IEEEJOURNAL OF SOLID-ST.iTECIRCUITS,VOL.SC-8,NO. 1,FEBRUARY 1973 71

Thermal Printer

THOMAS R. PAYNE AND HUBERT R. PLUMLEE

Absi’ract-This paper describes a method of printing using s6lid-state thermal printers. A matrix of silicon transistor-resistor heatingelements organized in a 5 )( 7 ~arrayis used to print on thermo-graphic paper at speeds up to 30 characters/s. Material processingcapable of achieving high-perf orrnance ( <100 mJ/element energydissipation for a clearly printed character) and high reliability( >25000000 character prints) is utilized.

A general description of the thermal printer is given followedby a more detailed analysis using mathematical models to investi-gate its electrical-thermal operation. The necessary electricalinterface requirements between the thermal printer and externallogic and the mechanical interface between the printhead andthermographic paper is discussed.

I. INTRODUCTION

s

OLID-STATE thermal printers have been developedfor use as printing devices in a wide variety of

applications. A thermal printer offers many distinct

benefits [1]. It operates silently with hard copy printout

without using inks, ribbons, or odorous chemicals. It

provides character-by -char:zcter readout with the last

printed character visible immediately. Because few me-chanical parts are needed! overall mechanical design

is greatly simplified. Depending on the particular modeof thermal printer used and its application, the additionaladvantages of high-speed output, small size, and low-power requirements are avaiiable. A line printer hasbeen assembled utilizing thermal printers that can print

over 300 character lines/rein; a later thermal printer

versicm will allow the spee,d to go over 1000 character

lines/rein. At the other extreme, a 10 character/s thermalprinter has been designed utilizing less than 1 W of

power (shown in Fig, 1). New design and processingtechniques have pushed the mean time between failures

(MT13F) to over 25000000 character prints.

II. GENERAL DESCRIPTION

A thermal printer is a printing apparatus ..that uses

thermal energy rather than impact energy as its printing

method. Thermographic paper used for printing has a

thermal sensitive coating that must be heated abovesome minimum temperature level to initiate a color

change. The heart of the thermal printer is the thermal

printhead that contains a matrix of heating elements

consisting of a single crystalline semiconductor body ina mesa shape. Each mesa contains a transistor–resistorpair, selectively energized so that the power dissipatedby the resistor causes the top surface of the selectedmesa to become hot. This mesa surface then produces

Manuscript received April 3, 1972; revised August 1, 1972.The authors are with Texas Instruments Incorporated, Dallas,

Tex. 75222.,.-,,,

I , ,“

HEATER ELEMENTSENERGIZED TO PRINT AN “E”

\

Fig. 1. 5 x 5 thermal printer with heater elements energizedto print an E.

a permanent localized dot on thermally sensitive paper

that is pressed against it. By selectively energizing groups

of heater elements, different groups of dots can be formed

on the thermally sensitive paper defining alphanumericcharacters. To print an E, for example, the shaded ele-ments are energized in the printhead as shown in Fig. 1.

Each heater mesa in the thermal printhead matrixis a separate piece of silicon approximately 2.0 roils thick

and 20 roils high by 16 roils wide. In a 5 X 5 matrix

this produces a character 100 roils high by 80 roils wide.

l’he mesas comprising the heater element array arelhermally isolated from each other to produce sharp

well-defined dot printouts (Fig. 2). Electrical connec-tions to the heater elements are made by a metallicconnecting pattern running beneath the mesas that ter-minate in bonding pads. The thermal printhead ismounted, using epoxy, to a ceramic support with ametalized pattern on the back side. An electrical con-

nection is made by bonding from the back side of the

silicon chip through a slot in the ceramic to the metaliza-

tion pattern on the ceramic. Since the leads and inter-connections of the printhead circuit are between the

silicon chip and ceramic support they are not exposed,resulting in a high-reliability structure. A Mylar cableis soldered to the ceramic that is used for external con-nections and the ceramic is mounted to an aluminumheatsink.

