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Synthetic vascular grafts are used in a wide range of clinical applications. However, they can become prone to occlusion over time, due to growth of vascular smooth muscle cells (VSMC). This phenomenon is associated with graft failure. Here, we describe novel techniques for packaging and integration of a miniature sensor of VSMC growth. The sensor was based on a microfabricated array of interdigitated platinum electrodes on a silicon substrate. It was assembled using flip-chip-on-flex technology to create a low-profile package that can be manufactured using industry-standard tools and is suitable for integration with a synthetic vascular graft. The packaged sensor responded to changes in solution impedance, and accurately detected growth of VSMC in vitro. We also demonstrated successful proof-of-concept integration of the sensor with a custom synthetic graft. Together these technologies have the potential to allow early detection of VSMC growth prior to graft occlusion, enabling more timely and effective clinical interventions to be made.

We present a sub-GHz, low profile Electrically Small Antenna (ESA), designed for UHF RFID miniaturised battery free Implantable Wireless Medical Devices (IWMDs). The custom ESA is a linearly polarised dipole, and its topology leverages a meanderline structure to miniaturise its form factor. Furthermore, the ESA utilises a receded ground plane to improve its gain performance and achieve resonance, at the desired sub-GHz design frequency of 915 MHz. The ESA's dipole characteristics provide an added benefit of 180° bi-directional RF signal propagation. The ESA's design was optimised to integrate the footprint of a UHF RFID sensor chip (SL900A). By integrating the UHF RFID chip on the ESA, a complete wireless battery free sensory medical device, with an integrated antenna, can be realised. The antenna has a formfactor of 12.75× 12.25× 0.29 mm3. A prototype of the proposed ESA was fabricated and encapsulated in Polydimethylsiloxane (PDMS). Measurements of the prototyped ESA's input reflection coefficient (S11) and farfield gain values, at 915 MHz, were -26.44 dB and -18.88 dBi, respectively and demonstrated significantly better gain and efficiency performance, when compared to peer reviewed work. The ESA can be used as an antenna for various battery-free subcutaneous implants with a connected sensor (e.g., temperature) or actuator (e.g., neurostimulator).

This article details and compares the technology options for post-processing foundry produced CMOS at chip-scale to enable More than Moore functionality. In many cases there are attractions in using chip-based processing through the Multi-Project Wafer route that is frequently employed in research, early-stage development and low-volume production. This article identifies that spray-based photoresist deposition combined with optical maskless lithography demonstrates sufficient performance combined with low cost and operational convenience to offer an attractive alternative to conventional optical lithography, where spin-coated photoresist is exposed through a patterned photomask. [2020-0249]

A 128 $\times $ 128 single-photon avalanche diode (SPAD) motion detection-triggered time-of-flight (ToF) sensor is implemented in STMicroelectronics 40 nm CMOS SPAD foundry process. The sensor combines vision and ToF ranging functions to acquire depth frames only when inter-frame intensity changes are detected. The 40 $\mu \text{m}\,\,\times $ 20 $\mu \text{m}$ pixels integrate two 16-bit time-gated counters to acquire ToF histograms and repurpose them to compare two vision frames without the requirement for additional out-of-pixel frame memory resources. An embedded column-parallel ToF and vision processor performs on-chip vision frame comparison and binary frame output compression as well as controlling the time-resolved histogram sampling. The sensor achieves a maximum 32.5 kframes/s in vision modality and 500 frames/s in motion detection-triggered ToF over a measured 3.5 m distance with 1.5 cm accuracy. The vision function reduces the sensor power consumption by 70% over continuous ToF operation and allows the sensor to gate the ToF laser emitter to reduce the system power when no motion activity is observed.

With their capacity for real time monitoring and spatial mapping, implantable sensors are becoming an increasingly important aspect of next generation precision healthcare. Microfabricated sensor systems are a popular choice, owing to their capacity for miniaturisation, repeatable mass manufacture, and numerous pre-existing sensor archetypes. Despite the drive for development, packaging these sensors for the environment within the body, as well as the implantation process itself, presents a significant challenge. This paper presents microelectronic test structures, which can be used to assess, compare, and optimise implantable packaging solutions in a standardised manner. The proposed structures are used to investigate: (i) the capacity of the material to be patterned, (ii) the permeability of the insulation material, (iii) adhesion of the encapsulant to the die, and (iv) the physical robustness of the package to implantation through a needle. They are used to characterise an example packaging strategy, using biocompatible epoxy-resin. In addition, a method of optimising the packaging performance using the test structures is presented.

