ME395 Final Project: Manuscript Szu-Kang Hsien

Ayodeji O. Coker

1. Name of the Project:

From Ink-Jet Technology to Nano Array Writing Technology

Since Ink-jet micromaching technology has been introduced 30 years ago, the ink jet printer has emerged as one of the mainstream printing technologies. From its market inception in 1985, the Hewlett Packard’s thermal ink jet technology has evolved progressively from a 12 nozzle 96 dpi (dots per inch) to a 300 nozzle 600 dpi print head [1]. Recently Epson introduced its modified micro piezoelectric printer, which has 1400 dpi resolution. Based on the micro piezoelectric printer technology and the new “Dip-Pen” Nanolithography technology, we come up with a design called “Nano Array Writing” that can write a resolution of 30 nm linewidth continuously.

2. Objective:

Using nanotechnology and microfabrication techniques to manufacture so-called “Nano Array Writing” to produce very thin linewidth (30nm) to replace the traditional lithography tools. One of the goals of this project is to create a process whereby we can control individually an array of AFM tips. We propose the use of a process developed by N.C Macdonald et al., in which the integration of micro-actuators and nanometer-scale tips (the AFM tip in our case) was employed to control things on a small scale.

3. Approach and literature review:

Due to the large demand of high speed microprocessors and large capacity of memory chips, the gate length of transistors need to shrink down in order to keep up with the pace.According to Moore’s Law, the number of transistors capable of being put on a processor should double every 18 months. So in order to keep with the demand, manufacturers need to find reliable and cheap lithography sources that can generate smaller wavelength every two or three years. Right now, manufacturers are using deep UV excimer laser (DUV) lithography tools to produce 0.1 m linewidth on top of the wafer but unfortunately, the current technique will likely hit its limit around 2003. There are many candidates that can write even smaller linewidth for examples X-ray, E-beam and the most recently talked about extreme ultra-violet(EUV) lithography onto a wafer, but X-ray and E-beam are not cost-effective and the EUV technology can only write down to 70nm. So we came up with a design that can combine the more established ink-jet technology to provide the ink and the traditional Atomic Force Microscopy (AFM) tips to write very small linewidth directly on the wafer without using the traditional lithography methods. First we like to talk about the revolution of printing technology from laser to ink-jet and secondly, the theory, operation of AFM and the fabrication of its silicon nitride tip and thirdly, using micromaching comb-drive actuators to control the x, y, z direction of the tip, and fourth, the dip pen technology and surface chemistry behind the writing and finally the structure of our newly designed devices.

1. Revolution of Printing:

Laser Printer:

The technology behind color laser printer is electrophotography or xerography and it was the invention of one man, Chester Carlson in 1938. The two ideas that he brought together were:  the formation of an electrostatic latent image using photoconductivity to selectivity discharge a surface charged insulator, and  “development” of this latent image by dusting with powers charged electrostatically. No chemical reaction are involved in processing. It is essential a dry photoelectric process. Unlike silver emulsions, the xerographic plate is not necessarily consumed in processing. The plates can be used over and over again for hundreds or even thousands of exposures. Since the first machine introduced 40 years ago by Xerox Corporation, technology has been improved.

Generally, six steps are involved in making a print by xerography [2], as we can see in Figure1:

  1. Sensitizing the xerographic plate by electrical charging.
  2. Exposing the plate to form a latent electrostatic image.
  3. Developing the latent image with fine particles.
  4. Transferring the developed image to paper or other materials.
  5. Fixing the image by fusing, and
  6. The plate is discharged, cleaned, and reused.

Laser printers have been growing very fast in the last 20 years and will continue be the dominant player in the high-end market segment. This is because printers based on electrophotographic are quiet and fast output, can handle multiple fonts, can produce pictorial information over a wide speed range, and can produce very good and vivid pictures. Even though Canon introduced the replacement cartridge concept and used cheap semiconductor laser in 1983, the price is still quite high for household users. Usually the price color laser printer is 5-10 times higher than the ink-jet printer but the gap has been narrowed quite fast.

