistor and R represents the resistance of the resistor after this thermal oxidation process. As can be seen in Fig. 4, the Ro/R ratio drops at a linear rate, which shows that the thermal oxidation processes oxidize the surface of the resistor at a speed, and to a depth from the surface, that is proportional to the thermal process time. It was further confirmed that entire surfaces of all the resistors were oxidized into electrically- insulating oxides by the thermal oxidation processes.

After the oxidation processes were finished, the resistors were placed in an air atmosphere of 350 degrees centigrade for a long time, and the resistance again measured. It was confirmed that the resistance of the resistor - 12 remained unchanged in that air atmosphere of 350 degrees centigrade. The heaters were also supplied with a thermal pulse of 350 degrees centigrade one hundred million or more times in an air atmosphere. It was further confirmed that the resistance of the resistor remained unchanged even when thus further heated in thermal pulses.

The present inventor further conducted the following measurements.

A Ta-Si-SiO alloy resistor was subjected to the above-described thermal oxidation processes so that the resistor was covered with a thermal lyoxidi zed insulation film of approximately 1,000 A thickness. The resistor was then placed in an electrolyte ink with between 8 and 9 pH. The resistor was tested for susceptibility to galvanic corrosion by application of a potential gradient of 30 V/50 microns for 10 minutes or more. It was confirmed that no changes were observed in the resistor. This shows that even though the insulation film 3' was formed only to an extremely thin 1,000 A thickness, the insulation film 31 was formed with no defects such as pinholes. The film 31 by nature can only be attained by the thermal oxidation processes and can be homogeneously formed.

Thus, the oxidized insulation film 3' can protect the non-oxidized portion of the heater 3 from galvanization by the electrolytic ink and lengthen the life of the heater 3.

Additionally, because of this extreme small thickness, the film 31 can transfer heat to the ink with a high efficiency sufficiently to boil ink with fluctuation nucleation boiling similarly to the case where the resistor 3 is formed with no such films. According to the fluctuation nucleation boiling, a multiplicity of small bubbles with a uniform size are generated across the entire surface of the heater at a uniform distribution. The number of bubbles rapidly increases. The bubbles couple to form a bubble film at the surface of the heater. It is therefore possible to eject ink with a high ejection frequency. Details of the fluctuation nucleation boiling are described in page 334 of Collection of Presentations from the 27th Japan Thermal Transmission Symposium 1990 - 5.

As described above, as shown in Figs. 2(a) and 2(b), the partition wall 8 covers all of the individual conductors 4 and further covers part of the heaters 3 connected to the conductors 4. The ink acts like an electrolyte with the same potential as the common conductor 5. The individual conductors 4 have a higher (or lower) potential than the ink. However, because the conductors 4 are separated by the ink with the partition wall 8, there is no possibility of the conductors 4 being effected by galvanization with the ink. On the other hand, the common conductor 5 does not need to be covered with the partition wall 8 because the conductor 5 and is the ink are at the same potential so that the conductor 5 will not corrode. Though the heaters 3 are partly covered with the partition wall 8, the heaters 3 are covered with the partition wall 8 only about 5 to 8 microns beyond the tip of corresponding electrodes 4. This will reduce thermal efficiency of the heaters 3 by only 10 to 15%. Thus, the abovedescribed arrangement allows construction of a head that is highly reliable in regards to electrolytic ink, while maintaining the high thermal efficiency of the heaters 3.

The partition wall 8 is made from a heat-resistant resin such as polyimide which has a thermal breakdown starting point of 400 'C or more. In order to perform the fluctuation nucleation boiling to provide a high frequently ejection operation, the heaters 3 have to be heated to about 310 OC. Considering the variations in the heaters 3 and in the driving circuit, the heaters 3 can be controlled in the range of 310 OC and 370 'C. The temperature at a part of the partition wall 8 nearest the resistor 3 will therefore reach between 360 OC and 370 'C even at maximum. During the life of the head, the resistor 3 will be energized by pulses of pulse width of about 0.2 gs at the maximum temperature about a hundred million times. Accordingly, the maximum temperature will last only for 20 seconds (= 0.2 lis x a hundred million) in total during the life of the print head. Accordingly, problems relating to the life of the partition wall 8 will not occur as long as the partition wall 8 is formed from a heat resistant resin, such as polyimide, which starts breaking down at temperatures of 400 degrees centigrade or more.