Robust and reliable micro-scale integrated electro- chemical sensors need a reference electrode that can provide a stable electrochemical potential. This can be achieved using a silver/silver chloride (Ag/AgCl) electrode, produced by the chemical chlorination of a thin patterned silver layer. Well understood and controlled processes are required to produce the Ag/AgCl electrode. This paper shows how previously reported test structures have been used to characterise and inform the fabrica- tion procedure. Wafer mapping of these structures was carried out using a Python controlled measurement system, consisting of a semi-automatic prober connected to an HP4062UX based analyser. The measurements were analysed to determine whether the chlorination process was affected by the test structure geometry. This was found to have no clear effect on chlorination.

The development of robust implantable sensors is important in the successful advancement of personalised medicine as they have the potential to provide in situ real-time data regarding the status of health and disease and the effectiveness of treatment. Tissue pH is a key physiological parameter and herein, we report the design, fabrication, functionalisation, encapsulation and protection of a miniaturised self-contained electrochemical pH sensor system and characterisation of sensor performance. Notably for the first time in this environment the pH sensor was based on a methylene blue redox reporter which showed remarkable robustness and accuracy and sensitivity. This was achieved by encapsulation of a self-assembled monolayer containing methylene blue entrapped within a Nafion layer. Another powerful feature was the incorporation, within the same implanted device, of a fabricated on-chip Ag/AgCl reference electrode – vital in any electrochemical sensor, but often ignored. When utilised in vivo, the sensor allowed accurate tracking of externally induced pH changes within a naturally occurring ovine lung cancer model, and correlated well with single point laboratory measurements made on extracted arterial blood, whilst enabling in vivo time-dependent measurements. The sensors functioned robustly whilst implanted, and maintained in vitro function once extracted and together, these results demonstrate proof-of-concept of the ability to sense real-time intratumoral tissue pH changes in vivo.

Recent advances in the fields of electronics and microfabrication techniques have led to the development of implantable medical devices for use within the field of precision medicine. Monitoring visceral surface tissue O2 tension ([Formula: see text]) by means of an implantable sensor is potentially useful in many clinical situations, including the perioperative management of patients undergoing intestinal resection and anastomosis. This concept could provide a means by which treatment could be tailored to individual patients. This study describes the in vivo validation of a novel, miniaturized electrochemical O2 sensor to provide real-time data on intestinal [Formula: see text]. A single O2 sensor was placed onto the serosal surface of the small intestine of anesthetized rats that were exposed to ischemic (superior mesenteric artery occlusion) and hypoxemic (alterations in inspired fractional O2 concentrations) insults. Control experiments demonstrated that the sensors can function and remain stable in an in vivo environment. Intestinal [Formula: see text] decreased following superior mesenteric artery occlusion and with reductions in inspired O2 concentrations. These results were reversible after reinstating blood flow or by increasing inspired O2 concentrations. We have successfully developed an anesthetized rat intestinal ischemic and hypoxic model for validation of a miniaturized O2 sensor to provide real-time measurement of intestinal [Formula: see text]. Our results support further validation of the sensors in physiological conditions using a large animal model to provide evidence of their use in clinical applications where monitoring visceral surface tissue O2 tension is important. NEW & NOTEWORTHY This is the first report of real-time continuous measurements of intestinal oxygen tension made using a microfabricated O2 sensor. Using a developed rodent model, we have validated this sensor's ability to accurately measure dynamic and reversible changes in intestinal oxygenation that occur through ischemic and hypoxemic insults. Continuous monitoring of local intestinal oxygenation could have value in the postoperative monitoring of patients having undergone intestinal surgery.

