Multitone ink jet printer and method of operation

1 2

drop generator-use time for the print head and, again,

MULTTTONE INK JET PRINTER AND METHOD imposes a limitation on the useful print head lifetime. OF OPERATION

DISCLOSURE OF INVENTION

TECHNICAL FIELD 5 The general purpose of this invention is to provide a

This invention relates generally to thermal ink jet new md improved thermal ink jet printer and method

printers and more particularly to a multitone ink jet of operation which overcomes the aforedescribed dis

printer and method having an improved grey scale advantages of the prior art and consequently provides a

operation. print head of decreased drop generator design complex

10 ity and characterized by an extended lifetime.

BACKGROUND ART T0 acC0mplish this purpose, I have discovered and

Thermal ink jet printing has been described in many developed an improved multitone ink jet printer and

recent technical articles, such as an article by Kuhn & method wherein a plurality of ink drop volumes are

Myers in Scientific American, 1985, at pages 162 through provided in a drop generator structure and are

178, and also in an article by J. B. Angell et al. also in 15 weighted in a predetermined binary sequence. The drop

Scientific American April 1983 at pages 44 through 55, generators which are each assigned a binary number

both incorporated herein by reference. corresponding to a specific ink drop volume are sequen

In the art of multitone ink jet printing, one approach tially fired at a chosen pixel as they come into alignment to printing a dot with one of eight grey scale levels is to with the pixel as the printhead moves with respect to employ a single ink jet drop generator and fire it from 20 the paper or vice versa. Thus, firing one to three binaryone (1) to seven (7) times at a given pixel in order to weighted drop generators produces 1 to 7 volume units provide the pixel from one to seven levels of ink drop of ink within the pixel. This process produces 1-8 levels volume. However, this approach suffers at least two 0f scaie. The total number of drop generators distinct disadvantages when used in a thermal ink-jet required in the print head and the total drop generator printer. The first of these disadvantages is that the sub- 25 uge ^ ^ ... to thereby maximize print head stantial repeated use of smgle drop generator and its lifetime with a minimum of associated drop generator associated heater resistor increases the wear and failure design complexity

rate (decreased lifetime) of the thermal ink jet print In m ^teraative embodiment of the invention, there

head. As used herein, the term wear is defined as the fa ... and ... a ... fof redud ^

accumulation^ofdrop ejection^cycles ma drop.genera- 30 ^ of mk ejected into a given pixel area

tor with finite lifetimes. Such lifetimes are typically ^ haiftoning pri„ting operation. This method

measured m tens of millions of cycles. ,? ... °.r e °. r , r . , „. ,

0 , . , . . . 3, . , mcludes ejectmg a drop of untoned liquid, termed ink

Secondly, when mk is ejected m a drop sequence ,. , „ ., * . * . M„ '. . , .

from a single drop generator, there is a certaki recovery veh«rle m the art' mt° a fea f als° time related to the bubble collapse associated with each 35 eJec^S °ne or more drops of ink with a fixed dye loadink drop ejection from the drop generator. This recov- mS mt° the same »"* °fthe ?aelIn this manner, the ery time obviously imposes a limitation on the maxi- dyf 18 dispersed over a larger area, and the objectionmum achievable rate at which pixels are printed using able °Ptlcal characteristics of smgle, small, high-conthis method of thermal ink jet printing. trast dots standing alone are eliminated m favor of a

Another approach to multitone ink jet printing in- 40 more uniform gray tone,

volves the use of multiple ink jet drop generators and BRIEF DESCRIPTION OF DRAWINGS firing these drop generators simultaneously in different

numbers to achieve different corresponding ink jet drop FIGS- 1A-1D are sequential isometric views looking volumes. To some extent, the use of these multiple drop UP through the paper at the bottom of a scanning printgenerators as contrasted to a single drop generator can 45 nead having the "1", "2" and "4" binary volumes for increase the lifetime of the thermal ink jet printer. One ejecting ink into a given pixel.

such approach is disclosed for example by T. Kawanabe FIG- 2 is a graph of optical dot density versus print

in U.S. Pat. No. 4,353,079 issued Oct. 5,1982. However, volume for the binary drop generator depicted in FIGS,

the thermal ink jet recorder apparatus of the Kawanabe 1A-1D.

patent identified above is possessed with certain other 50 FIGS. 3A and 3B illustrate isometrically, and in

disadvantages related to the requirement for simulta- cross-section respectively, a print head structure consti

neous firing of the multiple drop generators therein. tuting a preferred embodiment of the invention.