The transistor–resistor pair in each mesa and itsphysical layout are illustrated in Fig. 3. The activeelement in each mesa provides an amplifying functionthat allows the heating element array to operate directly

from low-power driving sources. Typically, the resistor

72 IEEEJOURNAL OF SOLID-ST.ITECIRCUITS,FEBRUAR1-1973

OXY

OXY

Fig. 2. Partial cross-sectional view of thermal printer mesa

Vcc = 15 v‘?

Fig. 3. Schematic and physical layout of transistor-resistor pairin each mesa.

k connected to a positive voltage source T~C,of about

15 V. The emitter is tied to ground or a voltage sourcevery close to ground and the base to a drive circuitthat acts as a current source in the ON state and low-impedancc sink in the OFF state. Because silicon has

a positive temperature coefficient, resistance is }owe~t,during the first part of the print pulse when the mesa

is cold and increases as the mesa heats up. Consequently,

power dissipation (collector current) is greatest at the

beginning of the print cycle when it is needed the most

and decreases as the mesa gets hotter as shown in Fig. 4.The energy dissipation for a single element print is ap-proximately 100 mJ. The maximum duty cycle and power

dissipation depends on the thermal printer application.Fig. 4 also indicates the temperature of an energized

mesa and the corresponding temperature of the paperin contact with the mesa. With presently available paper,

the thermal printer and paper must mate with a pres-

sure of approximately 10 psi for adequate printingdensity. A number of different pressure pads have beenused including pads fabricated of felt, Teflon, and BeCu.

When the paper temperature exceeds its threshold ofapproximately llO” C a black dot begins to form. Because

of the imperfect thermal coupling between the energizedmesa and paper, it is necessary to heat the mesa to ahigh temperature. The maximum peak temperature ofan energized mesa is typically between 200 and 250”C.

III. CURRENT CONDUCTION MODEL

As described in Section H, the collector current of a

mesa decreases with printing time until a plateau isreached as illustrated in Fig. 4. For a given mesa thisplateau is almost independent of the applied voltage Vc..

To understand this performance, a model for the powerdissipation of a mesa is helpful.

The collector resistor Rc is part of the silicon ma-terial and its cold resistance Rcz is determined by the

‘MAXNON LINEAR

SCALE

o—

1(5o mA/DIVl

o-

PEAKTEMPERATURE

-262° C

TIME (5ms/DIV)

Fig. 4. Typical surface temperature and collector current mea-surements of a powered thermal printer mesa.

dimensions of the mesa and the cold resistivity of thestai’ting silicon material. The resistivity of silicon in-

creases at approximately 0.7 percent of the resistivityvalue at 25 ‘C with 1‘C rise in temperature; therefore,the total resistance of Rc increases by this same per-centage. As power is dissipated in the mesa, its tem-

perature increases and the collector resistor also increasesreducing the power dissipation. For a constant applied

V,., the collector current decreases until a stable opera-

tion is established. The major concern here is to de-termine what parameters affect the final value of current.1~F.

It is assumed that the effect of the transistor in serieswith the collector resistor can be neglected and no cur-rent crowding exists, the entire silicon wafer is at auniform temperature and the temperature rise A!i” isproportional, 6, to the power dissipation P; therefore

R ., = RC1(l + b AT)

AT = 0P

P = Ic, v.., (1)

where b = 0.007/° C, the solution for ICF is

I “ +;d(ijlih~y +Kiij (2)‘F = o.014evc,

A comparison of the calculated and measured valuesof ICI, for a specific print mesa are given in Fig. 5. Thesmall discrepancy is due to the presence of the power

transistor in series with the collector resistor, which wasnot taken into account.

If the V.C is allowed to become sufficiently large invalue, (2) will reduce to

—..Icp - 41/o. oo7&e

and ICI. would be independent of VCC. For actual print-

heads, the value of Icr does become almost independent

of Vcr for values of VCCas low as 14 V. The power dis-

sipation of a print mesa at a given applied voltage isdetermined by the resistivity of the starting material,

dimensions of the mesa, and the heat sinking.However, the desired output of the print mesa is

heat, producing elevated temperatures that will causeprinting on thermal sensitive paper, Experimental op-erating temperatures of the thermal printhead mesaswere measured with an infrared microradiometer [2].

