Photolithography, as the name suggests, is a technique that uses light to create images. It is widely used in the semiconductor manufacturing industry and many other nanotechnology applications. To adapt to the trend of miniaturization in today's microelectronic products, related application fields increasingly require photolithography equipment with high production capacity.
This article explores the application of position feedback technology in modern lithography processes, as well as the advantages and potential of the latest grating systems and traditional laser scale systems. These features provide machine designers with great flexibility, enabling them to explore how to minimize the footprint of lithography equipment without compromising performance.
Semiconductor manufacturing
In photolithography, a layer of photosensitive photoresist material is typically deposited first on a silicon wafer. Then, a light beam passes through a photomask onto the wafer to project the mask pattern onto the photoresist. A developer is then used to dissolve the exposed photoresist areas. Finally, semiconductor, conductive, or insulating materials are selectively etched or filled into exposed areas on the wafer surface. In this way, multiple microelectronic feature layers can be constructed (typically requiring approximately 30 photolithography steps) (see Figure 1).
Figure 1: Silicon wafer under a microscope
Immersion scanning lithography machines contain a lens system that focuses a light beam through a photomask or "intermediate mask" onto a semiconductor wafer. They also include a set of sealing elements that seal a volume of liquid between the objective lens and the semiconductor substrate. Because the liquid has a higher refractive index than air, this allows for higher optical resolution and smaller feature sizes.
In immersion scanning, the light beam remains fixed, while the photomask and wafer must move in opposite directions due to the lens inversion effect. This requires precise position feedback to control actuators on the photomask and wafer motion platforms to achieve high-precision motion control. The light source can be made to blink at a specific frequency to expose different areas of the wafer each time.
The photomask is precisely aligned with the wafer substrate, allowing the pattern on each mask to be accurately etched onto the existing etched pattern layer. This step is crucial for manufacturing integrated circuits (ICs): the reference points on the wafer and photomask are automatically aligned with an error range of less than ±20nm, depending on the feature size of the IC, and biases in the X, Y, and θ (rotation) directions are corrected.
Linear gratings are required on each platform's long-distance incremental measurement system to ensure that position and velocity reach specified accuracy. High-precision grating feedback enables intermediate masks and wafer platforms to work in series, achieving planned scan trajectories with the required coverage accuracy. Laser rulers and some of the most advanced gratings can meet the demanding accuracy requirements of this semiconductor manufacturing process; for example, Renishaw's latest VIONiC series of gratings has an electronic subdivision error as low as <±15nm.
Flat panel display manufacturing
The traditional photolithography process used in flat panel display (FPD) manufacturing is also used in semiconductor chip manufacturing. A major driving force behind chip development is the increasing miniaturization of electronic devices. On the other hand, within the FPD industry, each generation of manufacturing technology is categorized according to the maximum physical size (in square millimeters) of the glass substrate that can be manufactured. For example, the tenth-generation (G10) FPD is cut from a 2880mm × 3080mm glass substrate. Thin-film transistors (TFTs) are essential display elements, with a critical dimension (CD) close to 3 micrometers, which has remained stable across several generations of manufacturing processes.
Each new generation of products requires the fabrication of larger substrates, necessitating increased productivity to create circuit patterns over a larger area of the substrate in a single exposure. Multi-lens systems have been proposed as a solution to this problem, enabling the coverage of even larger areas.
However, a major challenge for the FPD industry is manufacturing and handling increasingly larger photomasks, as photomask size must be proportional to substrate size. Maskless projection systems are gaining popularity as an alternative technology in FPD production. One such technology uses spatial light modulators (SLMs) to directly etch patterns onto the substrate in a manner similar to digital printing.
For example, a parallel lithography system, as shown in Figure 3, includes a set of SLM imaging units arranged in a parallel array. Each unit contains an SLM molding assembly, a spherical mirror, multiple light sources, and a projection lens assembly, as shown in Figure 2. The SLM molding assembly is a MEM (Micro-Electro-Mechanical System) device with thousands of controllable micromirrors. By tilting the mirror assembly, the incident light can produce a high-contrast light and dark mask pattern in the focal plane of the lens. Precise motion control is required to coordinate the imaging units and the larger substrate platform below them. In this case, the substrate moves along the X-axis, and the SLM units move along the Y-axis, similar to a printhead. Both platforms are supported by air bearings and driven by linear motors.
Figure 2: Spatial Light Modulator (SLM) Imaging Unit
Figure 3: Parallel lithography system with SLM imaging unit
A visual recognition system can be used to guide the movement of the imaging unit via reference marks on a substrate platform. Such systems can also be used with roll-to-roll flexible substrates.
In these manufacturing systems, position sensor feedback, in addition to providing data for linear motor commutation, also helps in precise position control. To achieve the alignment accuracy required by the FPD industry, i.e., <±2 micrometers, the encoder resolution must be significantly less than 1 µm. High-performance linear gratings and interferometric laser rulers are suitable for these applications, such as Renishaw's VIONiC gratings and RLE fiber laser ruler series.
Future high-throughput nano-etching technology
Modern photolithography involves scanning or stepping photomasks across the entire silicon wafer, with the long-term goal of achieving nanometer-scale resolution and high throughput at low cost. Maskless direct-write lithography eliminates the need for numerous expensive photomasks, which are precisely what limit the minimum achievable feature size of the latest microelectronic devices.
Near-field scanning lithography (NSOL) is particularly well-suited for these applications because it can overcome the Rayleigh diffraction limit of resolution. As shown in Figures 4 and 5, NSOL technology uses scanning probes with nanoscale apertures as "super-diffraction-limited" light sources on the mask, enabling direct writing of surface features within the optical near-field scale. The light emitted from these nanoscale apertures diverges significantly by up to tens of nanometers, therefore precise control of the gap between the mask and the substrate, maintaining it within tens of nanometers, is crucial for ensuring process performance.
Figure 4: Near-field scanning lithography equipment
Figure 5: NSOL mask with bow-shaped hole (bottom view)
An image can be directly constructed on the substrate by sequentially scanning each hole with a laser. Multi-axis piezoelectric platforms are used to position the substrate relative to the mask. The position encoder feedback for these platforms needs to be maintained within the sub-nanometer resolution range; therefore, laser interferometer-type systems are better suited for finer adjustments. Traditional high-performance gratings can be used for coarse adjustment of the commutation of the linear motor platforms.
The Importance of High-Precision Motion Platforms
The photomask motion platform is one of the core technologies of photolithography equipment. These advanced motion platforms use various types of motors, including voice coil motors (VCMs), to perform coarse (>100mm) and finer (<2mm) motion control. Motion command patterns are typically of the "acceleration-constant speed-deceleration" type. A typical mask platform usually has six degrees of freedom and uses multiple drive axes requiring high-precision position feedback. High-resolution, high-speed, and low-latency position encoders are crucial for dynamic platform positioning because they maximize bandwidth and minimize instability. Encoder selection is critical in these applications. Low encoder periodicity results in less interference with the input load of the servo loop, enabling finer speed control. Proper mounting using well-designed mounting tools (such as the Advanced Diagnostic Tool (ADTi-100) for VIONiC) further optimizes the overall performance of the encoder.
Summarize
Advanced grating technology can meet the stringent requirements of high precision, repeatability, and stability in photolithography processes. For certain feedback applications, machine designers should consider whether compact, advanced grating solutions can replace traditional interferometric laser ruler systems. Given the advancements in maskless lithography, multiple exposures using photomasks may one day no longer be necessary, but the future demands for measurement performance will certainly not decrease.
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