vacuum process considerations for large area flexible electronics

AIMCAL 2003 Fall Conference, Santa Ana Pueblo, NM

James R. Sheats 5 Sep 20031

Vacuum Process Considerations For Large Area Flexible ElectronicsJames R. Sheats*

Lost Arrow Consulting, Palo Alto, CA

AbstractRecently there has been a great deal of interest expressed in fabrication of thin film electronic products (including transistors, light-emitting diode displays, crossbar memories, and others) using roll-to-roll processing techniques. The advantage would be thin, lightweight, flexible products at lower cost. However, applying techniques so far used only for uniform large area coating of metals and insulators and relatively low-resolution patterning (as for example so-called flex circuits, and flexible photovoltaic cells) to the realm of micron-scale patterning of semiconductors with high yields of working products will demand significant innovations. This paper will attempt to provide an overview of these issues from the microelectronics perspective, in the hope of stimulating appropriate developments in web processing industries which could accelerate the advent of flexible microelectronic products.

IntroductionThe growth of interest in flexible electronics is well illustrated by the fact that two conferences devoted to this subject occurred within a month of each other earlier this year (3-4 February, Phoenix, AZ, sponsored by the U.S. Display Consortium; and 3-5 March, San Francisco, sponsored by Intertech Conferences); each of these drew several hundred attendees. An invitation-only National Science Foundation workshop was held in January on the topic of “Technological Challenges for Flexible, Light-weight, Low-cost and Scalable Organic Electronics and Photonics”, and the Materials Research Society semi-annual meetings in Boston and San Francisco now regularly include multi-day symposia devoted to flexible electronics, both organic and inorganic. The potential for flexible electronic products has also been highlighted by meetings related to materials printing; for example IMI’s (Information Management Institute) 1st Digital Electronic Materials Deposition Conference, 14-16 October 2002 (Palm Springs, CA), and TAGA’s (Technical Association of the Graphic Arts) 1st Microtech Forum, 7 Oct 2002 (Chicago).

This conference activity reflects actual research effort: both large companies and startups have extensive activities in the area. Among the most well-publicized and impressive from the standpoint of performance obtained are the polysilicon thin film transistors (TFTs) made by Seiko-Epson1 and Sony2 on glass followed by separation and transfer to plastic; these circuits formed the backplanes for successful flexible displays. Fabrication of amorphous silicon TFTs directly on plastic, first demonstrated publicly in 1994 at Iowa State University3 and subsequently at several other universities,4 is also seeing renewed interest at Sharp, Samsung, Philips, UNAXIS France, PARC, Iowa Thin Film Technologies, Ignis Innovations, Visible Tech-knowledgy5. Finally, the number of papers being published on organic TFTs, many of which are either made on plastic substrates or are intended to migrate there in the near future, continues to rise rapidly each year. 3M, Infineon, Philips, Xerox, Avecia and DuPont are among the large companies with active R&D in this arena, alongside startup Plastic Logic.

“Flexible” and “web” or “roll-to-roll” are often considered nearly synonymous; although all but one demonstrated realization of TFTs on plastic4 has used wafer-based tools, most workers in the field have web processing as their end goal. Some people have suggested that sheet-based processing may be the first step; much high-volume printing is done with sheets. However, the image of sheets of paper flowing through a printing press is still a far cry from plates of glass being processed as they are for flat panel displays today (and plastic plates would be handled very similarly). Certainly the equipment used for those displays is not well suited to handling thin plastic, and so much modification and innovation is required even for sheets. As usual, however, before one proceeds very far with a commercial venture, one has to answer the questions of 1) what is the product that customers really need and will pay for, and 2) what are the economics pertaining to the very best way to make this specific product? Unless the customer needs are put first, the technology is likely to follow a false path.

* Mailing address: 665 Lytton Ave, #6, Palo Alto, CA 94301; [email protected]



What is the point of flexible electronics?The vision that is held out in presentation graphics is nicely encapsulated by the concept shown on Universal Display Corporation’s website of a rollout organic LED display: nestled snugly inside a housing looking much like a slightly bulky fountain pen, it can be pulled out to reveal a display, probably a little wider than its height.

