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Technology Used in Flat Panel Display (FPD) Lithography Equipment

The liquid crystal panels used for televisions and smartphones are made by using technology that involves transferring ultra-fine pixel circuits to the glass substrate via exposure.
Canon strives for technological innovation as a top manufacturer of fine-pattern lithography equipment, which is essential for creating display panels that bring out the beauty of high-quality imagery.

2018/12/27Featured Technology

What Is Flat Panel Display (FPD) Lithography Equipment?

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FPD (Flat Panel Display) is a collective term encompassing such electronic device displays as LCDs (Liquid Crystal Displays) and OLEDs (Organic LED). FPDs of various sizes are used everyday in many devices including televisions, PCs, and smartphones.

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Some of the Many Products that Use FPDs

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An FPD comprises an array of one million or more ultra-fine pixels, each consisting of three colors: R (red), G (green), and B (blue). FPD lithography equipment is essential for creating these massive pixel array patterns.

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How FPD Lithography Equipment Works

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The FPDs for large televisions and smartphones are created by using a technology called photolithography, in which ultraviolet light is used to transfer an ultra-fine circuit pattern from an original photomask plate onto a glass substrate with µm (micron)1 level precision.
Canon's FPD lithography equipment uses a catoptric system with a large, high-precision concave mirror at its core for exposure and transferring during photolithography.

  • *1 µm (micron):
    micrometer, one millionth of a meter.

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High-precision concave mirror

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Main component units

During the photolithography process shown in the figure, the FPD lithography equipment has the most important role: it uses ultraviolet light to illuminate a photomask onto which a circuit pattern has been written, then transfers the ultra-fine circuit pattern from the photomask by exposing it onto the photoresist film applied to the glass substrate.

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Photolithography Process

The two main types of glass substrates used during FPD manufacturing are as follows:
Generation 8 (G8): 2,200 x 2,500 mm (mainly for television screens)
Generation 6 (G6): 1,500 x 1,850 mm (mainly for smartphone screens)
These substrates are much larger than the silicon wafers used for semiconductor IC manufacturing, which have diameters of up to approximately φ450 mm.

The mask stage on which the photomask is set and the plate stage on which the glass substrate is set must be aligned with a submicron2 level of precision while precisely overlaying many ultra-fine patterns. To achieve such a precise operation, it is necessary to minimize temperature changes because even a small temperature change can lead to fluctuations in thermal expansion and air density that can cause errors. Therefore, the lithography equipment is installed in a large chamber in which the temperature is controlled to within 23±0.1℃.

  • *2 Submicron:
    a dimensional range of 1/10 of a micron

Three Major Performances of Lithography Equipment

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Three Major aspects of performance are used as performance indicators for FPD lithography equipment.

1. Resolution
Indicates how fine the dimensions of a photomask pattern can be while still accurately transferring it, via exposure to ultraviolet light, onto the photoresist.

2. Overlay accuracy
Indicates how precisely a pattern can be overlaid on the pattern formed during the previous process following high-precision measurement of a reference mark and correction for various types of pattern distortion.

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The pattern is overlaid while precisely aligning it with the pattern formed during the previous process.

3. Tact time
Refers to the time required to process one plate; an important indicator of plant productivity.
With Canon's FPD lithography equipment, in addition to increasing the speed that the stage on which the plate is set can be driven, two different alignment mechanisms are used to reduce the time required for alignment measurement and further reduce the tact time.

One-Shot Exposure Optical Technology for Large Screens

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An Ultra-Precise Projector that Uses a Large Concave Mirror

In the world of photolithography, one-shot exposure technology is needed not only for semiconductor IC chips and other applications for which smaller surface areas are exposed to patterns, but also for such large surfaces as liquid crystal displays. In 1980, Canon recognized this need by independently developing its Mirror Projection Aligner (MPA) lithography equipment, which uses a mirror-projection optical method.
The basic configuration of this equipment comprises a large concave mirror, small convex mirror, and trapezoidal mirror created using the highest possible machining precision. The photomask mounted to the top of the unit is illuminated with intense ultraviolet light, which is reflected five times to accurately transfer the circuit pattern on the photomask to the glass substrate via exposure. Because this system is optically symmetrical, it offers two advantages in principle: no comatic aberration and no chromatic aberration, a potential problem for refractive optical systems employing lenses that can occur as a result of differences in wavelengths of light. Because an arc-shaped range makes it possible to obtain the best possible image-formation characteristics, Canon's equipment scans an arc-shaped exposure area to achieve high resolution performance over a large area.