We present a compact, Ultra High Frequency Radio Frequency Identification (UHF RFID) Bio-sensing system, which operates in the 865 - 868 MHz band. The proposed system was designed, using custom and commercial off-the shelf (COTS) components. The main system blocks consist of: (1) A compact, low power UHF RFID reader, with an effective isotropic radiated power (EIRP) level of 10.82 mW (+10.34 dBm), (2) A 16×15×1 mm3 custom designed electrically small antenna (ESA), fabricated on an FR4 substrate, and (3) A battery free UHF RFID sensor tag, with an internal temperature sensor. Wireless connectivity and power transfer, between the UHF RFID reader ESA assembly and the battery free sensor tag, were achieved at a distance of 31 cm. The reader assembly successfully recorded sensor tag localised ambient temperature measurements. Sensor tag temperature measurements were performed using its internal temperature sensors and compared to a reference temperature sensor. Both sensor tag and reference sensor measurements were performed concurrently, in a temperature-controlled environment. The reader-ESA assembly has a mass of approximately 20 g, suitable for portable applications. Low EM emissions, from the proposed system, are at a safer level for operation in proximity to sensitivity electronic medical devices, such as pacemakers. Healthcare practitioners can use this system, to remotely monitor infants localised body temperature in neonatal or transitional care wards, when performing routine patient checks.

Nafion is a solid electrolyte polymer that can be used as a sensor membrane in microfabricated electrochemical oxygen sensors. It allows ions to be transported between the sensor electrodes and removes the need for a liquid electrolyte. Here we used a series of small square Nafion test structures, fabricated on a variety of materials using standard thin-film patterning techniques, to optimize the design and processing of Nafion membranes. Measurements showed that the choice of photoresist developer is critical. Use of diluted MF-26A developer provided the most effective and manufacturable process. The underlying material also had an influence on robustness, with silicon dioxide and platinum giving the longest membrane lifetime under simulated conditions of use. Membrane size had no clear effect on lifetime, and under optimal processing conditions there were minimal failures even under continuous mechanical agitation for up to six weeks. We also developed test electrodes covered by Nafion, and showed that they were effective at supporting electrochemical oxygen detection.

This paper describes the use of open-source electronic design automation tools, which have been developed and adapted for maskless lithography, to enable automation of the design, layout and measurement of parametric test structures. Two test structures are presented as case studies for characterisation of microelectronic and micro-electromechanical systems technologies using electrical and profilometry measurements respectively.

Die-level thinning, handling, and integration of singulated dies from multi-project wafers (MPW) are often used in research, early-stage development, and prototyping of flexible devices. There is a high demand for thin silicon devices for several applications, such as flexible electronics. To address this demand, we study a novel post-processing method on two silicon devices, an electrochemical impedance sensor, and Complementary Metal Oxide Semiconductor (CMOS) die. Both are drawn from an MPW batch, thinned at die-level after dicing and singulation down to 60 µm. The thinned dies were flip-chip bonded to flexible substrates and hermetically sealed by two techniques: thermosonic bonding of Au stud bumps and anisotropic conductive paste (ACP) bonding. The performance of the thinned dies was assessed via functional tests and compared to the original dies. Furthermore, the long-term reliability of the flip-chip bonded thinned sensors was demonstrated to be higher than the conventional wire-bonded sensors.

Robust and repeatable processes are required to fabricate reference electrodes for micro-scale integrated elec- trochemical sensors. One method for this is to produce a “silver/silver chloride” (Ag/AgCl) electrode through chemical chlorination of a thin film silver layer. This paper presents test structures, which can electrically characterise the process to aid process development and in-line control of the chlorination process.

This work presents a system for the in-line wafer- level characterisation of electrochemical sensors. Typically, such sensors are first diced and packaged before being electro- chemically tested. By integrating their characterisation into the manufacturing process, the production of electrochemical sensors becomes more efficient and less expensive as they can be parametrically tested midway through the fabrication process, without the need to package them. This enables malfunctioning or failed devices to be identified before dicing and reduces costs as only functional devices are packaged (in many cases this can be more expensive than the sensor fabrication). This study describes wafer-level characterisation of a simple electrochemical sensor design using a photoresist hydrophobic corralling film for the electrolyte and a probe station for contacting to individual dies.

