Design and Fabrication of Monolithic Multidimensional ... - IEEE Xplore

JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 19, NO. 4, AUGUST 2010

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Design and Fabrication of Monolithic Multidimensional Data Registration CMOS/MEMS Ink-Jet Printhead Jian-Chiun Liou, Fan-Gang Tseng, Member, IEEE, and Chi-Ming Huang

Abstract—A monolithic ink-jet printhead is designed and fabricated with back-shooting type of thermal bubble nucleation for high-speed and long-life printing in this paper. It combines micromechanics, including heating actuators, temperature sensor, channels, and nozzles, with an integrated multidimensional data registration CMOS demultiplexer driving circuit, which includes D flip-flop signal processing along with bidirectional data transfer and 12-V power amplifiers in a printhead chip. Microelectromechanical systems fabrication processes are applied to define the ink-firing chamber, feed channel, and the orifice plate within this new micro injector structure. In this paper, the printing resolution of this monolithic ink-jet head is 1200 dpi, and the diameter of nozzle orifice is about 14 μm with a thickness of 30 μm. Both the silicon dry and wet etching processes are applied to the fabrication of orifice plate with the control of thickness within ±4 μm. The major advantage of the ink-jet chip assembly processes is that throughput is improved. The operating frequency of the monolithic ink-jet printhead developed in this paper is 24 kHz. The required voltage to start the bubble nucleation of printer head is 7.4 V, and the ink nozzle lifetime is 1.5 × 108 . The optimization design of this monolithic ink-jet printhead could provide better printing quality than the commercial ones. [2010-0112] Index Terms—Application-specific integrated circuit (ASIC), droplet, ink-jet printhead, microelectromechanical systems (MEMS), multifunction.

I. I NTRODUCTION

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OR A LIQUID ejector, there are two methods to drive a droplet. One is thermal bubble type. The other is piezoelectric type. Recently, a liquid ejector, which makes use of thermal energy as a driving force, has become a commercial product. Moreover, the process of it has been widely known and researched. For a traditional thermal bubble ink-jet head, there are four main components (thermal resistor chip, ink-

Manuscript received May 1, 2010; revised June 3, 2010; accepted June 5, 2010. Date of publication July 21, 2010; date of current version July 30, 2010. This work was supported by the National Science Council, Taiwan, under Grant NSC97-2120-M-007-010. Subject Editor Y. Zohar. J.-C. Liou was with the Institute of Nanoengineering and Microsystems, National Tsing Hua University, Hsinchu 30013, Taiwan. He is now with the Industrial Technology Research Institute, Hsinchu 31040, Taiwan (e-mail: [email protected]). F.-G. Tseng is with the Institute of Nanoengineering and Microsystems and the Engineering and System Science Department, National Tsing Hua University, Hsinchu 30013, Taiwan, and also with the Division of Mechanics, Research Center for Applied Sciences, Academia Sinica, Taipei 115-29, Taiwan. C.-M. Huang is with the Industrial Technology Research Institute, Hsinchu 31040, Taiwan. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JMEMS.2010.2055548

firing chamber, feed channel, and orifice plate) which have to be fabricated and assembled precisely, particularly for the process of a thermal resistor chip, the manufacture of nozzle plate, and the package between nozzle plate and chip. Nowadays, the main stream for a desktop printer is to do anything possible to reduce the production cost and increase the density of heater arrangement. Based on this concept, there are two issues that have to be concerned. One is that it would be more difficult to put the nozzle on the heater precisely. During assembly of the nozzle plate and the chip, precise alignment is required; thus, the time of assembly is increasing. Furthermore, assembly takes place individually, thus reducing the efficiency of the manufacture and increasing the cost. The other is that it would be more difficult to form ink slot based on reducing the chip size to lower the cost. For a color ink-jet printer with high resolution, three ink slots are formed on one chip. To reduce the area of the chip, the ink slot is a narrow and long rectangle, thus increasing the difficulty of the formation thereof. The sandblast process is introduced to form a through slot that passes the chip for ink feeding. Contamination and crack of the silicon wafer are commonly seen during the sandblast process. In addition, the sandblast ink slot on a chip increases the difficulty of the circuit layout. Recently, there is a trend for high speed, high resolution, and high printing quality on photo. However, for industrial application, it needs high flexibility to eject each kind of working fluids and high working life. The highest physical resolution is only 1200 dpi in commercial product, but the printing resolution could be 4800 dpi by the compensation of software. This compensation mechanism will lower the speed of printing because it needs to print on a local area again and again to reach its high resolution that it claims. Therefore, for looking after high speed and high quality, the physical resolution should be raised to a higher level. However, there are still three requirements even if the physical resolution is 1200 dpi. 1) The inaccuracy of alignment should be less than 2 μm. 2) The life of heater should be increased to ten times that of a traditional heater because the size of droplet continues to shrink and it will take more time to complete the compensation. According to one source, the typical traditional heater bubble lifetime for water-based ink-jets is 9 × 107 firing operations, depending on the input heat flux, from 6.4–8.0 V/nozzle [1]. According to Tseng’s type, during the testing, the microinjector ejected more than 14 million droplets continuously [2], [3].

