Introduction
The semiconductor industry stands at the forefront of technological innovation and drives advancements that power automation, consumer electronics, cutting-edge scientific research, and more. As the demand for smaller, faster, and more efficient electronic devices continuously increases, semiconductor manufacturing processes must also adapt. To address the precise alignment and handling requirements in semiconductor processing, nanopositioning equipment is becoming more frequently utilised.
In the context of semiconductor manufacturing, nanopositioning devices are integral to processes such as photolithography, wafer handling, and packaging, all of which rely on them to provide the precision necessary for fabricating devices on the nanometre scale. Improved precision can enhance the quality and performance of semiconductor devices as well as significantly improving production efficiency and yield.
This technical note will discuss where and how nanopositioning devices can be used within semiconductor manufacturing processes.
What is “Nanopositioning” Equipment?
Nanopositioning equipment encompass a range of devices that, as the name suggests, can repeatably resolve steps and hold position in one or more axes on the nanometre scale. The most ubiquitous nanopositioning devices derive from piezoelectric materials comprising Lead-Zirconate-Titanate (PZT) crystalline structures.
To turn a PZT crystal into a nanopositioning device, the inverse piezoelectric effect is utilised. This effect deforms crystalline materials by applying a potential difference across them. PZT material can then be built up into larger devices, where precision is achieved through complex mechanical design paired with sophisticated control systems and assembled with low-tolerance components.
The deformation caused in a PZT crystal is not completely linear with respect to the applied voltage. However, when paired with a position sensing device, the resultant change in position can be very linear and controlled via a PID control loop, with position resolution limited only by the resolution of the voltage supplied.
The types of piezo nanopositioning systems that we will consider in this article are:
Piezo Actuators & Flexure Amplified Stages
Piezo actuators consist of thin plates or discs of PZT material that are stacked together, with electrode layers in between each plate such that a single input voltage signal can expand them all synchronously. By stacking piezo discs together, actuators with travel ranges of between 1 to 200µm can be created. These actuators can be equipped with strain gauge sensors for use as positioning devices but can also be deployed dynamically as shakers for providing high frequency oscillations.
Flexure Amplified Piezo Stages
Alternatively, piezo actuators can be built into finely designed flexure arrangements to form single or multi-axis stages that can provide increased travel ranges of up to 800µm. A benefit of the flexure system is that in addition to amplifying the motion so that longer travel ranges can be reached, they can also guide the motion, optimising the straightness of motion to prevent undesired deviation in other axes. Also, due to their design, more advanced positioning sensors such as capacitive sensors can be used, allowing for sub-nanometre repeatability and a higher linearity.
Ultrasonic Piezo Motors
The inverse piezo effect can be taken one step further, and motors built using the same principle. By exciting a solid piezo element in a certain way at high frequencies, standing waves can be formed within the material itself. The resultant repeatable deformation of the piezo element can then be coupled to a driving rod to create motion over much longer travel ranges than a piezo flexure stage can obtain, while maintaining the same levels of precision of motion. Piezo motors can be used at the component level for integration into specific devices or can be built up into motion stages with small or large travel ranges, high speeds, and highly precise motion capabilities.
Other Technologies
Technologies such as voice coil and linear motor stages, as well as highly micro-stepped stepper motors can perform positioning tasks on the sub-micron level, with some even approaching motion and repeatability on the level of tens of nanometres (encoder dependant). These won’t be focussed on in this article, but can be viable alternatives in some situations.
Background Processes in Semiconductor Manufacturing
Semiconductor manufacturing processes have been developed over the years into a complex series of steps that turn a silicon ingot into many individual integrated circuits that are then used as the building blocks of modern electronic devices. These processes can be broadly assigned to two main categories, front-end and back-end processes.
Front-End Processes
Front-end processes focus on the production of silicon wafers, and the creation of the ICs onto these wafers. The stages involved are:
Growth & Wafer Cutting
Raw silicon material such as quartz is melted in a high temperature crucible and dopants are added in order achieve the desired material properties. The resulting mixture can be grown into single crystal cylindrical silicon ingots through the Czochralski method. Once an ingot is formed it is sliced into wafers of a flat disc shape, that are ready to then be further processed.