Real‐time monitoring of tumor microenvironment parameters using an implanted biosensor could provide valuable information on the dynamic nature of a tumor's biology and its response to treatment. However, following implantation biosensors may lose functionality due to biofouling caused by the foreign body response (FBR). This study developed a novel tumor xenograft model to evaluate the potential of six biomaterials (silicon dioxide, silicon nitride, Parylene‐C, Nafion, biocompatible EPOTEK epoxy resin, and platinum) to trigger a FBR when implanted into a solid tumor. Biomaterials were chosen based on their use in the construction of a novel biosensor, designed to measure spatial and temporal changes in intra‐tumoral O2, and pH. None of the biomaterials had any detrimental effect on tumor growth or body weight of the murine host. Immunohistochemistry showed no significant changes in tumor necrosis, hypoxic cell number, proliferation, apoptosis, immune cell infiltration, or collagen deposition. The absence of biofouling supports the use of these materials in biosensors; future investigations in preclinical cancer models are required, with a view to eventual applications in humans. To our knowledge this is the first documented investigation of the effects of modern biomaterials, used in the production of implantable sensors, on tumor tissue after implantation. © 2018 The Authors. Journal of Biomedical Materials Research Part B: Applied Biomaterials published by Wiley Periodicals, Inc. J Biomed Mater Res Part B, 2018. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1620–1633, 2019.

Electrical impedance tomography (EIT) is an imaging technique that reconstructs the conductivity distribution of an inhomogeneous medium and is capable of monitoring physiological changes in biological materials. This paper focuses on comparison of state-of-the-art regularisation methods in solving the image reconstruction problem in micro-scale EIT for biomedical applications. Since the quality of image reproduction is weak for micro-scale phantoms, it is vital to study the image reconstruction algorithm. Hence, we present three regularisation methods in this paper — Tikhonov, Gaussian-Laplace and L1 — for the image reconstruction of 700 pm diameter test samples. We verified our method using 500 μm × 250 μm rectangular Pt electrodes and compared the performance of these regularisation methods. The results suggest that Gaussian-Laplace regularisation provides better image reconstruction than L1 and Tikhonov algorithms.

This paper demonstrates a simple and low cost technology to reliably and accurately package integrated chips. Microchannels and cavities of minimum feature size of 500 μm can be reliably reproduced. In addition, the curing depth in relation to the exposure time was investigated. A simple microfluidic device, consisting of a 500 μm channel and 2 mm ports, was manufactured to demonstrate the possibilities of this technology. Extensive electrochemical experiments showed that the packaging material is a good insulator and leaves no residue on the chip.

Test structures were produced for optimizing the design and fabrication of a patterned solid polymer electrolyte in an electrochemical oxygen sensor. Measurements showed that choice of photoresist developer and the underlying insulator material affected durability of the polymer structures. Test electrodes covered by the polymer were effective at supporting electrochemical oxygen detection.

This paper presents three test structures targeted at characterising sensor packaging materials for liquid environments. The test structures enable the evaluation of: 1) the successful removal of packaging material on sensing areas, 2) the permeability of the packaging material to its environment, 3) electrical continuity through the packaging process, and 4) the ingress of the liquid environment between the packaging material and the chip surface. The paper presents an example of the evaluation of a UV curable resin as packaging process for a biomedical sensor.

Resistive electrical test structures have been designed to enable the characterization of optical proximity correction (OPC) applied to a right-angled corner in a conducting track. The OPC consists of square serifs that are either added to the outside corner or subtracted from the inner corner. Varying degrees of OPC can be applied by changing the size of the square serif or by changing the amount by which it encroaches on or protrudes from the corner. A prototype test mask has been fabricated that contains test structures suitable for on-mask electrical measurement. The same mask was used to print the test pattern in polysilicon and aluminium using an i-line lithography tool and results from these structures clearly show that OPC has an impact on the resistance of the final printed features. In particular, the level of corner rounding is dependent upon the dimensions of the serifs employed and the measured resistance can be used to characterize the effects of different levels of OPC applied to the inner corners.

This paper compares electrical, optical, and atomic force microscope (AFM) measurements of critical dimension (CD) made on a chrome on quartz photomask. Test structures suitable for direct, on-mask electrical probing have been measured using the above three techniques. These include cross-bridge linewidth structures and pairs of Kelvin bridge resistors designed to investigate dimensional mismatch. Overall, the results show very good agreement between the electrical measurements and those made with a calibrated CD-AFM system, while the optical metrology system overestimates the measured width. The uncertainty in each of the measurements has been considered, and for the first time an attempt has been made to describe the levels and sources of uncertainty in the electrical measurement of CD on advanced binary photomasks.

