Andreas Tsiamis

Development of Eectrilcal On-Mask CD Test Structures Based on Optical Metrology Features

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.

Electrical Measurement of On-Mask Mismatch Resistor Structures

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.

Comparison of optical and electrical measurement techniques for CD metrology on alternating aperture phase-shifting masks

This paper presents a comparison of optical and electrical techniques for critical dimension (CD) metrology on binary and alternating aperture phase shifting masks. For the first time, measurements obtained from on-mask electrical CD structures are compared with optical measurements made using a deep ultra-violet (DUV) mask metrology system. Initial results show that the presence of alternating phase shifting trenches between the chrome blocking features has a detrimental effect on the optical measurements. 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.

Comparison of Measurement Techniques for Linewidth Metrology on Advanced Photomasks

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.

Electrical test structures for the characterisation of optical proximity correction

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.

Investigation of electrical and optical CD measurement techniques for the characterisation of on-mask GHOST proximity corrected features

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.

Comparison of Optical and Electrical Techniques for Dimensional Metrology on Alternating Aperture Phase-Shifting Masks

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.

Electrical Test Structures for the Characterization of Optical Proximity Correction

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.

Electrical Test Structures for Investigating the Effects of Optical Proximity Correction

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.

Comparison of measurement techniques for advanced photomask metrology

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.