PAYNE AND PLUMLEE :THERMAL PRINTER

~~

L?e .150 OC/WATT

80

$

-u#

4<~ 60

/

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0 300

w3

$~0 7“ CALCULATED: Rc; 10012

2z MEASURED VALUES (O)ii e =207 “CIWATT at

Vcc = Is v

20

\

RC1-lOO n

00 5 10 15 20 25 30VCC (VOLTS)

Fig. 5. Comparison of the calculated and measured values off., versus Vc. as a function of thermal impedance.

The detector and amplifier system had a time responsefrom dc to less than 5 ~s and a temperature resolution

of 0.5°C over the normal operating temperature range.

The waveshape of the surface temperature (Fig. 4)is about the same anywhere on the mesa surface, al-though the value of the temperature does vary slightly

across the surface. The majority of the i power is dis-

sipated by the collector resistor and, as would be ex-

pected, the maximum temperature rise is over the col-lector contact region. However, some power is dissipatedin the transistor region and a second smaller tempera-

ture peak is found over the emitter contact region.It is desirable to develop an electrical technique to

indicate the operating temperature of a mesa. The mostpromising electrical parameter to indicate an operating

temperature is the change of resistance of the collectorresistor. Since the temperature at the end of a print

time is usually of the “most interest, it is necessary to

measure this change in resistance at the end of the printtime. If a single mesa is energized, its current can bemeasured with a sampling resistor in the VCCcircuit.

Good precision in the measurement of the current isnecessary to adequately determine the collector resistancechange for a temperature indicator. A voltage com-

parator preamplifier and an oscilloscope can be used

with good results.The determination of the operating temperature is

based on the relationships given in (1). The solution for

thermal impedance d is given by:

(3)

where RCI = V. C/ICI and R(7F = VCC/IcP. The tempera-ture rise AT, during the print time is given by:

AT = OICFVC,. (4)

A comparison of both thermal impedances and operat-ing temperatures as measured by this resistor chargeand by the infrared microradiorneter is given in Ta,ble I.

Good agreement is indicated.

is

in

is

73

Although the resistor–transistor system of the mesanot completely described by the simple model used

this report, it does appear that this electrical technique

adequate to indicate the thermal operation of a given

mesa. Measuring ICI, with VC, at 4 and 1,5 V, Fig. 6 gives

an estimate of the values of temperature rise AT of a

thermal printhead mesa at the 15-V value. In makingthese measurements, it is necessary that the drive cir-cuit be operated such that the power transistor of the

mesa is in saturation.

IV. THERMAL PERFORMANCE llJ’IODEL

The thermal model for the solid-state printheads isshown in Fig. 7. The various layers of the construction

were divided into separate mode regions and a general

finite-difference heat transfer computer program [3],

[4] was used to obtain transient operating temperature

solutions. Appropriate boundary conditions were usedto ensure proper heat flow between mesas for cross-

coupling studies. Not illustrated in Fig. 7 is a layer of

paper on the print surface that was included for heattransfer studies to the paper. Table 11 lists the valueof the physical properties used for the printhead ma-terials.

The thermal response of a printheacl with mesas of14 by 12 mik print surface, 1.5 roils thick, 3-roil separa-

tion between mesas, and 10-ms print time was determinedas a function of print time and mounting epoxy thick-ness. Fig. 8. is a plot of the peak mesa temperature,

normalized to an average of 1 W of power dissipated, as afunction of the mounting epoxy thickness. Curve A on

the figure represents the peak mesa temperature at the

end of the 10-ms print t~me. The curve indicates thata desired operating temperature can be established by

the appropriate mounting epoxy thickness that points

out how controlling this thickness under the individual

mesas of a printhead is used to obtain the best printingcapabilities. As can be seen in the figure, the theoreticalcalculations are supported by the available measuredvalues.