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Fig. 1. Concept of the smart printhead.

3) As the flow resistance of the microchannel increases, the efficiency of the bubble decreases. These two factors affect the frequency at which the ink is refilled. The flow regimes and heat transfer coefficients in the microchannel are closely related to the elongated bubble behavior. Based on these reasons, there have been some monolithic designs of ink-jet printhead to overcome these problems [4]–[12]. We provided a design of back-shooter ink-jet head based on microelectromechanical systems (MEMS) technology. This concept integrates the integrated CMOS circuit, MEMS technology, and electroforming technique into this design to fabricate multidimensional data registration driving circuit, heater element, microchannel, and nozzle array. It will decrease the inaccuracy of alignment between the nozzle and heater, and elon-

gate the life. The structure disclosed in this paper replaces the bonding process by the 3-D electroforming technique. The ink feed slot formed by electroforming is separated from the substrate; this makes the circuit layout more flexible and prevents contamination and crack during the sandblast process. The electroforming structure combines the ink feed slot, the ink flow channel, and the ink-firing chamber. In other words, there is no barrier layer of polymer material on the ink-jet head that may be corroded by strong acid solvent while the ink-jet head is applied to industry applications. Generally, thin-film processes include a passivation layer (SiN/SiC)/Ta to protect the chip from corrosion by using an ink solvent for industry applications. Using a photolithography process, we can make the chip and the nozzle plate into one component, which eliminates the alignment problems (that traditional machines have), with

LIOU et al.: DESIGN AND FABRICATION OF DATA REGISTRATION CMOS/MEMS INK-JET PRINTHEAD

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an alignment inaccuracy of less than 1 μm between the chip and the nozzle plate. In this structure, the batch processes are used, which could save more time.

II. D ESIGN OF THE M ONOLITHIC P RINTHEAD A. Design of CMOS Multiplexer Circuit This study develops the integrated multidimensional data registration demultiplexer driver ink-jet head by standard CMOS processes to reduce the connecting addresses needed for controlling the nozzle orifices which eject small size of droplets. The design concept can be easily scaled up for large array format ink-jet system without much change in the terminal numbers, owing to the 3-D hierarchy of control circuit design, which effectively reduces the terminal numbers into the cubic root of the total control unit numbers. The received printing data information has to be converted into data at an optimal transfer rate (frequency) in order to conform to the heater characteristics. To the end, the clock-control circuit divides the 8-b ink-jet printhead signals supplied to the data driver, with an aim of lowering the operation frequency. The serial/parallel-conversion circuit converts serial signals of a plurality of channels into parallel signals and supplies the parallel signals to the latch circuit. The latch circuit temporarily stores the received parallel signals and supplies them to the level shifter and the D/A converter at a predetermined time. Integration of logic function is enabling thermal ink-jet (TIJ)based products to move ever further up in the market. Fig. 1 shows the concept of the smart printhead, which includes a temperature-controlled sensor in chip to determine the printing system shutdown power for overheat on chip control block. The depicted integrated circuit TIJ transducer array serves beyond 450 jets and includes data interfacing, jet addressing, drop generation power pulsing, and bidirectional operation. The chip also includes output features that facilitate the electronic management of an assembly of multiple chips into larger arrays. The illustrated TIJ chip design allows beyond 450 jets to be operated with less than ten input lines [13]–[15]. The logic circuit is first translated into an ink-jet chip CMOS circuit, and the initial layout is done. We designed and analyzed the circuit for dc and transient performance by using the circuitlevel simulation program, SPICE, and then compared the results with the given design specifications [16]–[21]. An H-SPICE software analysis can help to verify that the transient performance has the best result. Transient performance problems are short lived. If a particular problem lasts for a very short duration, its severity might be averaged out or minimized by other performance problems in the entire analysis period. In this paper, we present an ink-jet chip CMOS circuit, and the enable gate is activated under the control of enable signal so as to drive a power transistor which allows for resistive heating of resistor. The functionalities of the shift register, transfer register, and enable gate are standard CMOS components that are well understood by those skilled in the art of CMOS circuit design. The techniques are developed based on observations from the results of H-SPICE simulations. These methods are incorporated into a performance- and power-constrained module generator.

