Semiconductor Lithography

Lithography is defined as "a method of printing from a flat surface (such as a smooth stone or a metal plate) that has been prepared so that the ink will only stick to the design that will be printed". In semiconductor device manufacturing, the stone is the silicon wafer while the ink is the combined effect of the deposition, lithography and etch processes that create the desired feature. Since lithography for device fabrication involves the use of optical exposure to create the pattern, semiconductor lithography is commonly called "photolithography".  As with the inspection and metrology techniques already discussed, photolithography is the technique of choice for patterning because it is optical, and thus enables small features and high wafer throughput. This contrasts with other techniques such as direct writing and imprint.

Figure 1 illustrates a typical photolithography process used to define shallow trench isolation features. Such a process consists of the following steps:

1. Substrate cleaning and preparation
2. Form layers of thermal oxide and deposit a layer of silicon nitride on the clean substrate
3. Deposit a carbon hard mask followed by a layer of anti-reflective material
4. Deposit a layer of photoresist
5. Pre-bake the photoresist
6. Align the substrate/photoresist and reticle and expose the photoresist using UV radiation and 4x-5x imaging. Step and scan to repeat
7. Post exposure bake
8. Develop the pattern in the photoresist and hard bake to remove remaining solvent
9. Perform etch to open dielectric anti-reflective coating (DARC) and hard mask pattern and remove photoresist and DARC
10. Perform etch to open trenches in substrate and remove hard mask
11. Clean surface

DUV technology for photolithography is exclusively based on projection optics since the pattern on the photomask is much larger than the final pattern developed on the photoresist. The optical system in a 193 nm photolithography tool is known as a catadioptric system. The term means that it uses both lens (refractive) and mirror (reflective) elements for directing and conditioning the light from the laser. Please see Deep UV Photolithography for additional information.

EUV lithography is being developed to fulfill single-exposure patterning requirements at feature sizes below 22 nm (Figure 3). Unique to this technology is the nature of the light source. Please see Extreme UV Photolithography for additional information.

Photo-molecular contamination is a significant issue for reliability and lifetime in semiconductor photolithography systems. Consequently, a great deal of care must be taken to prevent the exposure of the optics in these systems to adhesives, lubricants, and any other organic carbon as well as to siloxanes, phosphonates, or sulfates. The organics are absorptive in the DUV range and can react to form a variety of damaging contaminants after adsorption on the surface of optics and subsequent DUV illumination. Trace contamination by volatile organic compounds, condensable Si-O and inorganic compounds can lead to loss of light either in transmission or reflection and can cause wavefront distortion, significantly affecting performance and causing unplanned downtime. It can cause Strehl reduction (or optical imaging quality reduction), polarization changes and even de-tuning, i.e., shifts in the optical wavelength, to negatively affect the performance of a photolithography system. The scope of photo-molecular contamination covers DUV wavelengths (190 - 355 nm) and EUV wavelengths (sub-190 nm, typically 13.5 nm). Photo-molecular contamination mechanisms are complex and are highly dependent on power levels as well as type and concentration of chemical compounds. There are multiple aspects to consider and specific solutions must be understood and proven in specific applications. As wavelengths continue to shorten and power increases, prevention of this form of chemical contamination becomes increasingly important.

Optics fabricators have methodologies for eliminating or limiting the effects of photo-molecular contamination that can help achieve high performance, long lifetime optics and systems. These include proprietary optical materials and compounds, proprietary polishing, cleaning and coating processes, and cleanroom handling equipment and processes. This section describes the capabilities that set DUV optics apart from standard catalog optics, including:

• Extensive R&D in materials science
• Reliability and lifetime testing
• Initial design of the system
• System design for long life and minimal preventative maintenance cycles
• Sub-tier supply chain management and control
• Internal cleanroom environments and production control
• Preservation of cleanliness and packaging

Many optical materials have low transmission below 200 nm, and so UV fused silica or calcium fluoride (CaF₂) are favored for DUV transmissive optical substrates. Figure 4 shows the typical transmission for these materials, which extends below 200 nm and then steeply drops off. However, CaF₂ optics are subject to defects and slip-planes if not coated using specially optimized processes. Extensive research and testing have gone into the selection of polishing compounds and processes that are compatible with DUV wavelengths. Some polishing materials/compounds will absorb UV/DUV light and this can affect reliability and lifetime of the optical component. Others may contain chemical compounds that directly react with DUV light to cause damage and failures.

EUV places strict requirements on specifications for optical assemblies and vacuum control. EUV light sources require hard vacuum because all gases absorb 13.5 nm light. Process control equipment like MKS flow controllers, valves, and pressure gauges are used.

MicroPirani™/Piezo gauges contain two gauges in one package: a MEMS MicroPirani sensor and a Piezo sensor. They are designed for load locks, measuring pressures that range from atmospheric pressure down to medium vacuum (1000 to 1x10^-5 Torr). This wide range allows the gauge to be used in vacuum chamber applications requiring absolute vacuum/pressure switching capabilities.

Baratron capacitance manometers provide accurate, repeatable measurements of pressure which are stable over long periods of time. They are an ideal choice for measuring pressure and vacuum in many semiconductor and critical thin film applications, including photolithography for semiconductor device manufacturing and optical coating. Baratron manometers are constructed from corrosion resistant materials and are insensitive to the types of aggressive process gases, such as halogens, typically used in semiconductor etch processes. Baratron manometers for typical semiconductor applications have full scale pressure ranges from 20 mTorr up to 1000 Torr.

DUV lithography requires high velocity motion stages for wafer handling that have high accuracy and stability and fast step-and-settling times. Overlay (the relative position of one patterning layer to another), CD size, and throughput drive these requirements in the reticle and wafer stages, with typical overlay tolerances of 15% of the CD in 193 nm technologies. Throughput requirements (up to 200 wafers/hour) limit the maximum processing time to less than 20 s per wafer. This means that relatively high velocities and accelerations occur in the reticle and wafer translation operations. Motion control systems in these lithography tools must be able to achieve these velocities and accelerations with no impact on the vibration levels of the reticle or wafer, since this would impact the achievable CDs. Rapid step-and-settling requires active vibration isolation to minimize oscillation of the optics column and subsequent delay in illumination.

In addition to the higher velocities, throughput is also maximized by increasing the die size so that fewer dies are processed per wafer. However, this approach increases the requirements for positioning accuracy. Lithography applications require motion stage calibration to ensure repeatability in the positioning of many different stages in the fab.

MKS offers a series of high-performance air bearing stage solutions suitable for use in semiconductor photolithography applications and customized tools for automated manufacturing and process control. These extremely rigid structures can accommodate wafers with diameters up to 300 mm. Very high accelerations (up to 5G in some models) and velocities (400 mm/sec to 1000 mm/sec) are achievable while simultaneously retaining high repeatability (25 - 50 nm) and accuracy (0.2 - 0.4 .m). MKS performs high-accuracy calibrations on all stages that achieve operational stability and high positioning accuracy and enable the same level of accuracy from tool to tool.

The MKS Z Tip Tilt and theta stage provides high accuracy positioning for large dies and features and an autofocus capability to dynamically maintain the Z position of the wafer, enhancing the throughput of lithography systems. For example, the DynamYX GT stage provides six degrees of freedom for wafer positioning (XYZ, tip, tilt, theta) with XY position accuracy of ± 150 nm over 300 mm. It has flatness of ± 200 µm over this range and step settle for 25 mm (± 50 nm) within 200 ms. The DynamYX DATUM GT stage supports 600 mm panel sizes for panel processing, increasing process throughput in these large-scale applications.