Curve B in Fig. 8 shows the effect that the thermo-graphic paper has in reducing the normalized peak lmesatemperature. The variation in the mesa temperaturewith mounting epoxy thickness was calculated by as-

suming that the paper makes ‘(perfect contact” withthe printhead. Even under the conditions of perfectcontact, a comparison between Curves A and B indicates

that a relatively small amount of the dissipated energyis being transferred to the paper for thermal printing,and for the time of thermal printing that the mesa tem-perature is not greatly reduced by the presents of theprinting paper.

This low-energy transfer is more graphically illus-trated by Fig. 9. Shown in this figure is the percentageof the energy received by each component of the powered

printhead mesa, including the paper, at the end of the10-ms print time as a function of mounting epoxy thick-

74 IEllEJOURNALOFSOLID-STATECIRCUITS,FEBRUARY1973

TABLE ICOMPARISONOF THE THERMAL IMPEDANCIZOANDOPERATING T>~MP~RATUR~OF PRINTMES.YS AS MEASURED BY TH~COLL~CTOR RESISTOR

CHANGE T~CHNIQUIZAND BY INFRARED MICRORADIOMETER SURFACE MEASUREMENT TECHNIQUE

Collector Resistor Infrared MicroradiometerElectrical Measurement Change Measurement Measurement

—Mesa

(C&unn-Temperature Temperature

ICF at4V ICF at 15 V RCI Oatl.5V at 15V 0at15V(mA) (mA) (Q) (’c/w)

at 15V(“c) (“c/w) (“c)

1-1 32,2 63,8 124 1331–5 31.0 51.8 129 2292-3 33.5 64.2 119 1424-1 34.4 81.8 116 67

153203162107

12820213667

147196157107

Fig. 6. Indicated temperature rise of a thermal print mesa at15-V operation determined by Z,, measurements taken withV.. of 4 and 15 V.

SILICON MESA WITHSILICON DIOXIDE LAYER

EPOXY

a

ALUMINAEPOXY

WA

POLYCRYSTALLINESILICON

&

SILICON DIOXIDE

Sl~;;~N

EPOXY ALUMINA

@—

MST

&.p4MATERIA~ 03

QI-EPOXY92-ALUMINA ,2Q3-SILICONQ4-SILICON DIOXIDEQS-POLYCRYSTALLINE

THERMAL0, —SILICON MODEL

Fig. 7. Thermal analysis model appropriate for transient three-dimensional heat flow in thermal printer mesas.

ness. It is significant to note that, even under the as-sumed condition of perfect contact, <15 percent of the

dissipated energy is transferred to paper.Fig. 10 shows the temperature response of the print-

head for constant values of mounting epoxy thickness

along with experimentally determined response. It canbe seen that the agreement between the theory andexperiment is good. The experimental curve is lowerin the range O ~ PRINT TIME < 5 ms because the powerdissipation curve for the measured printhead was steeperin this range than the theoretical curve used in the cal-culation. It is evident that the printing mesa is quitethermally stable in less than 10 ms.

TABLE II

PHYSICAL PROPICRTIES OF MATERIALS USED IN THE THERMAL PRINTHEADS

Specific ThermalHeat Conductivity Density

Material (cal/gm. “C) (cal/s cm. “C) (gm/cm3)

Alumina 0.30 6.0 X 10-2 3.82Silicon 0,16 2,45 X 10-1 2.33Epoxy 0.20 6.6 x 10-’ 1,25Paper 0.35 8.3 X 10-4 0,67

AA-WITHOUT PAPERB-WITH PAPERo-EXPERIMENTAL

RESULTS

B

A

~/0

/Y

I I I I

zl- L00 lx 2x 3x 4x 5x

EPOXY THICKNESS

Fig. 8. Peak mesa temperatures as a function of mounting epoxythickness.

>C3Kwzw

50 -

POWEREO SILICONMESA

o—0 lx 2x 3x 4x 5x

EPOXY THICKNESS

Fig. 9. Energy received by each component of thermal printheadas a percentage of the total dissipated energy.