In contrast to this, it has been confirmed that a conventional partition wall, made from a photosensitive resist or other material with low heat tolerance, will be ruptured by galvanic corrosion after about-ten million ejections. By forming the partition wall 8 from a heat resistant resin, the head will be reliable even if the partition wall 8 is positioned imprecisely to partly overlap the heaters 3 in the widthwise direction of the individual ink channels 9. This provides some leeway in alignment precision when aligning components during assembly of the head.

According to the present embodiment, the ink supply channel 9 is formed to a height of less than 30 Rm. The nozzle 12 has a straight cylindrical shape. The nozzle 12 and the heater 3 are aligned so that the inner perimeter of the nozzle 12 at the end thereof nearest the heater 3, when projected onto the heater, does not extend beyond the perimeter of the heater 3 by more than 5 gm. The height or depth of the nozzle 12 is less than 80 gm. According to these sizes, a bubble generated on the heater 3 can grow to reach the uppermost aperture position of the nozzle 12 and connect with the outer atmosphere. This will prevent the is bubble from being collapsed.

This phenomenon will be described below in greater detail.

When the heater 3 is driven to generate bubbles by fluctuation nucleation boiling, the bubble expands upward without growing more than 5 to 10 gm beyond the edges of the heater 3 and the height of the bubble is about 30 ptm at the maximum stage of growth. It can therefore be understood that if the height of the ink channel 9 is more than 30 gm or if the perimeter of the heater 3 and the inner Perimeter of the nozzle 12 at the end thereof nearest the heater 3 are out of 5 gm alignment, the portion of the liquid, located above the bubble to be ejected, and the liquid remaining in the ink channel 9 be connected. This will prevent the bubble from growing to the uppermost aperture position of the nozzle 12.

The present inventor conducted the following measurements to confirm this phenomenon.

The present inventors fabricated a print head of the present embodiment, in which the ink ejection nozzles 12 had a straight cylindrical shape shown in Fig. 3 (a) More specifically, the heaters 3 were formed with an area of 50 x 2 tm The partition wall 8 was formed from polyimide to a height of 25 microns so as to provide the 25 microns high ink channel 9. The orifice plate 11 was formed by adhering and hardening a polyimide film, with thickness of about 50 17 - microns, to the surface of the partition wall 8. Ink ejection apertures or nozzles 12 were formed in the polyimide film 11 to a diameter of 50 microns directly above the thinfilm heaters 3 using the photo dry etching techniques.

The present inventors also fabricated a comparative print head. As shown in Fig. 3 (b), the comparative print head was similar to that shown in Fig. 3(a) with the exception that the ink ejection nozzle 12 opened in the orifice plate 11 to flare out at the end facing the heater 3.

Because polyimide is virtually transparent, generation of bubbles and ejection of droplets that occur when the resistors 3 are energized can be observed by filling the channels with water and energizing the heaters while photographing using stroboscopic photography. The observation results during and after energizing the heaters with a 2 microsecond long pulse are shown in Figs. 3(a) and 3(b).