The product is light, compact, rugged and yet highly functional, and certainly very relevant to the obvious trends of mobile, networked computing devices. Certainly if it were available at a price not significantly higher than more conventional displays that function with these mobile devices, it would be a hot seller. The critical question, however, is how much more expensive it can be before this sales volume drops. The present author is not a marketing executive, but many casual discussions in the industry have suggested that the answer is very little.

The single most important factor holding back the development of a flexible, web-based electronics industry is the cost of investment for even a minimum-scale facility. Details will be discussed later in this paper, but it is certainly not less than several million dollars just to get to the demonstration stage, and several times that for actual production (which is much less than current display facilities, but still a formidable quantity). For such investment to be justified in the face of nontrivial technology risk (that it is doable is not really in doubt, but the number of surprises and overruns lying in wait along the way certainly is), the market payoff has to be very clear, and for most applications of flexible electronics this payoff is in fact rather murky.

There are, nevertheless, applications for which the benefits can be fairly clearly identified. Outdoor advertising is one. Currently high-quality digital billboards are made with inorganic LEDs, a costly manufacturing proposition since each pixel must be assembled from a packaged LED. Nevertheless, such billboards are impressive revenue generators: for example, at the Metropolitan Atlanta Rapid Transit Authority, digital signs are being installed in all stations; the contractor covers the entire expense (including operation of MARTA’s information services) from advertising revenue (http://www.masstransitmag.com/script/search.asp?SearchSiteURL=/articles/2002/mt_06-02/mt_06-02_02.htm). Deployment worldwide is growing rapidly (http://www-ru.screens.ru/eng/index.html); the attractiveness of the business was underscored by a recent New York Times article (“Your Message Here, in a Flash”, by Michel Marriot, 28 Aug 2003) on the initiative of Matsushita and Magink.

There are other problems with LED billboards besides manufacturing cost. The structure is massive and its power requirements voracious (since as an emissive display it must compete with sunlight rather than using it). Installation requires specialized equipment and new structures capable of supporting the weight. Any display using a glass-based electronics backplane suffers from the same problems. A far more desirable approach would use plastic-based displays, which could weight little more than the vinyl sign material now used, and be installed with relatively similar equipment and training.

Manufacturing cost is certainly an issue. LED billboards in roughly a 10’x20’ or 12’x24’ size run into the hundreds of k$ ($500-$1500/sq.ft.), not including installation; MagInk proposes to sell this size (using a novel display medium driven by electronics on glass) for slightly under 100 k$ (http://signweb.com/moving/cont/mitsutesting.html), which is several times the cost of a conventional sign(and also brings with it the problems of weight). While revenue is certainly much higher with the frequently updateable display, arbitrary capital is rarely available for expansion into new technologies, so the upfront cost cannot be too high if rapid growth is desired. Given the requirement of very large areas (by comparison to most other electronic displays), roll-to-roll process technology applied to flexible display manufacture would appear to address an important impediment to the development of this industry. Novus Communications Technologies (www.novusct.com) has begun to address these issues, and seems committed to the implementation of flexible electronics for this industry.

Other types of advertising may be similarly relevant to electronic paper, a concept often discussed but so far not realized despite many demonstrations of excellent product prototypes. The E-book, desirable though it obviously is in principle, is not sufficiently superior to existing paper books that it can motivate the required investment in a short time (though eventually it will surely appear). But the ability to convey advertising updates in real time to a variety of print-like surfaces (kiosks, perhaps even objects within the home) has a value that is well understood by an industry that is already in the many billions of dollars with vastly slower updates.



This is not mere advertising for the application (pun duly noted!). The goal of the technologist with a background in processing appropriate to flexible substrates is to implement these technologies, and the logic of business, just as in physics, dictates that a low-energy path has to be found through the barriers.Once the infrastructure has been developed for one application, it is much easier to evolve it into others. Thus, anyone in the web processing arena with an interest in seeing the vision of flexible electronics become a real business must think about how to meet the needs of the most compelling customer.