Ink-Jet Printer:

Inkjet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because of its non-impact, low-noise characteristics, good printing quality at reasonable cost, color capability and versatility, its use of plain paper and its avoidance of toner transfers and fixing. Many types of ink jet printing mechanisms have been invented. These can be categorized as either continuous ink jet (CIJ) or drop on demand (DOD) ink jet [3], as we can see from Table 1.

Continuous ink jet printing dates back to at least 1929, where Hansell discloses an array of continuous ink jet nozzles where ink drops to be printed are selectively charged and deflected towards the recording medium. This technique is known as binary deflection Continuous ink jet and is used by several manufacturers, including Elmjet and Scitex. Hertz et al published a U.S. patent no. 3,416,153, in 1966 [4], discloses a method of achieving variable optical density of printed spots in CIJ printing using the electrostatic dispersion of a charged drop stream to modulate the number of droplets which pass through a small aperture. This technique is used in ink jet printers manufactured by Iris Graphics. Recently in 1994, L. Smith et al published a paper in Sensor and Actuators A [5] discussing the Continuous Ink Jet technology has an edge over the DOD with respect to high quality printing, as we can see in Figure2. The ink jet is ejected from the nozzle and breaks up into drops at its point of drop formation, which is situated close to the charge electrode A. When a signal voltage different from the ink potential is applied to the charge electrode, the issued drops will be deflected by the electrically charged. The charged drops will be deflected by the electrical field generated between the deflection electrodes B and caught by the knife edge C. Uncharged drops, will pass through unaffected and reach the printing surface of the receiving substrate D. The substrate is mounted on a drum that is rotated at high speed while the print head is moved along the axis of the drum. Although it has edge over drop demand technology, I think that this technology waste lots of ink on the electrode C and it is only feasible if the ink is quite expensive. So almost every company comes up with products by using Drop Demand Technology.

Kyser et al published a U.S. patent no. 4,189,734, in 1980 [6], disclosing a DOD ink jet printer which applies a high voltage to a piezoelectric crystal, causing the crystal to bend, applying pressure on an ink reservoir and jetting drops on demand as can be seen in Figure 3. The ink cavity is sealed and the orifice is completed by anodically bonding Corning 7740 glass plates to either side of the chip. Anodic bonding provides a simple, strong, hermetic seal, which requires no additional films or epoxies [7]. Many types of piezoelectric drop on demand printers have subsequently been invented, which utilize piezoelectric crystals in bend mode, push mode, shear mode, and squeeze mode. Piezoelectric DOD printers have achieved commercial success using hot melt inks (for example, Tektronix and Dataproducts printers), and at image resolutions up to 1440 dpi for home and office printers (Seiko Epson). Piezoelectric DOD printers have an advantage in being able to use a wide range of inks. However, piezoelectric printing mechanisms usually require complex high voltage drive circuitry and bulky piezoelectric crystal arrays, which are disadvantageous in regard to manufacturability and performance.
Endo et al published a British patent no. 2,007,162, in 1979 [8], disclosing an electrothermal DOD ink jet printer which applies a power pulse to an electrothermal transducer (heater) which is in thermal contact with ink in a nozzle. The heater rapidly heats water-based ink to a high temperature, whereupon a small quantity of ink rapidly evaporates, forming a bubble. The formation of these bubbles results in a pressure wave, which cause drops of ink to be ejected from small apertures along the edge of the heater substrate. This technology is known as Bubblejet.TM. (Trademark of Canon of Japan), and is used in a wide range of printing systems from Canon, Xerox, and other manufacturers and the ink flow comes out from the side of the shooting chamber which is also called the Sideshooter Ink-Jet as can be seen in Figure 4.

