New tests for evolving FPD technologies

Flat panel display (FPD) manufacturers are placing heavy bets on new technologies to fill the needs of demanding applications. These applications include larger and lower-cost laptop monitors, small area/low power panels for mobile phones and other portable devices, HDTV and widescreen formats for home television and high reliability daylight-readable displays for the ‘glass cockpits’ of fighter aircraft, battle tanks and warships.

Display technologies range from amorphous and low-temperature polysilicon (LTPS) LCD panels to emerging organic LEDs and others.

These emerging technologies promise to deliver higher value-added products, but they increase a display OEM’s investment in new tools and methods to shorten their time to market, start up new production lines and overcome lower than ideal yields.

All this requires more efficient testing with instruments and systems that provide higher throughput and accuracy, both in R&D and production areas.

Display device measurements use probers and parametric testers similar to those found in conventional CMOS and bipolar fabs.

Display OEMs and semiconductor fabs use distinctly different types of prototyping and production equipment. Typically, in display testing, the prober is physically larger and associated instruments are farther away from the device under test (DUT) than with conventional semiconductor wafers.

This greater distance leads to cabling length problems, such as higher parasitic capacitance and noise, which can reduce sensitivity, increase measurement settling times and lower throughput - just the opposite of what is needed.

To avoid these problems when testing new display devices, innovative measurement techniques and test equipment modifications are often required.

Amorphous silicon (a-Si), the traditional technology for AMLCDs, still holds the dominant share of the market for applications from mobile phones to PDAs and laptops, desktop monitors and most TV applications. This is the result of the technology’s high level of refinement and low cost, even though a-Si thin-film transistor (TFT) devices are slower, larger, block more light and require more external circuitry than newer LTPS LCDs.

Now in their fifth and sixth generations, a-Si substrate technologies are being used to create larger displays and manufacturers are striving to reduce costs even more by producing in higher volumes and improving yields.

Cost is a dominant concern, so production test time must be kept to a minimum. Typically, only essential characteristics are measured in a production environment, including: I_d-V_g curve sweeps, with up/down hysteresis; voltage threshold (V_th); forward (on) current level; leakage (off) current (I_L); switching (response) time; resistance and capacitance of contact chains.

These measurements are taken on a few test element groups around the outside edges of an LCD panel. Sometimes a few working pixels are also measured, to check for uniformity, and properties of the indium/tin oxide conductive layer may be spot checked.

Typical systems that characterise active elements in an FPD (Figure 1) include DC source-measure units (SourceMeter), a switch matrix (to allow testing multiple devices with one set of instruments), a probe station (not shown) and cabling between various components.

Figure 1a: Typical FPD production test diagram.

Figure 1b: OEL Leakage current tester.

Due to the large size of glass substrate panels, FPD production equipment also tends to be rather large and highly automated, including the associated probe station for making contact with LCD TEGs and working pixels.

This complicates placing measuring instruments close to the signal source. The natural inclination is to interconnect the probe card and instrumentation test head with a long cable, but this creates other measurement problems.

Parametric characterisation of LCD TFTs typically requires extremely sensitive measurements of the drain current during the off state. If the threshold voltage and subthreshold (leakage) current are too high, there is image ghosting, so I_L must be measured down to femtoamp levels.

Gate leakage current is also important in device performance, as are other low current phenomena.

Too often, when an FPD characterisation system is configured, the specifier tends to concentrate on DC parametric instrumentation while neglecting the rest of the system, such as cabling, probe cards, etc. In fact, these components of the system are the more likely sources of noise, given that poor quality or poorly shielded cables and high leakage switching systems often lie directly in the signal path.

For ultra-low current measurements, it’s important to have a tightly integrated parametric characterisation system, including not only the measuring instruments, but also the test fixture, probe station, switching system, connections, cabling, grounding and shielding.

Therefore, a test engineer needs to take a system-level view. Even with a properly configured system, these issues can affect measurement noise, accuracy and throughput rate:

• Cabling and the parasitic capacitance and shunt resistance it introduces;
• Grounding/shielding/guarding;
• Offsets and leakage in switch matrices;
• Probe card and test head design;
• Instrument noise and settling time;
• Environmental electrical noise levels and types;
• Test element group devices and associated test strategies.

Effective techniques must be employed to minimise errors, noise and excessive test time that arise from the problem areas just mentioned. Often, this requires unique solutions for unique devices, material and equipment problems.

Drawing on its 30+ years of experience in semiconductor, LCD and passive device testing, Keithley has developed solutions for testing everything from conductive coatings and insulating oxides to complete multi-element displays.

For example, the Model 4200-SCS semiconductor characterisation system provides a low-noise platform for LCD device testing. The modular design, local or remote pre-amplifiers and flexible GUI software environment allow it to be customised for typical FPD production testing.

Generally, system noise has the greatest impact on measurement integrity when the DUT signal is very small (ie, low signal-to-noise ratio). That’s because it’s difficult to amplify the signal without amplifying the noise along with it. Clearly, the key to low level measurement accuracy in FPD testing is to increase the signal-to-noise ratio.

The instrument’s noise specification is only about 0.2% of range, which means the peak-to-peak noise on the lowest current range is just a few femtoamps. Noise can be further reduced with proper signal averaging (through filtering and/or increasing the number of power line cycle integrations).

If this creates a test throughput issue, its remote low noise pre-amp option allows measurements down to the sub-femtoamp level.

To get that level of sensitivity, the pre-amps typically are remotely mounted on the probe station. With this arrangement, the signal travels only a short distance (just the length of the probe needle) before it is amplified. Then, the amplified signal is routed through the cables and switch matrix into the measurement hardware.