What are the needs for manufacturing of flexible electronics?Microelectronic devices are thin-film active devices whose operation is sensitive to topographic defects and material impurities as well as dimensional fidelity. Circuit element dimensions for the most advanced microprocessors are in the nanometer realm today, but useful circuits can be fabricated with features measuring tens of microns laterally and 0.1-1 micron vertically. Small defects such as pinholes or even misregistration from one layer to the next in such structures, however, can effectively ruin the entire circuit of which it is a part, unless redundancy has been built in by the designer. In addition, materials generally have a purity which is expressed in parts per million. The molecular composition and organization of interfaces from one layer to the next must similarly be controlled to minimize unwanted interface states (which tend to give rise to unwanted energy level shifts and traps for charge carriers). These constraints are in general quite different from those governing most of the products made with web processes.

The simplest active element for logic circuits is the thin film transistor, or TFT, based on amorphous silicon (a-Si) as the semiconductor and silicon nitride as the gate insulator; this structure is ubiquitously deployed in the backplanes of LCDs (liquid crystal displays)6. Both of these materials are deposited with PECVD in order to get acceptable electronic characteristics (especially the right amount of hydrogen, which must be present to passivate so-called “dangling bonds” of silicon with unsatisfied valence). Deposition temperatures above 200C have been used, but many authors have shown that properties do not start to degrade significantly until one goes below 150C4 provided the right plasma conditions are chosen.

A minimum of four lithographic “masking levels” are required for the fabrication of a) gate electrode, b) source and drain electrodes, c) isolation of individual transistors, and d) contact pads. Each of these must be aligned to within at most 30% of the smallest dimension in the two layers. Any mismatch between the edges of the source/drain and gate electrodes (which nominally are exactly over each other) leads to parasitic capacitance which slows the operation of the transistor and increases power drain.

Current LCDs typically have gate (or equivalently channel) lengths of around 2 microns. Thus, to satisfy existing a-Si TFT customers with devices on plastic, dimensional control would have to be on the micron scale, and defects small (and sparse) at that scale. However, products with slower switching speeds imply larger dimensions, and dimensional tolerance and defect specifications become more encouraging.

Thermal expansion and distortion. Many polymers have coefficients of thermal expansion of several tens of ppm, sometimes over 100. PET is unusually low at around 15-20 ppm (numbers found in reference sources vary substantially, based partly on variations in samples: polymers are rarely at equilibrium). PEN is similar but can handle substantially higher use temperature (up to around 180C, vs 120C for PET; higher for limited times such as would be encountered in soldering). Thus, for a temperature excursion to 150C, the expansion across a 12” wide web would be 750 microns, providing a challenge to maintain alignment of features using standard photolithographic masking techniques where the mask features are fixed. Of course, if the material returns to exactly the same relative locations after cooling, this is not a problem, but some distortion is likely. Heat-stabilized PEN as sold by DuPont-Teijin Films, for example, can be gotten to have stabilized shrinkage in the few ppm/hour or lower range, making it reasonable though still far from trivial to envision fabricating integrated circuits with conventional dimensions on wide webs. Larger features, such as would be needed for digital billboards, allow these constraints to be relaxed substantially.

There exist polymers which have suitably low CTEs. Polyimides (although they are generally not transparent) can be tailored over a wide range of specifications, and some curable (coatable) PIs have CTEs as low as 3 ppm (e.g. HD Microsystems). Standard substrate films such as Upilex (Ube) and Kapton (DuPont) are in the range of 11-20 ppm. Given that metals are also in this range, thermal expansion appears to be a manageable issue for amorphous Si devices, which do not need process temperatures above about 180C (and perhaps as low as 150C). The chief difficulty is with Si itself, with a CTE of 2.7 in this temperature range. Many workers who have made a-Si TFTs on PI have emphasized the value of island patterning (removal of the Si film except just in the active area of the transistor).



One proposed avenue for overcoming the thermal expansion/distortion issue is to avoid any thermal cycles, by using devices and processing techniques which do not require them. Organic materials can be used to make transistors,7 and organic TFTs with metal electrodes have been fabricated which have charge carrier mobilities (an important determinant of speed) comparable to those found with amorphous Si. All-organic TFTs, in which even the conductors are polymers, have also been made, though their performance is noticeably lower.

The advantage of organic devices from the process viewpoint is that they are ideally suited to printing technology; indeed, inkjet printing is now likely to be the dominant manufacturing technique for organic LED displays. The metal conductors can also be printed from precursor inks, and so conventional lithography with large area masks is completely eliminated. Even though some thermal cycling is required for curing metal precursor inks, it is possible to adjust for distortion dynamically, by altering the location of each ink droplet as required.

