multi layer thin film barrier for protection of flex

8/8/2019 Multi Layer Thin Film Barrier for Protection of Flex

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Multilayer thin film barrier for protection of flex-electronics

Source: Solid State Technology

Publication Date: 01-MAR-08

Author: Suen, Chyi-Shan ; Chu, Xi

EXECUTIVE OVERVIEW

Continuous evolution marks the electronics industrys progress toward even thinner, lighter and more

flexible products. Although tremendous progress has been demonstrated, one fundamental obstacle that

remains is the sensitivity of these electronics to the moisture and oxygen in the environment. So the

search for an inexpensive, yet robust method to protect these devices continues. The current conventional

encapsulation solutions, e.g., using a metal can or glass can with desiccant, although somewhat effective,

add weight, thickness and cost. Therefore, a thin film barrier is viewed as the only viable solution to protect

these devices from damage caused by moisture and oxygen while maintaining a thin and flexible form

factor and light weight.

Thin film encapsulation is gaining considerable traction in various markets for a variety of applications,

such as organic light-emitting diode (OLED) displays, thin film batteries, and thin film photovoltaics.

Theoretically, a single layer of some ceramic materials, if made perfectly, would have a barrier quality as

good as a sheet of glass. But this has never been demonstrated to be practical for commercial use

because imperfections occur during deposition. Among the most commonly seen imperfections are defects

caused by surface topography and particles. The temperature limitations of organic and flexible electronics


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also often limit the quality of barrier films that can be deposited. It is often the case that the device to be

protected is damaged by the high temperature required to deposit a good barrier. Some deposition

technologies known for depositing good quality film, such as atomic layer deposition (ALD), are often too

slow and too expensive for commercial production purposes.

Example: Barix

Vitexs Barix multilayer thin film barrier is comprised of alternating layers of organic and inorganic films

applied in vacuum. This coating is typically only a few microns thick and can be applied directly on the

device as encapsulation or on plastic film to be used as a substrate or laminating encapsulant.

The unique feature of this multilayer coating is how this organic layer is deposited. A liquid monomer

precursor is flash evaporated to vapor, which then flows into a vacuum chamber. This monomer vapor then

condenses and becomes liquid on the substrate surface. Because the monomer is deposited in liquid form,

it completely planarizes the surface topography and smoothes the surface. It is then polymerized by UV

and becomes a solid whose top surface is atomically smooth, creating an ideal surface on which to deposit

a barrier film.

Figure 1. Various structures: a) typical multilayer barrier coating structure; b) multilayer barrier covers

OLED cathode separator structure; c) multilayer barrier covers odd shape particle on device

surface; d) organic layer planarizes high-aspect ratio test structure.

An inorganic film, only a few tens of nanometers thick, is deposited on top of the polymer layer. Because

the polymer surface is so smooth, the inorganic film is grown with very few defects and is therefore an

almost perfect moisture barrier. These two processes are then repeated to achieve the desired level of


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barrier performance. Figure 1 shows what this barrier structure looks like and how it can effectively

planarize the surface topography and smooth surface defects.

Both organic and inorganic layers are deposited at low temperatures, usually less than 90C. Both

materials are highly transparent and their refractive indices are carefully matched to make the barrier film

suitable for applications such as OLED displays and photovoltaic modules to allow the maximum amount

of light to go in or out through the barrier film.

Figure 2. Calcium test yield vs. number of organic/inorganic pairs after high temperature and high

humidity test

Figure 2 is a yield study performed by Vitex using the so-called calcium test method. Each test group

consists of 54 calcium test samples. The yield criteria were set based on stringent tests that thin film

barriers need to pass for OLED displays. Although the yield is not at an acceptable level, it was shown that

it is possible to protect the device with only one organic-inorganic pair. It was observed that the yield loss

was mainly caused by the early failures related to particles so large that one layer of the organic film with

thickness used in this study was not adequate to cover. As the number of pairs of organic and inorganic

layers increases, the yield also improves. With a three-pair process, we were able to achieve almost 100%

yield. This study is particularly important because it shows the pathway of increasing robustness and

reducing production costs. Because different applications may have different yield criteria and may require

different levels of protection, the multilayer approach provides the flexibility to adjust the barrier

accordingly.