Almost at the same time, Vaught et al independently published the U.S. Pat. No. 4,490,728, in 1984 [9], disclosing an electrothermal drop ejection system, which also operates by bubble formation. In this system, drops are ejected in a direction normal to the plane of the heater substrate, through nozzles formed in an aperture plate positioned above the heater. This system is known as the Thermal Ink Jet and it is also called the Edgeshooter Ink-Jet since the ink shoots from the edge of the firing chamber, and is manufactured by Hewlett-Packard as can be seen in Figure 5. The resistor, probably made of doped poly-silicon will heat the ink at a very rapid rate in excess of 108C/sec. The ink does not boil, but explodes. A very thin layer of the ink on the top of the resistor undergoes a superheat vapor explosion. A superheat limit is the point at which the ink can no longer exist in a liquid state, which is about 340C. The explosive nature of the TIJ bubble proves to be beneficial for the operation of printhead since it clears the nozzle of printhead or static bubbles, which might otherwise cause reliability problems in printhead operation [1].

Thermal Ink Jet printing typically requires approximately 20 J over a period of approximately 2 s to eject each drop. The 10 W active power consumption of each heater is disadvantageous in itself and also necessitates special inks, complicates the driver electronics and precipitates deterioration of heater elements. But as we can see from Figure 6, with the combination of higher firing frequency and more nozzles in the TJI printhead results in very high print speed and it continues to outpace piezoelectric ink jet [1]. The superiority of TJI in print speed can now rival that of laser printers. The recently announced HP 2000C can print full-page color documents in about 43 second-other inkjet and laserjet printers take two minutes.

Other ink jet printing systems have also been described in technical literature, but are not currently used on a commercial basis. For example, U.S. Pat. No. 4,275,290 discloses a system wherein the coincident address of predetermined print head nozzles with heat pulses and hydrostatic pressure, allows ink to flow freely to spacer-separated paper, passing beneath the print head. U.S. Pat. Nos. 4,737,803, 4,737,803 and 4,748,458 disclose ink jet recording systems wherein the coincident address of ink in print head nozzles with heat pulses and an electrostatically attractive field cause ejection of ink drops to a print page. P. Krause et at of Germany in 1994 think of using <110> silicon wafer to produce “Backshooter” ink jet system that can reduce the manufacturing costs since it reduce large number of electrical interconnections [10] (at least one nozzle plus a common rear conductor), as in Figure 7. But I think that the price of its printer head will be high compared to “Edgeshooter” by HP and “Sideshooter” by Canon since it is using <110> silicon wafers

Each of the above-described inkjet printing systems has advantages and disadvantages. However, there remains a widely recognized need for an improved ink jet printing approach, providing advantages for example, as to cost, speed, quality, reliability, power usage, simplicity of construction and operation, durability and consumables.

2. Piezoelectricity:

Piezoelectric properties of a medium can be explained by the concept of piezoelectricity, which determines the distribution of the electric polarization and demonstrates how a piezoelectric field reacts to an electrical stress by emitting depolarization waves. The polarization field is linear with respect to mechanical strain in crystals belonging to certain symmetry classes. If there is no external field, crystal strain force is balanced by the internal polarization force. When this equilibrium is disturbed by the external field or mechanical force, the emitting depolarization field will create a rebalance force to maintain the initial equilibrium. If the external field is electrical field, a displacement will occur, but if the external force is from mechanical displacement, an electrical field will be produced. This is called the piezoelectricity. Table 2 lists several different kinds of piezoelectricity materials. Changing in voltage, gives rise to a corresponding force, F, and the resulting dimensional change L as we can see below:

V=Q/C=Qx/(orA)  V=dijFjx/(orA)

Typically values for L vary between 10-10 and 10-7 cm/V. Thus, to obtain displacements on the order of m, voltages need to exceed 1000 V, unless stacked actuators or mechanical motion amplification methods are used [10].