There are however also disadvantages to this solution (or family of solutions). For one thing, organic materials are generally less stable as active elements (electrical conductors, semiconductors, light emitters, etc.) than inorganic materials. In addition, the materials are far from optimized; problems remain with the selection of dielectrics, the control of interfaces, operating voltages and on/off ratios, and the availability of n-type semiconductors. While there have been (and continue to be) impressive advances in this field, it is unlikely that commercial products can be made with organic TFTs within the next few years.

The option of adjusting deposition locations according to local distortion is also not problem-free; it requires a sophisticated optical feedback system which adds cost and complexity to the process while limiting its speed. The method becomes inapplicable if a non-digital printing technique is employed (which would be attractive for throughput and system simplicity).

Topographic defects. As is familiar to AIMCAL and SVC conference attendees from other reports, coated webs typically have a density of micron or greater scale defects such as pinholes, scratches, particles, etc. in the many hundreds per cm2 or more, which would make it very difficult to construct integrated circuits. For example, Bishop reported8 36 defects with mean diameter 3.1 microns on a polyimide sample (Upilex), and several hundred on Mylar D, in an area of about 0.03 mm2. Similarly, in the same conference Fuse and Rimediotti of Galileo Vacuum Systems9 reported 10-100 micron-scale (1-100 microns) holes/mm2 in metallized web coatings, with a smaller number of larger defects.

In models of semiconductor yield statistics,10 one finds that an average of one fatal defect per chip reduces the yield to ~30%. In displays the backplane (the electronic portion) consists of largely empty space (with one transistor at each pixel, regardless of pixel size); however the effect of defects in the transistor area is essentially the same. Some interesting information on display yield issues (1992 vintage) is on the web at http://www.wtec.org/loyola/dsply_jp/toc.htm. Much relevant information is not public, but Nikkei BP made some available for this report, stating among other things, “one half of the array defects and 90% of the cell defects are linked to dust”. Photolithography was said to be routinely carried out in Class 10 cleanrooms, with Class 100 for CVD loading, and Class 1000 generally. An important problem operating on glass substrates is electrostatic charging, which of course can be expected to be worse with webs.

Bishop, in the paper cited8, gives an excellent discussion of the options available to the web process engineer for approaching the vastly more stringent cleanliness strictures that pertain to semiconductor device manufacture compared to conventional web coating applications. It is certainly possible to obtain extremely smooth, low-defect coatings from fluids, if enough attention is paid to filtering the liquid and avoiding subsequent particle formation by chemical reaction or introduction of dried flakes, etc. Such coatings will most likely be required as the foundation for vacuum-deposited device films. However, acceptable yield may be obtained only at very large volumes (the longer the coating flow is in place, the more stable it is and the larger the fluid volume the fewer particles, which tend to come in from surfaces).This is an important aspect since a nascent industry does not support the throughput of a mature coating or printing line.

In addition, vacuum chambers and processes will have to be designed with particle cleanliness in mind (especially PECVD, which is known11 to have a propensity for particle formation); the chamber then becomes the “cleanroom”. Of course, when lithographic processing is required it will have to take place under the usual low particle count conditions, and the issue of electrostatic contamination is one that will need careful attention.



Web mechanical and optical properties. Much effort has been put into developing transparent plastic substrates for flexible display backplanes. The entire business of FlexICs, Inc. (www.flexics.com) is built on the premise that transparency is a requirement. However, while current high-performance LCDs use a backlight, most developing display technologies with potential for flexibility are either directly emissive (e.g. organic LEDs, inorganic thin-film electroluminesence, etc.) or reflective. Thus there appears to be little value in transparency of the web for these applications.

More important is the need for a high degree of homogeneity of mechanical properties, so that tensioning of the web does not result in distortion and irreproducibility. As mentioned earlier, localized distortion is probably the single most serious concern facing the would-be manufacturer of flexible integrated circuits. Both silicon and glass substrates expand and contract significantly during processing, but there is very little local relative motion of atoms, and very little overall mechanical distortion (film deposition conditions areengineered and controlled to minimize stress, which can lead to long-range bowing or buckling). Thus a mask the size of the intended display, for example, can be used to pattern the features in succeeding layers which must be aligned to within a micron or better accuracy, without concern that the features on one side of the pattern will move substantially relative to those on the other side (30 cm or more away).