In the print head of the present embodiment, as shown in Fig. 3(a), between 2 and 3 microseconds after start of energization, a bubble with internal pressure of almost zero has been generated and the water in the nozzle 12 has just started moving at a speed of between 12 and 15 m/s. However, the water in the ink channel 9 has not yet started moving. After 6 microseconds elapses after start of energization, the tail end of the water body which will become the ejected droplet has approached near the uppermost aperture position - 18 is of the nozzle 12. On the other hand, a one-atmosphere pressure difference, between the outer atmosphere and the bubble in the nozzle, has started pulling the water in the ink channel 9 toward the heater 3. After 9 microseconds after start of energization, the pressure within the nozzle 12 has reached to atmospheric pressure, thereby reducing the pressure difference to zero so that movement of water in the ink channel 9 becomes sluggish. Thereafter, approximately 70 microseconds is required to refill the water in the nozzle 12. As was apparent from these observations, the portion of the liquid, located above the bubble to be ejected, was not -he ink channel 9. The connected to the liquid remaining in t, bubble grew to reach the uppermost aperture position of the nozzle 12 and connected with the outer atmosphere. Accordingly, the phenomenon of vacuum bubble collapse did not occur. The associated shock waves, which are a source of cavitation, also did not occur.

In contrast to this, when the nozzle base was greatly flared out as shown in Fig. 3 (b), the water mass to be ejected was completely connected with the water in the ink channel 9, which results in generation of a shock wave when the vacuum bubble vanishes about 9 microseconds after start of energization. This shock wave was not as strong as generating the phenomenon of rebound, which will generate secondary bubbles. However, this shock wave applies a partial shock to the central portion of the heater 3, which can destroy the heater 3, as described on page 41 of the February 1994 edition of the Hewlett-Packard Journal.

The present inventors further attained experiments to determine the life of the above-described heads of Figs. 3 (a) and 3(b) when filled with an electrolytic ink. In the experiments, the present inventors filled the electrolytic ink to a plurality of heads of each type of Figs. 3(a) and 3(b) and applied a great number of pulses to the heads.

The experimental results show that the head having the nozzle configuration of Fig. 3 (a) was able to withstand one hundred million pulses or more for ejecting the electrolytic ink. This is clearly superior to the configuration shown in Fig. 3 (b), which could only withstand from one million pulses or less to about ten million pulses. The life of the heads of Fig. 3(b) is thus widely distributed from one million pulses or less to about ten million pulses.

The present inventors directly detected the presence or absence of the above-described shock using an AE sensor (acoustic emission sensor) affixed to the underside of the head substrate. It was confirmed that the shock conventionally observed at time of bubble collapse was not detected at all, which shows that bubble collapse was eliminated. Even the shock detected with the bubble generation in the head of the present embodiment was one tenth or less of the shock - 20 detected during generation and collapse of the bubble in an open pool operation.

The present inventors further confirmed that when the orifice plate 11 was formed to a thickness of 80 microns or more, ink sometimes completely refilled the area above the heater 3 before the ink to be ejected separated from the nozzle 12. This generated shock waves and also associated cavitation that shortened the life of the heater.

Thus, according to the present embodiment, the ink supply channel 9 is formed to a height of less than the maximum height of the bubble, that is less than 30 gm. The nozzle 12 and the heater 3 are shaped and aligned so that the inner perimeter of the nozzle 12 at the end thereof nearest the heater 3, when projected onto the heater, does not extend beyond the perimeter of the heater 3 by more than 5 1m. With this arrangement, a bubble formed on the heater 3 can grow to the uppermost end of the nozzle 12 and can connect with the outer atmosphere. The bubble will not collapse and will not generate any shock wave. Because the depth of the nozzle 12 is less than 80 im, ink will not be completely refilled the area above the heater 3 before the ink to be ejected is separated from the nozzle 12. Shock waves will not be generated.

A method of producing the above-structured print head of the present embodiment will be described below.

21 - Using a slight modification of a standard bipolar LSI fabrication process, the drive LSI device 2 is formed on a surface of the silicon substrate 1. The S'02 f ilm 17 is formed to the surface of the silicon substrate 1 during this LSI fabrication processes.

An approximately 0.2 micron thick Ta-Si-SiO alloy thin-film resistor and an approximately 1 micron thick nickel thin film are formed over the S'02 film 17 using sputter techniques. More specifically, the alloy thin-film resistor is formed using reactive sputter techniques in an argon atmosphere containing oxygen. The nickel thin film is formed using highspeed sputter techniques in a high magnetic field. Then, the thin-film heaters 3, the individual wiring conductors 4, and the common thin-film conductor 5 are formed using photoetching techniques.