In particular, since these drop generators of the prior FIGS. 4A-4E illustrate schematically an alternative

art are simultaneously fired at a single location, the embodiment of the invention wherein untoned ink vehi

nozzles must be critically aligned with respect to each 55 cle is combined with an ink drop of fixed dye loading,

other so that the ink drops will properly register within This process is carried out in order to obtain a mixture

the pixel on the recording medium (paper). Further- of ink vehicle and ink on paper to produce a reduction

more, this alignment is predicated upon a particular in optical density of the fixed dye loading to thereby

spacing between the nozzles and paper, and maintaining eliminate grainyness of small dots,

this distance is critical to achieving a simultaneous com- 60 FIGS. 5A and 5B illustrate isometrically and in plan

bination of these drop volumes on the pixel. In addition, view, respectively, a three-color, eight-level halftone

since each drop generator in the Kawanabe recorder of printer in accordance with the present invention. U.S. Pat. No. 4,353,079 produces only one unit volume of ink, then anywhere up to seven drop generators must be fired simultaneously to achieve the variation of one 65

to seven levels on the grey scale. This requirement Referring now to FIGS. 1A-1D, there is shown in

significantly increases the complexity.cost and unreli- sequence the firing of the binary-weighted "4", "2" and

ability of printhead design, and it also increases the total "1" volume ink drops at the pixel p. The direction of

BEST MODE FOR CARRYING OUT THE
INVENTION

3 4

scan for a printhead 10 is from left to right, and the "4" means by which the ink bubble forms on the heater volume ink drop "a" has printed into the pixel at dis- resistor and provides the energy to eject a droplet of ink placement xj. As indicated in FIG. IB, the "2" volume is well known to those skilled in the thermal ink jet drop "b" is on its way toward the pixel where it subse- printing art, and is described, for example, in the Hewlett quently combines with the "4" volume drop to provide 5 Packard Journal, Volume 36, Number 5, May 1985 a "4"+"2" combined volume spot on the pixel at dis- which is incorporated herein by reference. These curplacement X2 as indicated in FIG. 1C. Also shown in rent pulses are applied by way of conventional surface FIG. 1C is the firing of the "1" volume ink "c" which metallization patterns (not shown), but typically concombines with the already deposited "4"+"2" volumes sisting of very thin conductive traces of aluminum or of ink to give a 7 volume ink spot or level eight on the 10 goid on the upper surface of the silicon substrate 30 and grey scale and completely covering the pixel as indi- deposited using standard evaporation processes well cated in FIG. ID at displacement X3. known in the art.

There is much less critical alignment with the drop Although the substrate member 30 is referred to

generator scheme of the present invention than with the herein as a "silicon substrate", it will be understood by

approach of firing seven drop generators simulta- 15 those skiUed m the m that the substrate 30 will typi

neously. Furthermore, since there are only three drop cally by a thin flhn composjte or layered structure

generators for the eight grey levels rather than the wherein a first layer of silicon dioxide S1O2, will be

seven drop generator scheme of the prior art, tins fea- QVm QJ. deposited on a silicon substrate surface to

ture results in a simpler electrical interconnect require- ide surface pasivation thereof; and then a resistive

ment for the printhead heater resistors, less complex 20 j such m tantalum-aluminum will be deposited on

driver electronics, a less complex plumbing scheme for ^ siQ j Next the conductive trace materia, wiu

feeding ink to the three drop generators, and a more bfi rated on the ta„talum-aluminum layer and

compact and rehaWe printhead. lithographically defined so that openings in this trace

Referring to FIG. 2 the scheme whereby combined ^/^ine the boundaries </the ifeater resistors, ink drops with total volumes T through 7 produce 25 „ . . . ^. , , . ,. * . uj-n- f u u Finally, a inert outer passivation or banner layer mateeight grey levels is descnbed. This figure shows how . . J', ... F., . ... .. e. ...