Fig. 2. Architect of 3-D driving circuit for printhead jets.

In the signal flow design, heaters are usually scanned over one by one without skipping the unactivated heaters. As a result, for the ink-jet chip with 450 jets, 1-D circuit architect needs 450 unit times for scanning all the jets. However, 2-D circuit architect needs 36 unit times, and 3-D circuit architect spends five unit times only. Therefore, the scanning time of the 3-D multiplexing circuit from the first address line to the 20th, as an example, takes only four units of clock time from the simulation result, which is much faster than that of the 2-D configuration with 20 units of clock time. Thus, the maximum scanning time for the 3-D circuit will be reduced to 30% of that in the 2-D case. To simultaneously write signals into the driving circuit, multiplexing data latches and shift registers are employed by the application of commercial available CMOS ICs. Small numbers of shift registers, control logics, and driving circuits can be electrically connected and integrated with jets using standard CMOS processes. Fig. 2 shows the driving circuit of the 3-D architect. The desired signal for “S” selections and “A” selections can be preregistered and latched in the circuit for one time writing. A smart CMOS circuit includes D flip-flop signal processing, along with bidirectional data transfer and 12-V power amplifiers, in a printhead chip. The input signals include DATA signal (for a selected switch action), CLK1 signal (to scan the DATA signal and describe the base clock on a printing chip system), CLK2 signal (to latch the DATA signal or to select), CTRL signal (to select enable type), and SETB (the time sequence to set up CTRL or power). The design flow is more complex than the traditional driver on an ink-jet chip; the research step is as follows: 1) make an application-specific integrated circuit (ASIC) design for addressing jets; 2) conduct simulation for the multiplexer in ink-jet chip; and 3) according to the simulation device size result for chip layout, tape out preprocess for CMOS circuit and postprocess for ink-jet process. The digital circuit is realized by a 0.35-μm double-poly double-metal (2P2M) CMOS technology with 12-V power supply. The acquisition time of SPICE simulation approaches 85 μs, as shown in Fig. 3. All jets must fire for 85 μs, and

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Fig. 3. SPICE simulation of jet addressing.

the SETB, CTRL, DATA, CLK1, and CLK2 signals control the addressing jets. The smart printhead total process flow is shown in Fig. 4. It is based on a 0.35-μm CMOS process. B. Structure of Ink-Jet Chip According to the previous discussion, there are three issues which should be solved, and we have to think of some ideas which could be applied to the design. The first one, which could avoid assembling the nozzle plate with the chip, is to fabricate the nozzle on the chip directly. However, it will change the direction of the droplet, differing from that of a traditional top-shooting ink-jet head. Therefore, this is a design of backshooting ink-jet head whose direction of bubble formation is inversed to that of flying droplet. Second, which could avoid the sandblast process to etch through the chip and generate some cracks, is to replace the method of liquid supplement, which supplies ink from the back side of a chip, with a new one, which supplies ink from the topside of a chip. Third, which could avoid some special working fluid to corrode the microchannel and ink chamber, is to make use of electroforming technique to form a metal structure. Therefore, as shown in Fig. 4(a), the first step is to manufacture some specific thin films to define heater elements and control circuit on the thermal bubble ink-jet chip. These thin films should contain a thermal barrier layer that can concentrate the thermal energy on the working fluid, a conductive layer that is responsible for sending an electrical signal to the heater elements, a thermal-resistor layer called heater element that is the key component to generate bubble, and a passivation layer that is a multifunction part which has to isolate those layers conducting electric current from liquid and deliver thermal energy.