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Extremely precise concave mirror

Diameter: 1.5 m
Surface accuracy: 10 nm RMS or less
Root-mean-square surface unevenness of 10 nm or less

  • nm:
    nanometer. One billionth of a meter.

An Illumination System Offering Large-Area Uniformity and High Intensity

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A Uniform, Bright Illumination Light

With FPD lithography equipment, there is a continuing need to shorten tact time and realize increased productivity. Canon's photolithography technology, which transfers photomask patterns onto a photoresist via exposure, utilizes an ultraviolet light illumination system with increased illumination intensity to reduce the exposure time and greatly shorten tact time.
In addition, because the MPA is a mirror optical system, no chromatic aberration occurs. Taking advantage of this, the system effectively utilizes ultraviolet light to achieve increased illumination intensity over a wide wavelength range (i-line: 365 nm to g-line: 436 nm) without sacrificing resolution performance. The MPA also combines three high-output ultraviolet lamps to further increase the illumination intensity.
However, increasing the illumination intensity by simply adding more ultraviolet lamps leads to more unenven illumination, making high-resolution performance impossible. Therefore, Canon's mechanism for combining the ultraviolet light from the three lamps uses a fly-eye lens and precise adjustment with other optical systems to achieve uniform and high-intensity illumination.

Hybrid Alignment Technology

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Quick, Accurate Measurement to Shorten tact time

To shorten the tact time, it is also necessary to develop faster alignment measurement technology. However, when a glass substrate udergoes such processes as heat treating, changes may occur in the substrate's absolute dimensions. This results in various distortions in the coordinates of the pattern formed during the previous process, and it is therefore necessary to measure many reference-point positions to accurately grasp these distortions.
In addition to the conventional method (AS: Alignment Scope), which uses the actual mirror-optical pass to see the overlay of alignment mark on the photo mask and the substrate, a new method (OAS: Off-Axis Scope) is now available for Canon's latest equipment. OAS can directly see the alignment mark near the substrate, without using mirror-optical pass. By using both AS and OAS methods simultaneously to see distortion, Canon's Hybrid alignment system enables further improvement of tact time and more precise measurement.

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Hybrid Alignment System

High-Precision, High-Speed Stage

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Quick Alignment of an Object as Heavy as a Small Truck with Submicron-Level Precision

Another means of shortening tact time requires high-speed driving of the plate stage on which the substrate is set. However, the plate stage that supports the large-surface-area glass substrate is very heavy, and requires precise, high-speed driving of the X-direction stage, which is as heavy as a small car, on the Y-direction stage, which can weigh as much as a small truck.
Canon uses such technologies as laser-interferometer-based measurement with nanometer-level precision, a powerful linear motor, and air bearings to achieve high-speed, high-precision driving control of the large stages.
As a result of these technologies, current equipment achieves reduced tact time through both submicron-level alignment precision and high-speed stage driving.
What's more, Canon's equipment incorporates calibration technology that makes fine adjustments to the scanning speed and scanning direction of the plate stage during transfer exposure to correct distortions in the pattern on the substrate that may result from such steps as heat treatment during the previous process.

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To account for pattern distortions resulting from the previous process, Canon technology performs fine adjustments to scanning speed and direction to correct the mask pattern on the photomask during the exposure process.

Nonlinear Distortion Correction Technology

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Flexible calibration even on distorted substrates

When a large-surface-area glass substrate undergoes such processing as heat treating during the previous process, changes can result in the substrate's absolute dimensions, which in turn leads to distortions in the patterns previously formed. Unless this issue is addressed, it is not possible to accurately overlay the distortion-free pattern on the photomask with the pattern formed on the substrate during the previous process.
Canon's FPD lithography equipment therefore incorporates not only a scanning-correction mechanism for making fine stage-driving adjustments, but also a magnification-correction mechanism for handling nonlinear distortions that cannot be completely resolved by scanning correction through independent adjustments in the X and Y directions.
The magnification correction technology makes possible enlarged or reduced-size projection of the pattern based on which an image is formed by dynamically adjusting the amount of bending of the X and Y-direction optical paths during stage scanning.
Using this technology in conjunction with the scanning-correction mechanism, it is possible to handle distortions of various shapes on the substrate and more accurately align it with the pattern on the photomask.

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