Thus fabricated head is located in an oven filled with air or oxygen gas, and the heaters 3 are subjected to thermal oxidation processes so that the insulation films 3' are formed to the surface of the heaters 3 in the following manner. entirely degrees processes conductors The abovedescribed monolithic LSI

head can not be subjected to the thermal oxidation processes at 400 centigrade or greater. The thermal oxidation will possibly oxidize also the nickel thin film 4 and 5. Therefore, in this example, thermal - 22 oxidization processes are performed by energizing the alloy thin film resistor 3 in pulses so that only the resistor 3 itself is pulsingly heated to about 550 to 600 degrees centigrade.

The most effective method of performing the thermal oxidization processes is to energize the resistor 3 in long pulses so that a high temperature is maintained at the resistor 3 for about one millisecond. This can easily be performed by pulsingly driving the resistors 3 using an external control device. That is, thermal oxidation processes are performed using a pulse width of about 1 ms that is about 10 3 longer than the pulse length (1 to 2 gs) used for actually driving the resistors. Even heating the resistors 3 during thermal oxidation processes to a temperature that is about 200 to 250 degrees centigrade hotter than the temperature used to actually drive the resistors 3 will require much less than the rated power of the drive LSI and so can be performed without any problem. During these pulse heating processes, the oven can be used to heat the silicon base 1 to a temperature of about 100 degrees centigrade.

These thermal oxidation processes increase the resistance of the thinfilm resistor 3 by from 30 to 40%. According to the present embodiment, the resistance of the thin-film resistors 3 is simultaneously measured and detected during the thermal oxidation processes so that all the 23 resistors 3 mounted to the head will have a uniform resistance. More specifically, in the thermal oxidation process, the resistance values of all the thin-film resistors are monitored while the thin-film resistors are pulsingly energized. Based on the monitored results, the pulses applied to the respective thin-film resistors are adjusted so that the resistance values of all the thin-film resistors will be substantially uniform. For example, the number of times at which the pulses are applied to the respective resistors may be adjusted.

The present inventors conducted the following experiments. While applying pulses to the resistors 3, the resistance values of the resistors 3 were monitored. The pulses were adjusted according to the monitored results. As a result, resistance values of all the resistors were adjusted to within +/- 1 %. This contrasts with the +/- 5% variation that can be found in resistance values of a row of resistors provided to tl,.e conventional print heads. Having the uniform resistance, all the resistors will heat the ink to a uniform temperature when actually driven then unnecessary heating will be eliminated. Therefore, the reliability of the head will be improved. Ink will not be scalded and the life of the resistors will be increased.

After the insulation films 31 are thus formed to the surfaces of the heaters 3, polyimide is provided on the - 24 surface of the silicon substrate 1, and a partition wall 8 is formed through etching the polyimide to define the individual ink channels 9 and the common ink channel 10. Then, a polyimide film 11 is provided over the surface of the partition wall 8, and ink ejection apertures 12 are formed in the polyimide film 11 directly above the thin-film heaters 3.

A second embodiment of the present invention will be described below in greater detail with reference to Fig. S.

This embodiment Is especially effective when the resistors 3 are not formed from Ta-Si-SiO alloy, but are formed from material wherein an insulation oxidation film 3' can be formed through subjecting the material to the thermal oxidation processes but the formed insulation oxidation film 31 is easily defected with pin holes. According to this embodiment, an additional insulation layer 7 is provided over both the heaters 3 and the conductors 4 and 5 for protecting the insulation f ilm 31. The thickness of the layer 7 is substantially equal to the thickness of the thin-film resistors 3. The layer 7 is therefore sufficiently thin to provide thermal efficiency as high as the case where the layer 7 is not provided.