This paper presents a comparison of optical and electrical techniques for critical dimension (CD) metrology on binary and alternating aperture phase-shifting masks. Measurements obtained from on-mask electrical CD structures are compared with optical measurements made using a deep ultraviolet mask metrology system. The results show that the presence of alternating phase-shifting trenches between the chrome blocking features has a detrimental effect on the optical measurements and that this effect strongly depends on the depth of the trenches. In addition, the optical metrology system appears to have problems with the measurement of the narrowest isolated features due to calibration related issues. Electrical CD measurements are seen as a way of probing the limits of optical tool calibration and for highlighting and managing the need to extend the complexity of the calibration schedule.

We investigate the sensitivity of marine cloud brightening to the properties of the added salt particle distribution using a cloud parcel model, with an aim to address the question of, ‘what is the most efficient particle size distribution that will produce a desired cooling effect?’ We examine the effect that altering the aerosol particle size distribution has on the activation and growth of drops, i.e. the Twomey effect alone, and do not consider macrophysical cloud responses that may enhance or mitigate the Twomey effect. For all four spray generation methods considered, Rayleigh jet ; Taylor cone jet ; supercritical fluid ; and effervescent spray , salt particles within the median dry diameter range D m =30–100 nm are the most effective range of sizes. The Rayleigh jet method is also the most energy efficient overall. We also find that care needs to be taken when using droplet activation parametrizations: for the concentrations considered, Aitken particles do not result in a decrease in the total albedo, as was found in a recent study, and such findings are likely to be a result of the parametrizations' inability to simulate the effect of swollen aerosol particles. Our findings suggest that interstitial aerosol particles play a role in controlling the albedo rather than just the activated cloud drops, which is an effect that the parametrization methods do not consider.

It may be possible to reduce global warming by increasing the reflectivity of marine stratocumulus clouds thereby reducing the amount of solar energy that is absorbed. Quite a small change to the reflectivity could stop further temperature rise or even produce a reversion towards pre-industrial values. This paper gives a brief account of the physics behind the Twomey effect and its application for marine cloud brightening by the release of sub-micron drops of sea water into the marine boundary layer using a fleet of mobile spray vessels. We argue that the mobility of spray vessels and the short life of spray are advantageous by allowing rapid tactical control in response to local conditions. We identify the main engineering problem as spray production, which in turn requires ultra-filtration of plankton-rich seawater. The proposed engineering solutions involving Rayleigh nozzles etched in silicon and piezo-electric excitation are illustrated with drawings. The results of a COMSOL Multiphysics simulation of drop generation are given, with nozzle diameter, drive pressure, excitation frequency and power requirement as functions of drop diameter. The predicted power requirement is higher than initially hoped for and this has led to a modified vessel design with active hydrofoils giving much lower drag than displacement hulls and turbines. The active control of hydrofoil pitch angle can be used for power generation, roll stabilizing and may also reduce hull loading similarly to the suspension systems of road vehicles. The need to identify unwanted side effects of marine cloud brightening has led to a method for using climate models to give an everywhere-to-everywhere transfer function of the effects of spray in each region on weather records at all observing stations. The technique uses individual coded modulation of the concentration of cloud-condensation nuclei separately in each of many spray regions and is based on methods used for small-signal detection in electronic systems. The first use in a climate model shows very accurate measurement of changes to a temperature record and that that marine cloud brightening can affect precipitation in both directions. Replication with other climate models will be necessary. The paper ends with tentative estimates for the cost of mass production spray vessels based on actual quotations for parts of the spray generation hardware and on the cost of Flower-class corvettes used by the Royal Navy in World War II which were built in similar numbers.