Wafer Handling
Once a silicon ingot has been cut into individual silicon wafers, these need to be handled and transported. Most important is moving a wafer into the correct position for the rest of its front-end processing.
Surface Preparation
The silicon wafer needs to be polished to a smooth and flat surface, and then chemically cleaned to remove any contaminants, ready for devices to be formed onto it.
Photolithography
A light sensitive layer is applied to the wafer and then ultraviolet light illuminates a mask to transfer the pattern on to the wafer surface for etching. Current state-of-the-art photolithography processes use deep UV light from excimer lasers at 193 or 248nm.
Etching
After a pattern is transferred onto the wafer, both wet and dry etching processes occur to remove the photo sensitive layer and transfer the pattern to the underlying silicon.
Thin Film Deposition
Usually a CVD (Chemical Vapor Deposition), or PVD (Physical Vapour Deposition) process is used to form or deposit a solid film on the wafer over the designated pattern. This is built up layer by layer, often incorporating other layers of doped or ionised silicon and finished with a protective inert layer.
Back-End Processes
Back-end processes involve the assembly, packaging, and testing of the ICs that are produced during the front-end processes. The stages involved are:
Wafer Inspection
Before any further processes can take place, the wafer needs to be inspected to find any potential irregularities in the final product. This can include looking at defects in the mask as well as on the wafer itself.
Wafer Probe & Test
After a physical examination, the wafers undergo electrical testing. Electrical probes are positioned on the device and send and measure signal responses to identify functional and non-functional chips.
Wafer Dicing
After working devices have been identified, the wafer needs to be sliced into individual chips. This process can involve physically cutting using diamond saws, or laser cutting when higher precision is required, when the chips are especially small.
Die Bonding
The die bonding process involves securing individual die (chips) to a substrate. This can use either soldering or an adhesive bond. Instead of die bonding, wire bonding can also be used.
Wire Bonding
An alternative to die bonding. Fine wires (typically gold) are used to connect bonding pads on the die leads on the substrate. The decision between die or wire bonding depends on many factors.
Encapsulation
Once bonded, the chips are encapsulated into a protective package, typically plastic or ceramic. This will protect the final device from the environment while still allowing for electrical connection.
Final Testing
The final ICs are visually inspected for defects, and some will undergo an electrical burn-in process to simulate long-term operation through elevated temperatures and voltages. Both processes will help to identify potential faults.
Piezo-Based Nanopositioning Devices in Semiconductor Applications
Now that we have explained what is meant by piezo-based nanopositioning equipment, and we know the step-by-step processes required to fabricate intricate semiconductor devices, we can go into more detail on how this motion equipment can be used. In all cases, the deployment of piezoelectric devices helps to improve the production process itself, by mitigating failed units and improving yield, and allowing for the realisation of smaller, faster, or more efficient devices.
Piezo Actuators
Due to intricacies in the inverse piezoelectric effect, piezo actuators by themselves can be used in multiple different ways. They can be used as precision positioning devices as mentioned above, offering travel ranges of between 1 and ~200µm depending on the actuator size. If constrained such that the crystal deformation cannot occur, they can also be used to generate and provide static force. Finally, when combined with suitable electronics, they can be driven at high frequencies, allowing for fast oscillation over sinusoidal or customised waveforms. Some of the less obvious uses for piezo devices within the semiconductor manufacturing process can take advantage of these properties.
The first of these takes place during the wafer preparation phase. After wafers have been sliced from the silicon ingot that is initially formed, the surfaces require polishing to ensure a smooth consistent facet for devices to be grown on. Piezoelectric actuators can be used as part of this controlled polishing process to apply a constant pressure onto the wafer surface. Doing so allows for a smoother final product with noticeably fewer defects and less instances of wafer warpage.