According to the first idea, the thickness of the local area of the nozzle position has to be reduced to a specific value, to 20–50 μm. The driving force cannot eject droplet until the thickness is diminished to a specific value. The TIJ drop carries away only about 0.03% of the ejection pulse energy as kinetic energy. The rest must be disposed as heat. During printhead reliability testing, TaAl thin-film resistors repeatedly failed in a fuse mode at the cathode side, from bubble nucleation simulation results, as shown in Fig. 5. A heater is located on the floor of an ink channel near the exit nozzle. A short (∼ 3 μs) voltage pulse is applied to the heater resistor, warming the ink in contact with it sufficiently for an ink component to boil. Film boiling begins ∼ 2 μs after the voltage pulse starts and is complete in ∼15 μs. This liquid-to-vapor transition results in a very great volume expansion (∼50 times) of the heated liquid. Compare the structures of an ink-jet printhead chip before [Fig. 5(a); thick Si/SiO2/TaAl/(SiN/SiC)/Ta/ink chamber] and after [Fig. 5(b); air/thin Si/SiO2/TaAl/(SiN/SiC)/Ta/ink chamber] back etching. The same voltage pulse is applied to the heater resistor. There is no bubble at 3 and 10 μs in the before-back-etching structure. Oppositely, water-based TIJ inks are heated to > 290 ◦ C in 2–3 μs in the after-back-etching structure. The left dashed line side is the analysis of temperature profile: about 100 ◦ C–150 ◦ C for the before-back-etching type and 150 ◦ C–500 ◦ C for the after-back-etching type. The right dashed line side is the analysis of bubble nucleation dimension: about 1–5 μm for the after-back-etching type. Therefore, as shown in Fig. 4(b), we make use of the etching technique to complete this requirement. In addition, for concentrating most of the driving force on the droplet, the position of the nozzle has to be placed near the heater. As shown in Fig. 4(c), the nozzle is formed by dry-etching instrument, such as inductively coupled plasma (ICP).


Fig. 4.

(a) Thin-film process. (b) Reduce the thickness of silicon substrate. (c) ICP etching to form nozzles. (d) 3-D sacrificial layer definition.

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Fig. 4. (Continued.) (e) Electroforming process. (f) Removing the sacrificial layer.

According to the second and third ideas, the mechanism of supplying liquid can be made of metal by electroforming technique and the direction of it is from the topside of the chip [Fig. 4(f)]. The firing chamber with microchannel and the mechanism of supplying liquid can be defined at the same time by making use of 3-D sacrificial layer composed of photoresist [Fig. 4(d)]. After the electroforming process [Fig. 4(e)], the sacrificial layer can be removed [Fig. 4(f)]. Finally, a highresolution monolithic ink-jet head is completed without any assembling process, as shown in Fig. 6. In this paper, the total resistance of the loop is about 37 Ω, which includes the power line, heater resistor, HVMOS turned-on resistor, and ground line loop, as shown in Fig. 6. These two heaters are parallel to form an individual actuator, and the size of the heater is 15 × 30 μm2 . The simulation task is typically divided into smaller subproblems like actuation and ejection. To model the actuation, it is necessary to find an appropriate pressure boundary condition to substitute the complicated bubble nucleation, expansion, and collapse [22]–[29]. Using this as input for the simulation package ACE+ of CFDRC, a computational fluid dynamics (CFD) simulation has been set up, and the droplet ejection process has been studied, as shown in Fig. 7. In practice, the isocontour of

f = 0.5 is typically applied to identify the interfacial location for computation and visualization purposes. To evolve the fluid distribution, the liquid volume fraction f is resolved by ∂f + ∇ • (ν f ) = 0 ∂t

(1)

where the symbol ν is the velocity vector and f is defined as the volume fraction of each computational cell taken by liquid, only present in the interfacial transition regions with f changing from zero to one. The average droplet ejection velocity of the computational result is 14.9 m/s, while the nozzle diameter is 4 μm by CFD simulation. The nozzle diameter on droplet volume and velocity results are shown in Fig. 15. III. FABRICATION OF THE M ONOLITHIC P RINTHEAD In this paper, the etching processes were carried out on a double-side polished 100 P-type silicon wafer with 400-μm thickness. The physical design rules of 0.35-μm 2P2M 12 V/12 V [Taiwan Semiconductor Manufacturing Company (TSMC)] (V ds/V gs) are high-voltage (HV) process and mixed-mode process. The asymmetric HV device structure (HV only applies


Fig. 5.

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Simulation of bubble nuclear formation. (a) Before-back-etching type. (b) After-back-etching type.