V. CIRCUIT DESIGN 7

It is necessary to select the proper resistance of the

mesa transistor to produce the correct operating tem-

peratures. Based on the models presented in Sections

III and IV, performance curves can be developed that

display the relationships between operating voltage,

PAYNE AND PLUM LEE:THERMAL PRINTER 75

<i=rnl-Wa=3

\

500

k=

5X

400 4X

300 3X

,. HK, EPOXY= 2X

200 ,&--ExpER}MENTAL/’

‘“y-----lo 5 10 15 20

PRINT TIME (ms )

Fig. 10. Temperature response of thermal printhead.

mesa resistance, surface temperature, print time, and

heat transfer coefficient (epclxy thickness). Of particular

importance is the selection of cold mesa resistance and

mounting epoxy thickness as shown in Fig. 11. Anotherrestraint on the total resistance is the area of the mesatop; the area must be such that the total printhead sizerepresents a standard print font. After the mesa re-

sistance is selected, the mesa transistor geometry can bedesigned to carry the necessary current. As thermalprinter mesas are heated to high temperatures, however,new operating conditions can develop, which are unde-

sirable and put a high-temperature limit on the operation

of the printheads if it is not compensated.

The primary upper limit results from developing athermal instability associated with the mesa powertransistor. The danger of operating in this mode resultsfrom the collector current conducting through only avery small region of the power transistor. Since high-

power densities can be developed even for small totalcurrents, very high temperatures can result causing

permanent damage to the silicon structure. If excessivetemperatures (which can cause damage) are not reached,

entering this unstable mode of operation is not harmful.

The onset of an unstable mode of operation, when V~~is being applied, can result from the following conditions.

If any region of the collector-base junction is heated to

a sufficiently high temperature, the silicon will becomeintrinsic in this region, and the junction will no longerexist. For the doping considered, this temperature is in

the 300–350”C range [5]. h this event the transistor

will act as a resistor and will not block high voltages.High currents that are conducted in local regions willresult and even higher temperatures will be developed.Local temperature can easily reach the eutectic of siliconmetals (5oO-1OOO”C) or even the melting temperatureof silicon (1400°C) in less than 1 ms. This thermalrunaway condition was eliminated in solid-statethermal printers by controlling the parameters leadingto its onset. The doping levels are chosen as high aspossible to ensure a high-temperature intrinsic pointabove 300°C, and the total power dissipation is main-tained sufficiently low such that an operating tempera-

ture above 300°C is not reached.

MESA TEMI?

,/, %

/“ Vcc

,/’

It,.120-

O“c

1,oo-RCI

75” c(n)

00-

60-

TQ--w ,0 3x 4X :x Lx +x 4X

EPOXY THICKNESS

Fig. 11. Windowgraph of mesa resistance, thermal coefficient(epoxy thickness), and temperature.

Another form of thermal instability can exist during

the TURNOFF phase. During the switching OFF operation,

the power transistor, which is at a high temperature,can be switched into a PARTIALLYON mode of operationsuch that the current is still flowing through a smallarea of the collector–base junction. This mode of op-

eration is temperature sensitive, but the transistor is

still under bias control and is not in the intrinsic con-

dition. Of course, if the transistor is operated in themode very long, excessive temperatures will be reached

due to the local heating and the intrinsic operation will

be entered as in the previously described instabilitymode. This instability is very common in power tran-sistors and is referred to as second breakdown for the

conduction mode of operation [6]. Secondary break-down was eliminated for thermal printing by designing

interface driver circuits to remove the available power

supplied to the mesas at the end of the print time, whichprevents partial conduction during the ~RNOFF phase.