Another use for piezo actuators occurs during the CVD process, where very thin films of gaseous chemicals are passed over the wafer post-lithography in order to build up the electronic structures that will eventually form devices. Small piezo actuators can be used as part of actuating valves that are used during this process to ensure that the chemicals are deposited as thinly as possible over the wafer. This leads to a higher level of control over the growth of the structures and less waste as the chemical levels are more precisely controlled.
Finally, the dynamic properties of piezo actuators can be used in the wire bonding process. Once the fabricated ICs have been tested and diced into individual dies, these will need to be linked to external circuitry in order to turn them into fully working parts of the electronic devices that we all use. Fine wires of aluminium or gold must be bonded to connection points on the die in order to then be interfaced with external devices. Ultrasonic frequencies are required to ensure that these bonds are as small as possible while remaining durable enough for end use. Piezo actuators can be used to create these ultrasonic frequencies in a precise and repeatable manner.
The most common use of piezoelectric devices within the semiconductor manufacturing process is in the multiple inspection routines that occur. Piezoelectric positioning systems can be of particular use during these inspection processes as these solid-state systems are free from mechanically moving parts. This removes internal friction and sources of vibration, both of which are undesirable for highly intricate positioning tasks.
Piezoelectric objective lens positioners can be easily added onto microscopes as autofocussing devices for these inspection purposes. These utilise monolithic piezoelectric actuators alongside mechanically amplified flexure guides, with either strain gauge or capacitive sensors added for position feedback. Due to the fundamental principles of the inverse piezoelectric effect, the resolution of such systems can be sub-nanometre as it is limited purely by the noise of any input voltage signal, and the repeatability of the sensor designs is often in the single digit or low tens of nanometres. Moreover, the lack of any internal friction in this design allows for high duty cycle operation without mechanical wear and tear. When paired with suitably robust electronics, dynamic scans of up to 100Hz can be achieved over the full travel range.
The piezo device’s high resolution paired with fast dynamic scanning makes them ideal for inspection processes throughout the entire semiconductor manufacturing progression. Initially they can be used for inspecting and monitoring the wafer slicing process to ensure a consistent wafer thickness and to continuously monitor for warping defects. This is followed by defect inspection on the mask – looking for small discrepancies that would affect the pattern exchanged onto the wafer, and then also for seeking residual defects on the wafer itself after the lithography process. Because of the layer-by-layer build-up of devices onto the wafer, the scans need to be fast enough to catch any defects early, but also accurate enough to resolve infinitesimal inaccuracies in the structures.
Sometimes inspection processes are taken one step further and utilise electron beam microscopy to inspect particularly small (i.e. sub-nm) chip architectures. Multi-axis piezo wafer positioners can accompany this e-beam inspection, allowing the electron beam to reach parts of the die that optical methods fail to see.
Outside of inspection processes, the other biggest use for piezo actuators is in the photolithography process within large and intricate industrial fabrication machines. Such machines employ a large quantity of optics and alignment tools that can make use of the precision of piezo actuators and tip/tilt mechanisms. Multi-axis piezo positioning stages are routinely used within DUV lithography tools for aligning the 300mm silicon wafers onto wafer chucks with extreme precision. This requires high repeatability and sub-nm resolution of payloads up to 20kg in mass. As the chuck is used to position and orientate the wafer during the photolithography process, ensuring that this alignment is achieved with minimal deviation is crucial for minimising defective devices.
Assemblies Incorporating Piezo Motors
Ultrasonic piezo motors work by taking advantage of the deformation of monolithic piezo elements due to standing waves created by high frequency potential differences. When coupled with a motion axis, the advantages of this design mean that ultrasonic motors can provide a fast response with short settling times, can hold position without energy consumption and without a brake, exhibit no backlash, and can drive systems at constant speeds from nm/sec up to hundreds of mm/sec. What’s more, all of this can be achieved over theoretically unlimited travel ranges as the travel range limitation is purely down to the length of the guiding rod and other mechanical integration factors.