Fig. 7. Simulation of droplet ejection process.

Fig. 6.

Schematic view of the ink-jet printhead.

on the drain side, not on the source side) is recommended for lower Ron (turn-on resistance of MOSFET) and higher current drive purpose. The NMOS devices in the n-well technology are formed in the lightly doped p-substrate, while the PMOS devices are formed in the more heavily doped n-well. The starting material is a heavily doped 100 p+ wafer with a thin (5–10 μm), lightly doped p-type epitomical layer at the surface. For AlSiCu deposition and patterning processes as metal 1, and then intermetal dielectric deposition and via patterning, TaAl

(29 Ω/) for the heating resistor and Al (7000 Å) for the interconnection line were sputter deposited and patterned by wet chemical etching. Si3 N4 (4000 Å)/SiC (2500 Å) and Ta (4500 Å) were then deposited by plasma-enhanced chemical vapor deposition and sputtering, respectively, after thin films were deposited on one polished side and deep chemical wet etching was done on the other side. The etching solution is a 25 wt% tetramethylammonium hydroxide solution, which is isotropic in nature. For manufacturing the ink-jet chip with driver circuits, it needs a silicon membrane with 30-μm thickness and good surface quality of the etched surface to provide good status of ejection droplets. The standard of this etching mechanism is that the deviation of the etching depth is within ±2 μm. Moreover, we expect that the surface roughness could be controlled under some specific value. After a progress of etching technique, the etching depth deviation can be controlled under 4 μm, and the values of the depth are arranged from 364–368 μm (as shown in Fig. 8).

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Fig. 8. Etching depth and roughness.

On the other hand, the roughness of the etched surface is from 34–45 Å (as shown in Fig. 8). This etched surface is not only free pyramid but also ultimately smooth like a polished surface. Under the same method of measurement, the roughness of a polished wafer is about 30 Å. This measurement instrument, whose method belongs to contacting the surface like α-stepper, is very sensitive to surface condition. Moreover, there is a standard glass ball with 40 Å to modify the sensitivity. Therefore, these results could be trusted. There is another important issue that could not be ignored. This issue is how to decide the timing to stop etching if we expect that the thickness of the nozzle is 30 μm. However, there is no etching stop layer to reach this requirement, and 75% of the etchant is water, which is so easy to vaporize at 80 ◦ C condition that could make the etching rate unsteady. Thus, the etching rate has to be controlled very stably during the process. We can find that the etching rate is from 22.73 to 24.36 μm/h in 14 h. The variation is about 1.63 μm/h. The wet etching process is linear to the etching time. This is a qualified stability to manufacture the ink-jet printhead. After the thickness definition of the silicon plate, the next step is to drill through the silicon wafer by using the ICP dry etching to defined nozzles. For a better nozzle structure, the etching process will be carried out on the topside of the silicon wafer. We use the photoresist of AZ4620 as an etching mask to define the pattern of nozzle. As shown in Fig. 9, the nozzle has to be defined precisely between two heaters. The depth of the nozzle is about 30 μm, and the diameter of the nozzle is about 14 μm. The next step is defined the micro channel and ink chamber. The MESD (Multi Exposure Single Development) technique is developed to form the 3-dimension sacrificial structure for the electroforming process. Two kinds of photo-resist are used to form the structure by laminating. The composition of these two types of photo-resist is the same but with different thickness. The thickness of first photo-resist is about 20 μm and which laminating onto the substrate, after the first laminating step, the

Fig. 9.

Fig. 10.

ICP to form nozzles.

Three-dimensional sacrificial structure.

photo-resist should be exposed by UV light source for curing purpose. The thickness of second photo-resist is 40 μm and which laminating onto the first photo-resist, and the substrate is exposed under UV light again. After two times of the exposure steps, the substrate is then proceeding with the development step. As shown in Fig. 10, the pattern that we desired to achieve will appear after the development step. After the 3-dimension sacrificial structure is formed, the electroforming process is used to define the ink feed slot, ink flow channel, and the inkfiring chamber. The sulfa-mate nickel is used as the electrolyte here, the temperature of the electrolyte is about 55 ◦ C, and the current density is about 10 ASD. The electroforming process


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Fig. 11. Logic analysis signal of 3-D driving circuit.

will be stopped once the deposition thickness reach about 50 μm. Finally, the sacrificial layer will be stripped and leave the nickel structure on the substrate.