This arrangement also benefits test throughput by reducing measurement settling time because cable lengths, and therefore parasitic capacitances, are greatly reduced. Throughput is further enhanced with matrix switching systems, that allow connecting multiple DUTs to the test system. The company’s low-leakage matrix switching cards are designed specifically for ultra-low current measurements.

Earlier polycrystalline silicon required high deposition temperatures that were impractical for LCD on glass manufacturing. However, today’s LTPS technology has overcome many of these manufacturing problems and its inherently higher speed provides visible benefits to displays.

Another advantage of p-Si on glass is that driver chips can also be produced during the same process, saving cost and space and improving reliability. As new, lower cost production methods are developed for still lower temperatures,p-Si displays will continue to gain in sophistication and market share.

They are rapidly becoming smart, high value-added displays that will eventually include memory and CPUs, in addition to array drivers.

These ‘System On Glass’ FPDs will require less power, produce brighter images, have faster response, provide higher resolution and require less external circuitry than current generations of either a-Si or p-Si technology.

LTPS displays require more tests because they incorporate other control devices in addition to the pixel TFTs and they are intended for operation at video rates. These tests include measurements on the driver ICs, digital tests with clock signals and checking high frequency operation.

As a result, high test throughput is even more important than in traditional a-Si products. Given that p-Si active devices are smaller and operate more efficiently at lower currents, testing them may require higher measurement sensitivity than a-Si devices do.

Otherwise, similar parametric testing is performed on p-Si FPDs, with all the same measurement problems associated with a-Si technology. However, additional signal sources and instruments make the integration of LTPS test system components more of a problem. These include parametric tester interfacing issues, synchronisation problems, and software compatibility.

As high-speed AMLCD panels for video applications evolved during the 1990s, the company developed complete high-speed process monitoring system solutions that help display OEMs improve device yields and more easily control product quality. LTPS allows drivers and other advanced circuitry to be integrated on glass, so TEG systems are being enhanced with pulsed and RF capabilities to support specific types of high-speed functional testing.

These capabilities can be combined with TEG testing earlier in the production process, avoiding expensive packaging steps for bad devices or making it economical to implement corrective actions and repairs.

Various test platforms are available that allow LTPS display OEMs to optimise measurement sensitivity and throughput. When ultra-low current measurements are not an issue, the S400 automated parametric test (APT) system can increase throughput in a fully cabled application due to its ultra-low parasitic switching matrix and sensitive SMUs.

Alternatively, when both low-level measurements and high throughput are required, the S600 APT system allows both good low-current performance and high-speed pseudo- and (in many cases) true-parallel testing operation.

To extend measurement sensitivity in S400 applications, the 4200-SCS can be connected through the system matrix with up to four separate pixel probe cards. With this arrangement, it is possible to achieve current sensitivity at the DUT in the range of 1-2fA. Many other hardware variations are possible to optimise the system for the particular application.

These display devices are rapidly approaching commercialisation, including both active and passive variations. Figure 2 illustrates a typical OEL device. Two technologies have emerged, which seemingly fill different application niches.

Light emitting polymer (LEP) devices are being developed based on large molecule polymer technology created by Burroughes and others at Cambridge University in Britain. This technology is being pursued for small area and low speed/resolution applications, such as mobile phones, digital ‘ink and paper,’ fabrics, greeting cards, window/POP advertising devices, etc.

Figure 2: Typical OEL device construction. The organic layer is positioned perpendicularly between striped cathode and anode. From the anode side, the organic layer has a hole transport layer, emission layer and electron transport layer. The structure closely resembles light emitting diodes with PIN junctions.

The stated advantage of polymer displays is the ability to spin the layers on the glass substrate and, in some cases, to pattern the films with photolithography. Thus, it is theoretically easy to achieve simple, low-cost active matrix displays, and (at higher cost) higher resolution displays with fine stripes of different emitters for full colour images.

OLED devices based on Kodak’s small-molecule technology are compatible with most semiconductor processing techniques, but their manufacture is considerably more complex than that of LEPs.

Nevertheless, they are the front runners for high information content, video-bandwidth displays, such as monitors and TVs. In these applications, they will almost certainly disrupt the dominance of silicon-based LCDs, as there are many researchers and start-up companies working on this technology. (No one has ever succeeded in integrating an array of inorganic LEDs with a density as high as that possible in an active matrix OLED display.)

Material lifetime is still a key issue limiting widespread application of OEL FPD technology, so developmental testing is focused on evaluating materials, processes and devices for light output vs operating life.

The test systems now required for device characterisation during product and process development can be used later, with minor modifications, to monitor ongoing processes and help manufacturers climb up the yield and quality curves.

The development of such systems for OLED applications is possible with the company’s instrument level solutions such as the SourceMeter family of current and voltage sources, which also includes precision current and voltage measurement capabilities.

In addition, these core I-V tools provide system-level compatibility with other instruments and high-density switching systems, such as the series 2000 family of DMMs, the series 7000 family of switching mainframes and the series 2700 integrated data logging family.

These instruments can be combined in a wide variety of configurations to create manual and automatic test systems for lifetime, I-V and light-current-voltage (LIV) characterisation.

One of the issues associated with OLED testing is the higher level of capacitance in these structures. Although many of the measurements required are the same as for AMLCD devices, the test system and methodology must be able to handle higher capacitance without adding excessive test time.

Also, given that OEL FPD pixels are active light-emitting devices, it is important to characterise LIV properties under both DC and pulsed DC operation. This creates additional test complexity.

By combining its experience in conventional semiconductor and specialised laser diode testing, the company has developed I-V and LIV characterisation instruments and systems for OLED and LEP researchers and process development engineers. For electrical testing needs on emerging production systems, Keithley instruments and system platforms are being tailored for advanced device measurements, while still meeting the requirements for high throughput and tightly integrated functional test systems.