Two different driver circuits have been developed

for use with thermal printers classified. as: 1) strobed

input circuit and 2) simultaneous inpub circuit.The strobed input thermal printer uses an X–Y (.row–

column) addressing scheme that is basically time sharing

of information across the rows, one row at a time. In-formation is addressed to the thermal printer by select-ing one row, energizing the appropriate columns in that

row depending on the character to be printed, and thenproceeding to the next row. After all of the rows have

been selected, the paper is moved and the process re-

peated to print the next character. Fig. 12 illustratesthe necessary timing requirements to print an E. As canbe seen, it takes 50 ms to print one complete characterbefore the paper is moved to print the next character.Using a strobed input addressing technique allows fora simple addressing and decoding scheme between thethermal printer and external logic. In addition, onlyA7 + Y logic signals are required for an XY-dot array

that keeps the number of pin corrections to a minimum.A circuit schematic of a 5 x 5 strobed input thermal

printer is shown in Fig. 13. The thermal printhead itself

76 IEEE JOURNAL OF SOLID-STATECIRCUITS,FEBRUARY 1973

R3 ~ Cc cccR4 ____IT_ 12345

R5 _____17 RI

R2

cl ~ R3

C2 J—hl_Jl R4

C3 _rL_n_n R5

C4J—u—L—nC5~

Fig. 12, Timing requirements necessary to print an E in astrobed input thermal printer.

is organized in a matrix where each set of five tran-

sistors has their bases common (rows 1–5) and in theorthogonal direction, each set of five transistors hastheir emitters common (columns 1–5). Even though a5 X 5 matrix is described, the same approach applies

to any size matrix. The interface drivers are dividedinto two sections: the row drivers and the column drivers.

The output signals from the row drivers can drive from

one to five loads depending on how many mesas are

energized, but since they arc base input. loads, the cur-rent requirement is low. The column drivers must sink

the full column–emitter currents from the heater tran-sistors, but because of the orthogonality of the columnsto the rows, each column driver only has to carry oneheater transistor current at a time.

During the ON state the input voltage of the columndrivers is at most 3 V~I: drops above ground and therow driver is at most 3 VB~ drops, one VcE (SAT) drop,

and a small resistor drop, above ground. This low voltage

allows an MOS transistor driving the driver circuits tohave a maximum IVns I and, therefore, a maximum

IIDSI.

To solve the problem of turnoff, each row driver not

only drives its own common base line, but also drivesa transistor, which has its collector tied to the precedingcommon base line, into saturation. In this way, at the

end of each row cycle the bases of the five respectiveheater transistors are clamped to ground through Vc~

(SAT), producing fast turnoff. At the end of row 5, the

column signals from the MOS logic turnoff, turning thecolumn drivers OFF and clamping the emitters of theheater transistors OFF. With this technique no standby

power is required and, therefore, battery operation can

be used.The simultaneous input thermal printer uses an ad-

dressing scheme where all the heater elements are se-lectively energized at the same time, which permits

a complete character to be printed in one 10-ms time.Each heater transistor in the printhead matrix has itsbaselead brought out for external connection while the

collectors are tied common to VCCand the emitters tiedcommon to ground. The bases are then connected to ex-ternal drive circuits that provide the control function.

A circuit schematic of a 5 x 7 thermal printhead and

a MOS compatible drive circuit is shown in Fig. 14. Thedrive circuit shown is typical for one heater element

with 35 required for a 5 x 7 matrix. When the drivecircuit is energized, (MOS is ON), transistor Q1 is on

and transistor Q2 is off, which provides drive currentfrom V. through Q 1 and Rn to the base of the heater

element transistor. As soon as the drive circuit is de-energized ( MOS is OFF’), transistor Q 1 goes off, which

cuts off base drive to the heater element transistor, and

Q2 goes on, which clamps the base to ground through

Vrl} (SAT), producing fast turnoff.

For reasons of both economy and performance, the

drive circuits for thermal printers are separated from

the printhea(l matrix and packaged in standard plasticdual-in-line IC packages. In the case of the 5 X 5strobed input thermal printer, two 14-pin packages areused; one for the row drivers, and one for the columndrivers. The simultaneous input thermal printer re-quires five 16-pin packages with seven drive circuits per

package.