The insulation layer 7 can be formed from any insulating material with good sealing and covering characteristics. For example, the insulation layer 7 can be formed from a S'02 layer, a Ta20, layer, or a Si 3N4 using RF sputter - 25 techniques, a Si 3 N 4 layer using plasma CVD techniques, a A1203 using Zorger coat techniques, or an SOG film using commonly used semiconductor processes. The present inventors confirmed that it is effective to cover the entire surface of the heater 3 with the insulation layer 7.

Because of the small thickness of the layer 7, energization power required to induce fluctuation nuc1eation boiling is still small. For example, when energization power is applied in pulses of 2 microseconds, the energization power required to induce the fluctuation nucleation boiling need be only about 1.5 the power required for a naked, or protection layerless, heater. This is still one seventh to one tenth the amount of energy that must be applied for driving conventional heaters with a thick two-layer construction. It can be seen that the heater according to the present invention has excellent heat efficiency. This excellent heat efficiency allows forming the drive circuit and the heaters integrally on the same silicon substrate at a high density. This allows manufacturing a high-speed fullcolor ink jet printer with this high-density head.

As described above, according to the embodiments of the present invention, the extremely thin thermal oxidization layer 31 (and an additional thin insulation layer 7 formed thereon) separate the resistor 3 from the electrolytic ink. The heat-resistant walls 8 separate all the individual 26 - is electrodes entirely from the electrolytic ink. The nozzles 12 are formed in a shape that prevents bubbles generated by nucleate boiling from vanishing so that the thin insulation layer 31 is protected from destruction by cavitation. The thin insulation layer 31 allows almost complete prevention of damage to the heater 3 by galvanic corrosion without reducing the heat efficiency of the heater 3. This allows manufacture of a highly reliable high-density head and a high-speed fullcolor ink jet printer capable of printing with an electrolytic ink.

A long line head can be produced by connecting ends of two print heads of the above-described embodiments along a nozzle aligned direction. In this case, the nozzles of each print head can be slanted toward the connecting ends. Also in this case, the nozzles and the heaters should be positioned so that the inner perimeter of each nozzle at the end thereof nearest the corresponding heater, when projected onto the heater, does not extend beyond the perimeter of the heater by more than 5 pm.

- 27 Although not shown in the drawings, in an ink ejection printer, an image recording medium may be supported at a position confronting the plurality of nozzles of the print head of the present invention. A relative motion is attained between the print head and the image recording medium in a direction orthogonal to a direction along which the plurality of nozzles are aligned.

As described above, according to the present invention, a plurality of heaters are provided on a silicon substrate in a plurality of individual ink channels. Each heater is constructed from a thin-film resistor and a thinfilm conductor. The thin-film resistor is formed with an electrically- insulat ion layer at its upper surface. The electrically-insulat ion layer is formed through subjecting the thin-film resistor to a high- temperature thermal oxidation process. The thin-film resistor formed with the electrically-insulation layer may be covered with an additional insulation layer of a thickness substantially equal to the thin-film resistor.

- 28

Claims

1. An ink ejection print head for ejecting ink droplets to print an image, the print head comprising: a silicon substrate; a partition wall provided on the silicon substrate for defining a plurality of individual ink channels; a plurality of heaters provided JLn the individual ink channels, each heater being made from a thin-film resistor and a thin- film conductor formed on the silicon substrate, a surface of each thin- film resistor having an electrical insulation layer formed by high- temperature thermal oxidation of the thin-film resistor; and an ejection nozzle portion formed with a plurality of nozzles at positions in correspondence with the plurality of heaters.

2. An ink ejection recording head as claimed in claim 1, wherein each of the plurality of nozzles extends in a direction substantially perpendicular to an upper surface of a corresponding heater.

3. An ink-ejectIon recording head as claimed in Claims 1 or 2, wherein the plurality of thin-film conductors include a common elect--rode connected to all the plurality oil heaters and a plurality of individual electrodes connected to the corresponding heaters, wherein the partition wall is made from a heat-resistant resin provided on the substrate, the partition wall covering entire portions of all the individual electrodes to define the individual ink channels.