Today's Optical Proximity Correction (OPC) is becoming increasingly complex and necessitates that we use smaller and smaller grid sizes to produce the fine patterns required. These small grids lead to significant overhead in data handling and, more importantly, for the tools that will write and inspect the mask, together making the mask extremely expensive. For two dimensional structures, such as corners, we have very complex structures using either additive or subtractive OPC features to produce the desired shape. However, it is unclear whether these structures need to be so perfect for the electrical task they are intended to perform. In previous work we have created a number of corner type electrical test structures and applied varying degrees of OPC to both the outer and inner corners of the structures, then printed these on doped polysilicon and the electrical effect of the OPC was investigated. This work showed that the electrical effect of OPC on the outer corner was minimal, whereas the inner corner shape had a marked influence upon the electrical resistance of the circuit feature. However, technology continues to move forward and polysilicon gates are being replaced by metal gates for 32nm node. Therefore, in this work we replace the polysilicon with a metal and investigate the size and position of OPC applied to both the outer and inner corners of the structures. The data obtained using the metal structures suggests that as was the case when using polysilicon, OPC on the outside corner has little impact upon a simple circuit's performance, while care should be taken with OPC on the inner corners, particularly with regard to the size of the OPC serifs used.

Electrical test structures for the characterisation of Optical Proximity Correction (OPC) have been fabricated in thin aluminium using i-line lithography and reactive ion etching. Initial electrical measurements are presented which show an increase in the resistance of a right angled section of Al track as the level of OPC on the inside corner is increased. Structures with OPC applied to the outer corner do not show the same change in resistance. SEM images of similar Al test structures clearly show the effects of applying OPC and suggest that inner corner serif OPC leads to a narrowing of the conducting track.

Today's Optical Proximity Correction (OPC) is becoming increasingly complex and necessitates using smaller and smaller grid sizes to produce the fine patterns required. These small grids lead to very high overhead in data handling, as well as for the tools that will write and inspect the mask; which together make masks extremely expensive. For two dimensional structures such as corners, we use complex structures incorporating either additive or subtractive OPC features to produce the desired shape. It is unclear though, how precisely the final structures must match the original design to perform their intended electrical functions. In this work we have created a number of corner type electrical test structures and applied different degrees of OPC to both the outer and inner corners of the structures. These features were then printed on doped polysilicon wafers, and the wafers were etched and electrically tested. The electrical effect of OPC on the outer corner was found to be minimal, whereas the inner corner shape had a significant effect upon the electrical resistance of the circuit feature. The data suggests that OPC on the outside corner has little impact upon a simple circuit's performance, but care should be taken with OPC on the inner corners, particularly with regard to the size of the OPC serifs used.

The effects of the GHOST proximity correction process on chrome-on-quartz photomasks can prove difficult to quantify and so they are not routinely characterised. This paper presents a methodology for addressing this issue using electrical test structures designed to measure dimensional mismatch. In the past these have been used successfully to characterise standard GHOSTed photomasks, which displayed systematic offsets that were not seen on an unGHOSTed mask using the same design. In order to investigate this further, a second mask was fabricated using a variation of the GHOST process which increased the resolution of the secondary exposure to be the same as the primary pattern. This enabled the source of the previously observed systematic offset to be determined as test structures on the new mask did not show the same overall dimensional bias. However, the range of mismatch in some of the structures was increased as a result of the new process.

Electrical test structures have been designed to enable the characterisation of corner serif forms of optical proximity correction. These structures measure the resistance of a conducting track with a right angled corner. Varying amounts of OPC have been applied to the outer and inner corners of the feature and the effect on the resistance of the track investigated. A prototype test mask has been fabricated which contains test structures suitable for on-mask electrical measurement. The same mask was used to print the structures using an i-line lithography tool for on-wafer characterisation. Results from the structures at wafer level have shown that OPC has an impact on the final printed features. In particular the level of corner rounding is dependent upon the dimensions of the OPC features employed and the measured resistance can be used to help quantify the level of aggressiveness of the inner corner serifs.

This paper presents results from the use of electrical measurements to investigate dimensional mismatch in an advanced photomask process. Test structures consisting of matched pairs of Kelvin resistors have been measured and the results analysed to obtain information about the capability of the mask making process. The mask plate used in this work has an array of over 350 sets of mismatch test structures, providing an unprecedented volume of data. A second mask, which was underexposed, has allowed the relationship between mismatch and the location of nearby mask writing pattern boundaries to be investigated for the first time.