VI. PROCESS TECHNOLOGY

Fundamental to the processing of solid-state thermalprinters is the standard technology of fabricating semi-

conductor integrated circuits. The original silicon sliceis formed by taking advantage of the fourfold symmetryof (111) planes intersecting the (100) plane at 54.74°

for controlled isolation etching [7]. By using an orienta-

tion depend ent etch (ODE), which etches silicon muchfaster in the [100] direction than in the [111] direction,

the etch depth can be controlled by the width of the open-ing in the oxide mask (Fig. 15). ODE etches are used,which etch silicon at a rate 50 times greater on the (100)

plane in the [100] direction than on the (111) planein the [111] direction. The etching of isolation moatsin the thermal printhead slice is accomplished by align-ing the isolation etch oxide pattern parallel with the

(111) planes that intersect the (100) surface plane at

an angle of 54.74°. Due to the slower etch rate of the

(111 ) planes, the vertical etching (100) is bounded by(111) planes, which form the walls of the moats. It is

these moats, that isolate the printhead mesas from each

other to produce sharp thermal printouts. Precision lapstops (exact thickness indicators) are designed into thedielectric isolation material by opening and etchingthrough several different known window widths utilizingan appropriate mask.

Utilizing the planar process the transistors, resistors,and isolating junctions are formed in the surface of thesilicon wafer. After the slices are electrically tested, they

are background to the exact required thickness andscribed to produce individual bars for the assemblyprocess. After the assembly process, each individualthermal printer is given an electrical check, a visual

inspection, and a print test to ensure quality of print.

VII. PAPER INTERFACE

In the application of thermal printers either the paper

or the thermal printer can be moved after each character

PAYNE AND PLUMLEE :THERMAL PRINTER 77

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1 x“ !

RI

R2

R3

R4

R5

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I ---- ———. . _-.. -_— -—- —-- --- -._— ———

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II

IIII

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_ac:(Typ)’I

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150(TYP) ELEMENTS(TYP)

II I

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485 551 =I

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II , _==-–--=l==:-–- =-L-_-_-= = :1= = = ❑ = : k_---:-: =IL

I ROW DRIVER II,. I I I IL ——— —–— ——— —--J I

I I

~ -=——___

I

IIII

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ITHERMAL I

PRINTHEAD

/

---._— ——— -i----- ---1

:COMP. VALUES TYP) I

I

ICOLUMN DRIVER ,

— —cl C2 C3 c+ C5

Fig. 13. Strobed input thermal printer circuit schematic.

print. It is not necessary to disengage the paper from

the thermal printer during the time of travel betweenprints, but if engaged, the paper should be kept flush.Thermal energy or heat must be transferred from theheated printing mesa to the thermographic paper in order

to initiate printing. Most clf this energy goes into the

heating of the paper since its thermal sensitive coating

,is quite thin and only a small amount of energy is re-

quired to cause a color change once the coating is abovesome minimum temperature level. The heat transferis influenced by the printhead/paper interface. The paper

and its coating will have some roughness that will affect

the thermal coupling. It is imperative for the paperto be lined up parallel to the printhead with a sufficientpressure for optimum heat transfer. The ideal pressurebetween the silicon printhead and most papers is 10psi, * 0.5 psi, with a feltlike material as a recommendedpressure pad. The feltlike pad gives a backing that can

conform to small nonuniforrnities and ensures a couplingwith the roughness of a paper surface.

Many companies produce thermographic papers and

their surface abrasiveness, residue, and sensitivity canvary drastically. Although silicon is quite resistant toweai, the abrasiveness of paper coatings can be excessivecausing accelerated printhead wear. Coatings can alsobe too soft causing a residue buildup on the surface

of the printhead reducing the efficiency of the heat

transfer from the mesas to the paper. The sensitivity orrequired temperature for printing of a thermal sensitive

paper must be within an acceptable operating range ofthe printhead. Coatings that are very sensitive normallydo not have good shelf life because partial color changecan be accumulated at temperatures as low as room

temperature over long periods of time. Since pressurerequirements, abrasiveness, residue, and sensitivity ofa paper are coupled together, the entire thermal print-ing system must be considered to match a thermographicpaper and a thermal print system.