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Enabling ultra-thin and flexible OLED displays

OLED displays have the advantages of self-emitting light, fast response time, low power consumption and

wide viewing angle and are considered as the most promising next generation display to replace LCD.

In TIMEmagazines November 12, 2007 issue, LG. Philips flexible OLED display was selected as one of

the best innovations of 2007 in the computer category. This 4-in. QVGA (240 RGB 360) display was a

joint collaborative effort of LG. Philips, Universal Display Corp. (UDC), and Vitex Systems.

LG. Philips first built the amorphous silicon thin film transistors (TFTs) on stainless steel foil; UDC then

deposited the OLED with its FOLED and PHOLED technologies; and then the display was encapsulated

by Vitex with its Barix encapsulation. The resulting display has total thickness of 0.15mm and is flexible.


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Figure 3.a) Samsung SDIs 4-in. bendable AMOLED display; b)schematic cross- section view of the

display structure (not to scale, courtesy of Samsung SDI).

Additionally, Samsung SDI demonstrated a bendable 4-in. WQVGA (480 RGB 272) AM OLED display

with low temperature polysilicon (LTPS) TFTs by using a glass substrate. The glass is etched back to

50m thick after the completion of OLED construction and thin film encapsulation. This etching step is

used to reduce the thickness and achieve the desired bendability of 10cm radius or less. The display,

including encapsulation, was


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Different substrate materials, such as stainless steel and glass, were used to build the ultra-thin and

flexible displays. However, because of the relatively high temperatures involved in depositing TFTs, the

multilayer thin film encapsulation was the common choice and was proven essential in order to achieve the

thickness, weight, flexibility, and durability goals.

Protecting thin film batteries

As displays and electronics become thinner and more flexible, so does the requirement for power supplies

to have a similar form factor. Therefore, thin film and flexible batteries have become indispensable. A

lithium phosphorus oxynitride (LiPON) thin film battery is considered a prime candidate for flexible

electronics because it has the advantages of ultra thinness, high energy density, fast charging, and high

temperature stability.

However, the use of lithium as the anode material makes the battery sensitive to moisture so it needs to be

protected by a hermetic seal. Just like glass encapsulation used in the OLED displays, a mechanical

encapsulation using metal or glass lid greatly diminishes the inherent merits of a thin film device. Thin film

encapsulation is therefore necessary in order to fully exploit a thin film batterys advantages.

However, using a thin film barrier to encapsulate LiPON batteries is more challenging than encapsulating

OLED displays because the device surface topography is typically greater and the surface is rougher. The

thin film encapsulation structure needs to be adjusted in order to be effective. Figure 4a is a lithium test

sample with thin film encapsulation after 24 hrs in the damp heat test at 85C/85% relative humidity (RH).

The sample was encapsulated with the typical barrier structure used to protect OLED devices that has

been developed over the past several years.

Figure 4.a) Lithium test sample coated with non-optimized barrier structure after 24 hrs in

85C/85%RH; b) similar test sample coated with optimized barrier structure after 200 hrs in 85C/85%RH.


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The barrier was not able to adequately protect the lithium because much of the lithium (metallic color

portion) reacted with moisture and disappeared in a short period of time, although the identical barrier

structure was proven to be effective for over 1000 hrs in the same test conditions for OLED devices. After

adjusting the barrier structure to overcome the rough surface and extreme topography, Fig. 4bshows a

similar lithium test sample after 200 hrs in the same damp heat test condition. Significant improvement

was made simply by adjusting the barrier structure. The same coating structure was then used on real

LiPON batteries for a customer (results are not shown).

Encapsulating thin film photovoltaics

Thin film photovoltaics, such as copper indium gallium selenide (CIGS), have the potential of low

manufacturing cost and a flexible form factor. These types of thin film solar cells are gaining considerable

attention that is exacerbated by the shortage of silicon material. Many companies are attempting to make

CIGS solar cells on stainless steel foil in a roll-to-roll format in order to make the cell flexible while

maintaining low manufacturing cost.