This paper reports the measurement results from a set of electrical, on-mask test structures based on industry standard test feature layouts normally used to investigate process proximity effects and improve optical proximity correction (OPC) models. The electrical test structures were fabricated on a binary photomask using the GHOST proximity correction technique to compensate for typical e-beam induced proximity errors. This is one of the first times that electrical test structures have been used to evaluate GHOST exposure. The test structures were measured electrically and optically with a dedicated photomask metrology tool and the results from the two techniques are presented.

This paper compares electrical, optical and AFM measurements of critical dimension (CD) made on a chrome on quartz photomask. Test structures suitable for direct, on-mask electrical probing have been measured using the three techniques and the results show very good agreement between the electrical measurements and those made with a calibrated CD-AFM system.

The National Institute of Standards and Technology (NIST) has a multifaceted program in atomic force microscope (AFM) dimensional metrology. Three major instruments are being used for traceable measurements. The first is a custom in-house metrology AFM, called the calibrated AFM (C-AFM), the second is the first generation of commercially available critical dimension AFM (CD-AFM), and the third is a current generation CD-AFM at SEMATECH - for which NIST has established the calibration and uncertainties. All of these instruments have useful applications in photomask metrology. Linewidth reference metrology is an important application of CD-AFM. We have performed a preliminary comparison of linewidths measured by CD-AFM and by electrical resistance metrology on a binary mask. For the ten selected test structures with on-mask linewidths between 350 nm and 600 nm, most of the observed differences were less than 5 nm, and all of them were less than 10 nm. The offsets were often within the estimated uncertainties of the AFM measurements, without accounting for the effect of linewidth roughness or the uncertainties of electrical measurements. The most recent release of the NIST photomask standard - which is Standard Reference Material (SRM) 2059 - was also supported by CD-AFM reference measurements. We review the recent advances in AFM linewidth metrology that will reduce the uncertainty of AFM measurements on this and future generations of the NIST photomask standard. The NIST C-AFM has displacement metrology for all three axes traceable to the 633 nm wavelength of the iodine-stabilized He-Ne laser. One of the important applications of the C-AFM is step height metrology, which has some relevance to phase shift calibration. In the current generation of the system, the approximate level of relative standard uncertainty for step height measurements at the 100 nm scale is 0.1 %. We discuss the monitor history of a 290 nm step height, originally measured on the C-AFM with a 1.9 nm (k = 2) expanded uncertainty, and describe advances that bring the step height uncertainty of recent measurements to an estimated 0.6 nm (k = 2). Based on this work, we expect to be able to reduce the topographic component of phase uncertainty in alternating aperture phase shift masks (AAPSM) by a factor of three compared to current calibrations based on earlier generation step height references.

This paper describes the design and measurement of electrically measured test structures for the characterisation of dimensional mismatch in an advanced photomask making process. Test structures consisting of pairs of Kelvin connected bridge resistors have been fabricated on a chrome-on-quartz photomask plate. These have been electrically measured on-mask and the results used to obtain information about dimensional mismatch in the mask making process.

The standard approach to generate the data required for automated proximity correction is to measure a set of patterned features using an optical tool or a critical dimension scanning electron microscope (CD-SEM). This paper describes the design of a set of on-mask electrical test structures to perform the same task which has a number of attractions. The electrical test structures are based on the Kelvin bridge resistor to measure the widths of isolated and densely packed lines and spaces. The results from these measurements can be used to extract information about proximity effects in the mask making process and to generate rules or models for the correction of mask designs. Electrical results from a test mask, fabricated without any correction for e-beam proximity effects, are presented and compared with optical measurements of the same structures made with an industry standard mask metrology tool.

Simple electrical test structures have been designed that will allow the characterisation of corner serif forms of optical proximity correction. The structures measure the resistance of a short length of conducting track with a right angled corner. Varying amounts of OPC can be applied to the outer and inner corners of the feature and the effect on the resistance of the track measured. These structures have been simulated and the results are presented in this paper. In addition a preliminary test mask has been fabricated which has test structures suitable for on-mask electrical measurement. Measurement results from these structures are also presented. Furthermore structures have been characterised using an optical microscope, a dedicated optical mask metrology system, an AFM scanner and finally a FIB system. In the future the test mask will be used to print the structures using a step and scan lithography tool so that they can be measured on-wafer. Correlation of the mask and wafer results will provide a great deal of information about the e ects of OPC at the CAD level and the impact on the final printed features.