optical density of a pixel is increased from the reflectiv- "a ^h as silicon carbidej SiC or silicon nitride, S13N4,

ity of the untoned paper ("white") to the reflectivity of or tantalum Pentoxlde> Tatfs. or combination thereof in

ink covering the paper ("black") as successively larger successive » f°TM f °» *e surface of the alumi

volumesofinkareapphed to the region ofasmgle pixel. 30 num trace and the exposed heater resistors m

The curve in the figure obeys a typical relation between °rde_r to Provlde a Sood «°latlon bamer layer between

spot area and drop volume on a coated paper, and the *e ***** resist0rs andJhe lnk mL the reservoirs above

total optical density of the pixel is computed from the these heater "»*ors- 71118 lnert bamer 18 neceS"

reflectivities and areas of the tones and untoned regions sarv due to corrosive nature of the ink and

within a pixel boundary. Next to each print volume 35 also because of their cavitation produced wear during

derived from the "1", "2", and "4" binary-weighted dot lnk ejection from the drop generators. Therefore, as

printers is shown schematically the combination of used herein> "sillcon substrate" is intended to mean a

drops producing this volume. thin 61111 resistor silicon substrate of the above type of

Referring now to FIGS. 3A and 3B, the partially layered structure, cut-away isometric view of a printhead employing the 40 Referring now to FIGS. 4A through 4E, there is three drop generator scheme of the present invention illustrated an alternative embodiment of my invention includes a silicon thin film resistor substrate 30 which is which includes an additional drop generator which is fabricated using silicon planar processing and thin film indicated as drop generator 60 in FIG. 4A. This generadeposition techniques which are well known to those tor 60 produces untoned droplets of fluid which is the skilled in the art. The silicon substrate 30 includes a 45 ink without toning dyes. This liquid is known in the art common ink feed-hole 32 in the form of a cylinder or as the ink vehicle. The volume of the untoned drop will slot through substrate 30 and configured using diamond be between the "1" and "4" volume. When a single saw blade or laser drilling techniques. As is also well toned ink drop 64 is ejected from the printhead onto the known, the thin film resistor silicon substrate 30 is con- paper 65 as indicated in FIG. 4B, it will typically exhibit structed to have a plurality of heater resistors 34,36 and 50 an optical density profile in the form indicated in FIG. 38 thereon, and these resistors are configured to corre- 4C. In this Figure, reference number 66 indicates the spond to the different "1", "2" and "4" volume ink optical density profile across the diameter of printed dot channels 40, 42 and 44 in the barrier layer 46. This 67. As a result of the sharp steep profile 66 of optical barrier layer 46 isolates individual drop generators to density at the edge, the dot 67 in FIG. 4C, when seen at reduce crosstalk and is important to obtaining long 55 a normal viewing distance, will produce a grainylike operating life for the thermal ink jet resistors. Typically characteristic as perceived by the eye and will consethe barrier layer 46 is fabricated from a well known quently result in undesirable shading or grainyness for commercial polymer material with the trade name certain image printing and background application"VACREL", nickel, glass, or some other material im- s—particulary in the reproduction of highlights in conpervious to attack by the liquid ink contained therein. 60 tinuous-tone images. Thus, it is frequently desirable to

The printhead in FIG. 3A further includes a drop remove this grainylike characteristic and this may be

generator orifice plate 48 typically constructed of accomplished by the utilization of an untoned liquid

nickel and includes the plurality of orifices 50,52 and 54 vehicle drop 68 as indicated in FIG. 4D. This vehicle

for ejecting the "1", "2" and "4" binary ink volumes, drop 68 is ejected onto the same pixel area as the toned

respectively. These orifices 50, 52 and 54 will emit the 65 ink drop 64 and this produces a certain quantity of

"1", "2" and "4" volumes of ink when their correspond- vehicle volume plus "1" toned ink volume in area 69 as

ing thin-film resistors 34, 36 and 38 respectively are indicated in 4E and having an optical density profile 70

heated by the application thereto of current pulses. The which has a maximum value in the center portion 72