For crystalline quartz, the piezoelectric can be written as:

d11 d12 0 d14 0 0

dik= 0 0 0 0 0 0

0 0 0 0 0 0

We can assume that our PZT film is about 2 m, and the voltage added on this thin film can’t exceed 10 V since the break down electric field is around 5000 (V/mm) and in more humid environment this break down field will be even lower. Using the equation above we can get F will be 0.11 N (PZT film dimension 100m*100m, r=3000, d11=370 pC/N, d31=110 pC/N). And the displacement of the crystal, L= LV/(td31), will be around 0.2m. So we can stack around 5 PZT crystals together in order to have 1m pressing into the ink. The pressure applying to the liquid can be up to 107N/m2 or approximately 100 pounds per square inch.

They are many piezoelectric film reported in the last 20 years. The three most popular with the sensor industry are ZnO, AlN and Pb(Zr,Ti)O3 (PZT) Thin films. Because PZT has high piezo-coupling, high piezoelectric constant and very high dielectric constant compared to ZnO and AlN, there have been enormous papers on this material. Many methods and techniques have been reported such as e-beam evaporation, RF sputtering, ion-beam deposition, MOCVD, magnetron sputtering and sol-gel. In our device, we can choose e-beam evaporation or RF sputtering because the equipment is quite convenient.

3. AFM working methods and the fabrication of its tip:

In 1981, Dr. Gerd Binning and his colleagues at IBM Zurich Research Laboratory developed the first scanning tunneling microscope (STM) that is the first instrument capable of directly obtaining three-dimensional images of solid surfaces with atomic resolution. But the STM can only study surfaces, which are electrically conductive to some degree. Based on this design, Binning et al. developed an atomic force microscope (AFM) to measure ultra-small forces (less than 1 N) between the AFM tip and the sample surface. AFMs can be used to measure all kinds of surface either conducting or insulating. AFM then become a very popular surface profiler for topographic measurements on micro-to nanoscale [11].

Like the STM, the AFM relies on a scanning technique to produce very high resolution, three-dimensional images of sample surfaces. AFM uses the motion of a very flexible cantilever beam with low vertical spring constant (0.05 to 1 N/m) and a very small mass (on the order of 1 mg) to measure the ultra-small force between the AFM tip and sample surface. The AFM combines the principles of the STM and the stylus profiler as shown in Figure 8. Usually the tip is only a couple of microns long and often less than 10 nm in diameter at the end and the cantilever is about 100-200 m. Today the most advanced tip can be microfabricated from silicon nitride using photolithography techniques, as we will discuss below. The lever deflection has been measured by several different methods including capacitance-detection, optical interferometry detecting and laser beam deflection. And I think that by using the capacitance-deflection, we can control which AFM will go down to the surface and write the linewidth.

Generally, there are three types of methods the AFM used to measure the profile of the surface, one is the “repulsive mode” or “contact mode”, the second one is the “intermittent-contact mode”, and the last one is the “attractive force imaging” or “non-contact imaging” as we can see from Figure 9 [12]

  1. Contact Mode

Conceptually, contact mode is the most straightforward AFM imaging mode. The cantilever tip is held close to the sample surface and the sample surface rastered underneath the tip. As the sample surface is moved, the change in topography results in a change in tip-sample interaction. Thus, the force incident on the cantilever tip is altered and the equilibrium between the elastic force of the deflected cantilever and applied force changed. The AFM operates in either constant height or constant force mode. The constant force mode is generally the preferred mode of operation. The total force exerted by the tip on the sample being within the limits. However, the response time is quite slow and it affects the scanning speed.

  1. Non-contact Mode

Non-contact AFM is one of several vibrating cantilever techniques in which an AFM cantilever is vibrated near the surface of the sample. The cantilever is held 5-10 nm away from the surface, within the region where long-range Van Der Waals forces dominate. Although the attractive force imaging exerts no normal pressure at the interface, the scanning speed is slow and the main disadvantage is that in humid conditions the tip will condense water so this method is seldom used outside the research environments.