There are several options available for patterning which do not require mechanical fidelity over the whole device. Inkjet printing (there are other digital printing techniques) has already been mentioned. This is certainly an effective, perhaps the “ultimate” way to control pattern placement. However, as discussed above, it is not without its own problems: it is essentially a serial technique, as opposed to the parallel pattern transfer of conventional optical lithography, and so is inherently slower; there is always a trade-off. The suitability of inkjet printing to OLED manufacture is driving the development of reliable, high-throughput multi-head systems, but they will come at a cost; the most prominent commercially available machine today (Litrex) costs in the vicinity of $1M for a printing rate of 0.67 cm/sec on a 30 cm wide web (calculated from data given for glass plates), assuming a resolution of 200 ppi (about 127 microns).

At the other end, a relatively conventional approach to lithography is taken by Anvik (www.anvik.com), which manufactures a step-and-scan projection optical aligner with a 1:1 mask:substrate magnification that is slightly adjustable, so that distortion can be taken into account as long as it is over a length scale larger than the field of view of the lens (5 cm). For this system, the web is detensioned and held in place by a standard vacuum chuck during exposure. This system also costs around $1M, but has a throughput of 4 ft/min. and resolution of 10 microns or better. On the other hand, one requires separate systems for resist coating and development.

From the perspective of web process and machine suppliers, the challenge is to deliver systems that can maintain uniformity in all aspects of operation. The process can then be designed to accommodate the specific amount of stretching that is introduced. The same principle applies to many other physical parameters. Plastics typically expand upon absorbing water (this is one of the main problems with polyimide). As long as it is uniform, the process can take it into account. Without homogeneity in materials, microelectronics cannot be developed.

Cost Issues The ultimate goal is to make a product at a cost that is saleable. It is a trivial observation, but must be kept clearly in mind: flexibility per se has, as was discussed earlier, relatively little market value. Web processing has been alleged to be a path to highly economical production, but this viewpoint has been challenged as cost models start to be evolved and published.

The best starting point at this time is probably the cost model developed by Abbie Gregg, Inc. under US Display Consortium auspices for organic LEDs.12 They found that active matrix backplanes (using excimer laser-annealed polysilicon TFTs) could be made at close to half the cost of current glass-based production. While there are many somewhat arbitrary assumptions, since the process has never been implemented and not all of the equipment is “off-the-shelf” with specified prices, this is both an encouraging sign and a challenge. The result is clearly not “vastly” cheaper, and the investment price tag is hefty: in the vicinity of$250M for an output of a little over 5M ft2/yr. However, it is an improvement over the predictions of FlexICs of a 60% decrease in facilities cost (http://www.flexics.com/news/mrs2002.pdf).

The cost is uncertain by at least +/-30% because it is not completely broken out between TFT and OLED parts, and there are some discrepant numbers



The AGI study, while based on a computer model which must be purchased to see its inner workings, lists all of the basic process steps considered, and provides an excellent foundation for further work. It can be considered to be something like a “worst reasonable case”, however, since it was intended to be a useful guideline for the entire USDC community (technical contributions were made by members of 8 companies besides AGI), and hence rather conservatively stated. There are clearly many avenues for improvement, and many of them would begin with innovations of web processors. Machine throughput is of course the dominant focus of process engineers in all industries; the cost of integrated circuits is almost entirely understandable in terms of throughput per unit capital cost.

For the polysilicon TFT-based OLED displays modeled by AGI, 20% of the total tool budget was devoted to PECVD (for silicon oxide, silicon nitride and silicon), at a cost of about $9M apiece (approximately 8 systems for the modeled throughput, running at 2 ft/min. using a 2 ft wide web). Lithography was the bottleneck in terms of single tool speed, at 1.2 ft./min. for exposure.

If flexible silicon electronics (either amorphous or polycrystalline) are to become competitive solely on the basis of cost, the overall factory performance will have to increase by a factor of 5 or so from what was calculated in this study (nominally 10x better than the glass competition). This seems feasible though it represents a nontrivial challenge, and certainly not all of the necessary tools have been demonstratedcommercially. However, it is in all probability not the individual tool cost that is currently limiting the implementation of this technology. Rather, the decision to spend well in excess of $100M capital alone obviously has to be based on a great deal of risk mitigation that has yet to be carried out; some smaller steps are required first.