One of the most unique aspects of ultrasonic piezo motors is that they can act in two separate driving modes. The so-called AC mode generates standing waves continuously within the piezo elements to provide a high frequency from the piezo and allowing for high-speed operation of up to 500mm/sec with precision typically in the region of 10s to 100s of nm, defined by specifications of the encoder that is used. The DC mode is more precise. Instead of generating a standing wave, the piezo elements inside the motor are instead treated as singular piezo actuators and provide the unlimited resolution motion required to position down to the single nm level. When using suitable control electronics these two modes can be combined to provide the ultra-precise motion of a piezo actuator over many times the travel range and at high speeds.
As with piezo actuators and flexure stages, the predominant use of piezo motors within the semiconductor industry is for inspection and metrology tools within the manufacturing process.
As has been alluded to in this paper already, the photolithographic mask must be checked for small discrepancies and deviations in the intended pattern, as any unwanted artifacts in the photoresist image can then in turn transfer to the wafer itself. In order to prevent this, it is important to not only identify these features, but then correct for them to improve the fidelity of the transmitted image and further improve yield and quality in future manufacturing runs.
Customised multi-axis stages built up from piezo motors can be used in these optical wafer inspection techniques to run metrology sequences on wafers after the lithography process to determine key parameters such as the critical dimension (a comparison of the position of a feature on the wafer versus the position of the same feature on the mask). By measuring these parameters, it is possible to verify compliance to the specification, and to further optimise the processes should any variation be seen.
These processes typically have incredibly tight tolerances and require high-measurement accuracy while also measuring over tens to hundreds of mm depending on the specific feature identified. This process can be slow and once a position is found and recorded; stages need to be able to hold position with incredibly low drift in the order of 1nm over the process timescales. Other error sources in the form of external vibrations and temperature drift can also affect such measurements so it is important that these factors can be mitigated.
The distinct features that a piezo motor stage can offer are invaluable for making these measurements. The frictional coupling of a piezoelectric motor to the friction rod that it moves can be of the order of tens of Newtons and when held in such a way, sub-nanometre positional drift can be achieved. Furthermore, precision under temperature and vibration changes can be managed through sophisticated control algorithms in combination with the two motion modes (AC and DC).
Ultrasonic piezo motor stages can also be used in combination with electron beams, to translate and manipulate the wafer after epitaxial growth to identify the most miniscule aberrations. This process is performed under ultra-high vacuum conditions (≤1×10-9 mbar) and inspects the wafer under such a high precision that even regular process speeds make it a slow procedure. The high speeds of piezo motor stages are of relevance here, as is the fact that they can be used at a high duty cycle even in these ultra-high vacuum conditions – all while maintaining the precision required.
Importantly, these processes must also be kept free from any other contamination from the measurement instruments. Like piezo actuators and flexure stages, the only moving part in a motion stage built with ultrasonic piezo motors is the piezo itself. Operation of such devices is therefore clean and does not impart organic contaminants onto the wafers which is a vitally important feature as devices get smaller and smaller.
Summary
Semiconductor manufacturing processes are vital to ensure that current and future electronic devices can fill the needs of the digital world. The relentless pursuit of smaller and smaller devices results in a requirement for increasingly precise and sophisticated motion solutions. Pro-Lite Technology are proud to be able to work with partners who have been providing cutting edge precision motion solutions for the semiconductor industry for decades. This expertise can be a differentiating factor for manufacturing processes, not only for all of the current needs in semiconductor manufacturing, but to support all of the technological advancements to come.
Citations
Nanomotion: Clean, Stable 1nm Motion Systems: https://www.nanomotion.com/wp-content/uploads/Clean-Motion-Systems-for-1nm-Precision.pdf
Piezo-based, high dynamic range, wide bandwidth steering system for optical applications: https://www.nanomotion.com/wp-content/uploads/Image-Stabilization-Steering.pdf
Piezoelectric Actuators in the Semiconductor Manufacturing Industry: https://www.piezosystem.com/request-whitepaper/
Beyond Microscopy – MIPOS in Industry Applications: https://www.piezosystem.com/wp-content/uploads/2022/10/MIPOS-Piezo-Focus-Positioners-Whitepaper.pdf