4. An ink ejection recording head as claimed in claim 3, wherein the partition wall further covers portions of the thin-film resistors.

5. An ink ejection recording head as claimed in any preceding claim, wherein the partition wall further defines a common ink channel provided on the silicon subszrate in fluidly connecL-ion with all the individual ink channels.

6. An ink ejection recording head as claimed in any preceding claim, wherein the partition wall forms the individual ink channels to a height of less than 30 pn.

7. An ink ejection recording head as claimed in any preceding claim, wherein each heater and a corresponding nozzle are formed so that an inner perimeter of the nozzle when projected on the heater as aligned with the heater is within 5 Im of the edge of the heater.

8. An ink ejection recording head as claimed in any preceding claim, wherein the heat-resistant resin for forming the partition wall has a thermal break down starting temperature of 400 degrees centigrade or more.

9. An ink ejection recording head as claimed in any preceding claim, wherein the ejection nozzle portion has a thickness of less than 80 microns to provide the depth of the ejection nozzle to less than 80 microns.

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10. An ink ejection record head as claimed in any preceding claim, wherein each of the plurality of thin-film resistors is made from a Ta-SiSiO alloy thin-film resistor.

11. An ink ejection recording head as claimed in any preceding claim, wherein each of the thin-film conductors is made from a nickel metal thinfilm conductor.

12. An ink ejection recording head as claimed in any preceding claim, wherein the thin-film resistors and tle thin-film conductors are covered with an additional insulation layer having a thickness substantially equal to a thickness to which the thin-film resistors are formed.

13. An ink ejection printer for printing an image with ejected ink, the printer comprising: a print head including a silicon substrate, a partition wall provided on the silicon substrate for defining a plurality of individual ink channels, a plurality of heaters provided in the individual ink channels, each heater being made from a thin-film resistor and a thin-film conductor formed on the silicon substrate, a surface of each thinfilm resistor having an electrical- insulation layer formed by high-temperature thermal oxidation of 7-he thin-film resistor, and an ejection nozzle portion formed with a plurality of nozzles at positions in correspondence with the plurality of heate-rs; support means for supporting an image recording 31 - medium at a position confronting the plurality of nozzles of the print head; and motion means for attaining a relative motion between the print head and the support means in a direction orthogonal to a direction along which the plurality of nozzles are aligned.

14. A method of fabricating an ink ejection head, the method comprising the steps of:

providing a plurality of heaters to one surface of a silicon substrate, each heater including a thin-film resistor and a thin-film conductor; subjecting each thin-film resistor to a thermal oxidation process, thereby forming an electrical- insulation layer on an exposed surface of the thin-film resistors; form.1-ng a partition wall on the surface of the silicon substrate, the partition wall being formed with a plurality of ink channels in correspondence with the plurality of heaters; and forming an orifice plate on the surface of the silicon substrate, the orifice plate being formed with a plurality of nozzles.

15. A method as claimed in claim 14, wherein the thermal oxidation process heats the plurality of thin-film resistors in oxidising atmosphere.

16. A method as claimed in claim 15, wherein the 32 plurality of thin-f ilm resistors are pulsingly energized in an oxidizing atmosphere.

17. A method as claimed in claim 16, wherein the pulse width is longer than that applied to the thin-film resistors for ejecting ink.

18. A method as claimed in claims 15, 16 or 17, wherein the substrate formed with the plurality of heaters is heated in an oven filled with an oxidizing gas.

19. A method as claimed in claims 15, 16, 17 or 18, wherein the thermal oxidization process includes the steps of:

monitoring the resistance values of the respective thin-film resistors while the thin-film resistors are pulsingly energized; and adjusting the pulsingly energization applied to the respective thin-film resistors based on the monitored results, thereby controlling the resistance values of all the thin-film resistors to be substantially uniform.

20. An ink ejection print head substantially as described with reference to the accompanying drawings.

21. A method of making an ink ejection print head substantially as described with reference to the accompanying drawings.