IV. R ESULTS A. Logic Analysis of CMOS Multiplexer Circuit In the logic analysis equipment, we observe the ASIC input and output relationship, as shown in Fig. 11. The input signal includes “SETB, CTRL, DATA, CLK1, and CLK2.” The output signal matches an ASIC design waveform. The enlarge part of Fig. 11 shows the four conditions of CLK1 and CLK2 in Fig. 3. They are 0, 0, 0, 1, 1, 1, and 1, 0 that correspond to “Type 1,” “Type 2,” “Type 3,” and “Type 4” output signals in Fig. 11. The ASIC design and logic analysis result matched. From the verification measurement, the use of CMOS logic circuit in the ink-jet chip is a success. For the four scanning types are fitted selection functions of multidimensional driving circuit architect. In this paper, we will use two methods for the evaluation of this ink-jet head. First, the open pool observation to observe the formation of bubble and the bubble growth cycle was used. Second, the behavior of droplet ejection was also observed by image capture system. In this paper, the bubble formation is shown in Fig. 12. The bubble occupies the firing chamber, which value of applied voltage is called begin voltage (V b). The V b of this design is about 7.4 V, while the firing frequency is 5 kHz and the pulsewidth of the applied signal is 2.4 μs.

Fig. 12. Bubble formation by open pool testing.

B. Observation of Bubble Formation In this paper, we observed the cycles of bubble formation by image capture system; the moving pictures of bubble formation are captured by using high-speed photography at 5000 frames per second. The camera could capture seven 576 × 385-pixel frames in one image series. The exposure time of each frame was set to 10 μs, and the interframe time was set to 200 μs. As shown in Fig. 13(a), the applied voltage of bubble nucleation is 7.1 V. In Fig. 13(b)–(d), the bubble volume is increasing with the increase of applied voltage. After that, we would like to observe the whole process of bubble formation. It would be from the nucleation to dissipation. The operational parameters will be fixed at an applied voltage of 7.4 volt, an operation frequency of 5 kHz, and a signal pulsewidth of 2.4 μs. In this paper, the bubble nucleation

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Fig. 13. Formation of bubble with different applied voltages. (a) 7.1 V. (b) 7.2 V. (c) 7.3 V. (d) 7.4 V.

is at 2 μs and is getting bigger. At 5.8 μs, the volume of bubble reaches maximum. Therefore, the time of bubble formation is 5.8 μs in this study. Continuously, the bubble is dissipating and disappears at 7.4 μs. C. Observation of Droplet Ejection In this paper, we would like to know the behavior of the droplet ejection and its life under severe driving conditions. We observed, measured, and estimated the velocity and volume of droplet. In Fig. 14, the behaviors of droplet ejection are shown, and the operation frequency is 5 kHz. From the results of Fig. 15, the computed velocity of the droplet is 10 m/s with a nozzle diameter of 14 μm (smaller ejector nozzles for smaller drops) based on estimation from experimental result, and the estimated volume of the droplet is about 4 pL, which is smaller than that provided by the state-of-the-art commercial ink-jets at the time of experiment. The commercial ink-jets (HP 51645A) with 27-μm nozzle diameter provide the 27-μm droplets, of which volume is about 10 pL based on [3, Table I]. The drop diameter is getting closer to nozzle diameter, where the nozzle diameter is expected to become smaller in the future. It decreased the inaccuracy of alignment between the nozzle and heater, and elongated the life. Orifice plates are mounted to ink-jet pens and include orifices through which ink drops are expelled by any one of a number of drop ejection systems. With the inaccuracy of alignment between the nozzle and heater, some of the ink that is ejected through the nozzle does not reach the printing medium (e.g., paper, polymer, etc.) but instead collects on the outer surface of the nozzle plate. Some of this residual ink accumulates or puddles adjacent to the edge of the orifice and may alter the trajectory of the subsequently ejected drops, thereby reducing the overall quality of the printed image and the ink nozzle refill lifetime. The inaccuracy of alignment should be less than 2 μm. Using a photolithography process, we can make the chip and the nozzle plate into one component, which eliminates the alignment problems (that traditional machines have), with an alignment inaccuracy of less than 1 μm between the chip and the nozzle plate. The velocity range of the commercial inkjet printhead is 8–12 m/s for HP 51626A by Tseng et al. [2]. The time of tail severed from the orifice of monolithic head is 10 μs shorter than the commercial ink-jet head. The tail cutoff times of the commercial ink-jet (HP 51626A) is about 53 μs [3]. Tails (or ligaments) and satellites are a major issue in the design of high-quality ink-jet systems and pose special

Fig. 14.