Although the solar cells are made on flexible metal foil, they are usually protected by rigid glass cover at

the end of the process in order to meet the lifetime and moisture resistance requirements because some of

the materials used in the cells are sensitive to moisture. Such a solution, although effective, not only

diminishes the flexibility, but also adds weight and cost to the module.

Some companies try protecting the cells with plastic film instead of rigid glass to maintain the flexibility, but

can only maintain short lifetimes because of the poor moisture barrier property of the plastic film. Figure

5a shows a flexible CIGS solar cell protected with Vitexs Flexible Glass (polyethylene naphthalate (PEN)

coated with the Barix barrier) on top. The cell was tested in a damp heat test at 85C and 85%RH for over

1000 hrs.


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Figure 5. a) Flexible CIGS solar cell after 1000 hrs of damp heat test at 85C/85%RH; b) open circuit

voltage Voc after damp heat test.

Due to the test and equipment limitations, it was not possible to measure the cell efficiency, so only open

circuit voltage (Voc) was measured. Figure 5b shows the cells open circuit voltage over time after being

stored in a damp heat test chamber. The Voc started to drop in


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Barrier stability under UV and sunlight

Although the barrier has been proven effective under high temperature and high humidity test, more

stringent tests are required because solar cells used for rooftops or solar farms need to be extremely

reliable. The solar panels have to survive 30 years of harsh environments, such as UV exposure and

extreme temperature.

UV damage to the organic material is an obvious concern so the organic material used in the Barix

encapsulation was tested in various UV and sunlight irradiation to verify stability. A 0.5m thick film of this

organic material was deposited on 2-in. square glass substrates with the standard process.

Transmission spectra were measured by using a broadband spectrophotometry from n&k Technology.

Samples A and B were then both exposed to UVB fluorescent lamps in an arrangement that complies with

IEC 61646 for three weeks. The dosages were calculated to be equivalent to 15kWh/m2 in wavelengths

between 280 and 385nm and 5kWh/m2 in wavelengths between 280 and 320nm. Sample A was then

removed for transmission measurement while sample B was removed to be exposed to 100X solar

irradiation (100kW/m2) by a sunlight spectrum simulator for two weeks for an equivalent dose of

33,600kWh/m2. The transmission spectrum was then taken on sample B.

The table shows the average transmission from 350nm to 1000nm for all the samples before and after the

tests. It can be seen that the transmission changes after such high dose UV and sunlight exposure are

minimal. Barrier integrity under similar exposure will be studied in the near future.

Barrier stability at high temperature

A lamination process is often used to attach the barrier film to the device to be protected, such as a solar

cell. This lamination process typically applies higher temperatures, such as 130C to 150C to melt EVA or

thermal plastic in order to bond the films. Therefore, to demonstrate the applicability of our technology for


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these markets we have to make sure that our barrier film is not damaged by the high process

temperatures. The calcium test is again used to verify the barrier performance in this study. We have

verified that the barrier performance is maintained after being tested at 140C for over 500 hrs.

Conclusion

The multilayer barrier developed has been proven effective in protecting various organic and flexible

electronic devices including OLED, thin film batteries, and solar cells. Accelerated aging tests at high

temperature and high humidity for thousands of hours have been performed and the positive results

demonstrated. Film properties under severe UV exposure and high temperature were studied. Yield data

demonstrate that the Barix process is a production-worthy process.

Acknowledgments

The author would like to thank H.K. Chung, PhD, and D.W. Han, PhD, at Samsung SDI for their continuing

support for the development of thin film encapsulation. The author would also like to thank LG. Philips,

UDC, and other unidentified collaboration partners for supplying the solar cells, thin film batteries, and test

devices. Barix is a trademark of Vitex Systems Inc.

Chyi-Shan Suen received his MS in material science and engineering and MBA from U. of California,

Berkeley and is the director of sales and marketing at Vitex Systems Inc., 2184 Bering Drive, San Jose,

CA 95131 USA; ph 408/325-0366, e-mail [email protected].

Xi Chu received his PhD in material science and engineering from Northwest U. of California and is the

director of process engineering at Vitex Systems Inc.

multi layer thin film barrier for protection of flex

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