The quality of a printed character is very difficultto measure. Many factors affect print quality such asgranularity, contrast, and bridging between dots. Equip-ment such as a reflectometer can be used to measure

78 IEEEJOURNAL OF SOLID-STATECIRCUITS,VOL.SC-8,NO. 1,FEBRUARY 1973

{

Ovcc I

Vcc 2

Vss

~’ +

QIRc

Q2

d=THE;MAL

PRINTHEAD

4r ~SIMULTANEOUS

DRIVE CIRCUIT

VmMOS LOGIC

Fig. 14. Simultaneous input thermal printer

darkness of a print as compared to

MESA

circuit schematic.

ts background.

However, subje~tive judgement of print quality by the

human eye has proven to be quite sensitive and consistent

under proper conditions. Therefore, at present the best

evaluation of the results of a print system is the human

eye.

VIII. CONCLUSIONS

It has been shown that solid-state thermal printerssuited for production can be designed for a wide varietyof applications. Quite standard silicon production tech-nology can be used to produce the thermal printheadsin production volumes and with high reliability. Proper

design of the printheads are made with electrical and

thermal performance models. Although the printhead/

paper interface is of prime concern, proper matching

of the paper, pressure pad, and transport systems can

be made. Several drive circuits with either strobed or

simultaneous inputs can be selected to meet the needsof the application.

With present improvements in both performance andreliability,

to becomesolid-state thermal printers are now readya. widely used printing system.

Fig. 15. Mesa configurations using orientation-dependentdefinition.

ACIKNTOWLEDGIWEiNT

etch

The authors would like to thank Dr. W. W. 130yd for

his work on the thermal performance modeling; Dr.

M. F. Judy for his contributions in material and process

technology; R. Yeakley for his work in applying the

dielectric isolation process to thermal printers; and C. E.

Avery, Manager of Thermal Printers at Texas Instru-

ments Incorporated, whose support made this paperpossible.

REFERENCES

[11 Mr. H. Puterbaugh and S. P. Emmons, ‘(A new principle,”in AFIPS Con}. Proc., vol. 30, pp. 121–124, 1967.

[21 B. R. Pagel and L. R. Reid, “An infrared microradiometer,”IEEE Trams. In.stmm. Meas., vol. IM-15, pp. 89-93, Sept.1966.

[31 F. Kreith, Principles o~ Heat Transfer. Scmnton, Pa.: In-ternat. Textbooks, 1968, pp. 166-188.

[41 K. W. Lallier and B. R. Pamani. “A three dimensional heattransfer computer program ~or aerospace application,” IBM,Fed. Syst. Div., Oswego, N. Y., IBM 63-825-862A, Sept. 1964.

[51 W. R. Runyan, Silicon Semiconductor Technology. NewYork: McGraw-Hill, 1965.

[61 H. A. Schafft, “Second breakdown—A comprehensive review,”P~OC.IEEE, VO1.55, pp. 1272-1288, Aug. 1967.

[71 K. E. Beanand P. S: Gleim, “The influences of crystal orienta-tion on slhcon semiconductor processing,” Proc. IEEE, vol.57, pp. 1469-1476, Sept. 1969.

A MicrocircuitTone GeneratorforPush-ButtonDialing

MICHAEL C. J. COWPLAND

Absfract—A new microcircuit tone generator fabricated withtantalum thin film and a custom IC in a dual-in-line plastic pack isdescribed. Novel designs of filter, amplifier, and amplitude limitinghave been used that enable operation over a wide frequency range,wide supply voltage range, and wide temperature range while

Manuscript received April 5, 1972; revised July 26, 1972.The author is with Microsystems International, Ltd., Ottawa,

Ont., Canada.

maintaining excellent stability. Interfacing of the tone generatoris very flexible enabling it to be used in most telephone and datasystems. Frequency selection switches with up to 150-Q resistanceare permissible, without pulling effects, so that low-cost conductiverubber push-button pads and electronic switching can be used di-rectly. The feedback filter configuration used in the oscillatordesign is gain insensitive so that only frequency has to be trimmed,making rapid automatic trimming possible in manufacture. Theprimary application for this device is for telephone systems, but it isalso useful in the general purpose low-frequency oscillator area.