Behaviors of droplet ejection and droplet velocity evolution.

Fig. 15.

Effects of nozzle diameter on droplet volume and velocity results.

requirements both to ink developers and the printing system. For reducing both the ligament length and satellite droplet problems associated with producing high-velocity ink droplets from an ink-jet head, printing at relatively high ink-jet head transport speeds comprises driving the ink-jet head. The monolithic ink-jet chip has several advantages over previous printing systems in creating high-quality images by using very small individual ink drops of low volume and high velocity. From Fig. 15, the droplet velocity by simulation (computed) is 13 to 15 m/s under 10-μm nozzle diameter. Fig. 15 shows the effects of the nozzle diameter on droplet


volume and velocity results from computational results, and they are compared with those derived from experiments. It is observed that there is an increase of the droplet volume with an increase in nozzle diameter. The surface tension force acting on the fluid–air interface is lower for a larger nozzle; thus, more volume of fluid can be ejected for the same thermal energy. The operational energy parameters are fixed at an applied voltage of 7.4 V. A low surface tension makes it easier for a stream of ink to break up into a series of droplets. An increase in surface tension requires an increase in the electrical driving voltage to generate a droplet. The decrease in the velocity of the droplets may be due to the increase in the droplet mass with an increase in the nozzle diameter. The experimental drop size distributions were described adequately by the upper limit number and volume distributions. In this paper, the highest operation frequency is 24 kHz, the volume of droplet is about 4 pL, and this nozzle can be fired continually for more than one-half hour. In other words, the life of monolithic head will be longer than 1.5 × 108 times.

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V. S UMMARY An innovative monolithic multidimensional data registration CMOS/MEMS ink-jet printhead has been fabricated and tested. The fabrication process combines the 0.35-μm CMOS semiconductor thin-film process and micromachining techniques of the (100) silicon wafer. The 3-D electroforming process is also involved here to replace the sandblast and chip bonding process that is commonly used in commercial ink-jet head. Optimization of this structure will be continued to achieve a high-resolution, high-nozzle-density, and high-print-quality ink-jet printhead in the future.

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[17] [18] [19]

ACKNOWLEDGMENT The authors would like to thank the Chip Implementation Center, TSMC, Hsinchu, Taiwan, for organizing the chip fabrication. R EFERENCES [1] J.-C. Liou, C.-J. Chen, and M.-H. Chuang, “Lower power supply thermal bubble printhead chip with MEMS technology increasing thermal energy effect,” in Proc. Int. Conf. Digit. Printing Technol., 2004, pp. 863–866. [2] F.-G. Tseng, C.-J. Kim, and C.-M. Ho, “A high resolution high frequency monolithic top-shooting microinjector free of satellite drops—Part I: Concept, design, and model,” J. Microelectromech. Syst., vol. 11, no. 5, pp. 427–436, Oct. 2002. [3] F.-G. Tseng, C.-J. Kim, and C.-M. Ho, “A high resolution high frequency monolithic top-shooting microinjector free of satellite drops—Part II: Fabrication, implementation, and characterization,” J. Microelectromech. Syst., vol. 11, no. 5, pp. 437–447, Oct. 2002. [4] J. Chen and K. D. Wise, “A high-resolution silicon monolithic nozzle array for inkjet printing,” IEEE Trans. Electron Devices, vol. 44, no. 9, pp. 1401–1409, Sep. 1997. [5] F.-G. Tseng, C.-J. Kim, and C.-M. Ho, “A novel microinjector with virtual chamber neck,” in Proc. IEEE Micro Electro Mech. Syst., Jan. 1998, pp. 57–62. [6] S.-W. Lee, H.-C. Kim, K. Kuk, and Y.-S. Oh, “A monolithic inkjet print head: DomeJet,” Sens. Actuators A, Phys., vol. 95, no. 2, pp. 114–119, Jan. 2002. [7] J.-D. Lee, H.-D. Lee, H.-J. Lee, J.-B. Yoon, K.-H. Han, J.-K. Kim, C.-K. Kim, and C.-H. Han, “A monolithic thermal inkjet printhead