A crude but useful estimate can be made of the minimum investment required to demonstrate the viability of roll-to-roll fabrication of amorphous silicon TFTs as follows (items listed in thousands of dollars):

PECVD/Sputtering: $1,000Contact Aligner: 300Resist coater: 250Development: 150Pretreatment, plasma etch: 1,000Misc.: 300

Total $3,000,000These estimates are for a 12” web, and are purchase costs only rather than installed cost; in addition, lower-cost items have to be considered, as well as some analytical and test equipment (or outsourcing costs), and basic facilities. Staffing would probably be around 10 engineers and technicians for 18 months, adding not less than $2M. Thus, the total for a “proof of principle” is certainly over $5M (quite possibly closer to $10M, once all the unexpected difficulties have been surmounted). Even for a well-funded R&D lab in a large company such as DuPont or 3M (which might be expected to have many of the skills needed for web process development), one has to have a very compelling market case to justify a commitment of this magnitude. Typically exploratory research projects are undertaken with a budget closer to $1-2M to get to a go-no go decision point for serious development. Similar amounts might be obtained from early-stage venture capitalists.

The foregoing example is based on amorphous silicon, but organic electronics could probably not be demonstrated in a roll-to-roll process for significantly less. Metal deposition is still required for devices of commercially meaningful quality, and as we have seen, the inkjet printer would be comparable in cost to vacuum web equipment. Moreover, the greatest uncertainties are probably around the web handling and defect issues, where the performance of vacuum and fluid-based processes are equally unknown. In this respect, however, wet processing has an advantage, because there exist laboratory-scale coaters with price tags substantially lower (perhaps even an order of magnitude) than those quoted above, which would give meaningful results on which to base scale-up decisions.

Many of the unknowns will undoubtedly be reduced to acceptable levels by experiments that are being undertaken today, especially as a result of OLED development. However, it will take many years to see flexible electronics emerge in the marketplace based on this path. The alternative is for a few people in the web processing arena to focus their attention strongly on lowering the key barriers by a concentrated effort, which could be undertaken with a sufficient market motivation.



References

1. S. Inoue, S. Utsunomiya and T. Shimoda, SID Digest, 2003, 984-987 (Proc. of Society for Information Display Symposium, Baltimore, MD, May 2003), and references therein.

2. A. Asano, T. Kinoshita and N. Otani, SID Digest, 2003, 988-991 (Proc. of Society for Information Display Symposium, Baltimore, MD, May 2003), and references therein.

3. A. Constant, S.G. Burns, H. Shanks, C. Gruber, A. Landin, D. Schmidt, C. Thielen, F. Olympic, T. Schumacher and J. Cobbs, Proc. 2nd Symp. Thin Film Transistor Tech., Electrochem. Soc., 94-35, 392-400 (1994).

4. See J.R. Sheats, Proc. SPIE, 4688, 240-248 (2002) for references and a discussion of web process equipment relevant to microelectronics.

5. Much of this work has been described in the last two SID conferences, as well as MRS (Materials Research Society) proceedings of the last two years.

6. S.S. Kim, Information Display (SID magazine), Sep. 2001, 22-26.

7. C.R. Kagan and P. Andry, eds., “Thin Film Transistors” (Marcel Dekker, 2003).

8. C.A. Bishop, 45th Annual Technical Conference Proc. (Society of Vacuum Coaters), 476-481 (2002).

9. A. Fusi and F. Rimedotti, 45th Annual Technical Conference Proc. (Society of Vacuum Coaters), 509-513 (2002).

10. T. Kim and W. Kuo, Proc. IEEE, 87, 1329-1344 (1999).

11. Donald L. Smith, “Thin-Film Deposition” (McGraw Hill, 1995), pp. 536 ff.

12. A. Gregg, M. Strnad, L. York, USDC 2nd Annual Flexible Microelectronics and Display Conf. (Phoenix, AZ, 3-4 Feb 2003); “Flexible Microelectronics and Roll-to-Roll Processing Study”.

vacuum process considerations for large area flexible electronics

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