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Jian-Chiun Liou received the Ph.D. degree from the Institute of Nanoengineering and Microsystems, National Tsing Hua University, Hsinchu, Taiwan, in 2009. He joined the Printing Technology Development and Manufacturing Section, Optoelectronics and Systems Laboratories, Industrial Technology Research Institute (ITRI), Hsinchu, in 1999, where he focused on ink-jet printing system. Since 2005, he has been a Project Leader working on a new MEMS architecture design and display application in the Electronics and Optoelectronics Research Laboratories, ITRI. His research interests are in the fields of ASIC design, optical MEMS technology, integration of ink-jet printhead processes, and display technology. He is the holder of 23 patents on ink-jet printheads and has written more than 15 SCI Journal papers and 30 conference technical papers on MEMS, optical-N/MEMS, and display-related fields.

Fan-Gang Tseng (S’97–A’99–M’01) received the B.S. degree in power mechanical engineering from National Tsing Hua University (NTHU), Hsinchu, Taiwan, in 1989, the M.S. degree from the Institute of Applied Mechanics, National Taiwan University, Taipei, Taiwan, in 1991, and the Ph.D. degree in mechanical engineering from the University of California, Los Angeles, in 1998. For one year, he was with the Information Sciences Institute, University of Southern California, Los Angeles, as a Senior Engineer working on a new microfabrication process called EFAB. In 1999, he became an Assistant Professor in the Engineering and System Science Department, NTHU, where he has been a Professor since 2006. He is also with the Institute of Nanoengineering and Microsystems, NTHU, and the Division of Mechanics, Research Center for Applied Sciences, Academia Sinica, Taipei. His research interests are in the fields of bio-MEMS/nano and nano-/microfluidic systems. In addition to his more than 125 refereed (SCI and EI) journal papers and 170 conference publications, he has been awarded more than 30 patents and was invited to contribute to the MEMS Handbook (CRC Press, 2000, 2002) on “Micro Droplet Generators” and Protein Microarrays (Jones and Bartlett Publishers, 2002) on “Protein Microarray Technology and The Micro Fluidic Aspects.” Prof. Tseng was the recipient of Mr. Wu, Da-Yo Memorial Award from the National Science Council, Taiwan (2005), NTHU Outstanding Teaching Award (2003), NTHU New Faculty Research Award (2002), NTHU Nuclear College Outstanding Teaching Award (2002), Research Award from the National Science Council, Taiwan (2000), and NTHU Academic Booster Program Award (2001). His research work has won several awards including the overall conference Best Paper Award at the 15th National Conference on Theoretical and Applied Mechanics in Taiwan (1991), two Best Paper Awards at the Nanotechnology and MEMS conferences in Taiwan (2003, 2005), 2nd Place National Innovative Award (2006), Best Poster Award at microTAS in Sweden (2004), two Best Student Paper Awards at the 25th and 26th National Conferences on Theoretical and Applied Mechanics in Taiwan, Undergraduate Research Creativity Award from the National Science Council (2005), and the TECO Technology Student Award, among others. He has also actively participated in international MEMS activities. He has served on program committees and as session chairs for numerous international MEMS conferences such as IEEE Transducers (2001), IEEE NEMS (2005, 2006), IEEE Nano (2007–2009), IEEE Nanomed (2007), and many more. He is a member of the American Society of Mechanical Engineers (ASME), American Association for the Advancement of Science (AAAS), and American Chemical Society (ACS). Currently, he serves as an Editor for the Open Nanomedicine Journal, and a reviewer for more than 20 international journals including Applied Physics Letters, Biosensors and Bioelectronics, Lab on a Chip, Microfluidics and Nanofluidics, and Biomedical Microdevices. He has also been a consultant to four U.S. companies, three Taiwan companies, and many institutions in the Industrial Technology Research Institute in Taiwan.

Chi-Ming Huang received the M.S. degree from the Institute of Applied Mechanics, National Taiwan University, Taipei, Taiwan, in 2002. He joined the Printing Technology Development and Manufacturing Section, Optoelectronics and Systems Laboratories, Industrial Technology Research Institute (ITRI), Hsinchu, Taiwan, in 2002. After four years, he joined the Electronics and Optoelectronics Research Laboratories, ITRI, as an Engineer working on a new microfabrication process and MEMS architecture design. His research interests are in the fields of MEMS systems. He is the holder of several patents and has written papers related to ink-jet printheads.