Currently semiconductor industry drives the 193nm lithography to its limits, using techniques like double exposure,
double patterning, mask-source optimisation and inverse lithography. These requirements trend to full in-die
measurement capability of photomask metrology for registration. Especially, overlay becomes more and more critical
and must be ensured on every die. For this, Carl Zeiss SMS has developed the next generation photomask registration
and overlay metrology tool PROVE which is already well established in the market. To ensure in-die measurement
capability, sophisticated image analysis methods based on 2D correlations have been developed.
A key component for registration tool users is the cross site manufacturing flexibility given by the matching capability of
all its metrology tools. Therefore all PROVE tools offer a tool matching procedure based on 2D Golden Grid
references. In this paper we first review the optimal length standard and golden grid matching procedures of modern
registration metrology tools. Systematic errors in fleet matching based on illumination differences, thermal expansionbased
issues or line width roughness are addressed. The tool matching performance of PROVE tools is demonstrated by
comparing up to 7 different tools. All tools are well within accuracy and long-term repeatability specification which
considerably reduces the statistical error contribution of the tool matching performance. For grid matched tools the final
cross tool registration error is shown to be below 1nm.
The image placement is and remains an important aspect of photomask metrology. Not only the position accuracy of
features for an individual mask - representing one layer in a complete chip design have to meet stringent requirements,
the complete mask set for all layers have to match in order to get a functional device. At a time were registration and
overlay errors were counted in micrometer it was enough to compare one mask with another by a so called overlay
machine. This approach works sufficiently until placement specification reached the "nanometre range" and the
development of dedicated 2D coordinate measurement systems became necessary. Since then, pattern placement
metrology tools became "enabler" for the continuous improvement of pattern placement accuracy on photomask and the
improvement of the final wafer overlay error. This paper reviews and discuss current trends of pattern placement
metrology on photomasks, highlighting the major error drivers and will focus on current and future requirements for in -
die registration.
Following the international technology roadmap for semiconductors
the image placement precision for the 65nm technology node has to be 7nm. In order to be measurement capable, the measurement error of a 2D coordinate measurement system has to be close to 2nm. For those products, we are using the latest Vistec registration metrology tool, the LMS IPRO3. In this publication we focus on the tool performance analysis and compare different methodologies. Beside the well-established ones, we are demonstrating the statistical method of the analysis of variance (ANOVA) as a powerful tool to quantify different measurement error contributors. Here we deal with short-term, long-term, orientation-dependent and tool matching errors.
For comparison reasons we also present some results based on LMS IPRO2 and LMS IPRO1 measurements. Whereas the short-term repeatability and long-term reproducibility are more or less given by the tool set up and physical facts, the orientation dependant part is a result of a software correction algorithm.
We finally analyse that kind of residual tool systematics and test some improvement strategies.
For next generation photo mask lithography the tolerance range for pattern placement and critical dimensions (CD) is further shrinking. Improved optical resolution and precision of a metrology system are required to qualify the lithography tool and monitor the photo mask process. Edge detection methods in transmitted light mode for pattern placement and CD measurements are advantageous if the tightened resolution and precision requirements can be met. The new LMS IPRO2 using an illumination wavelength range of 360 to 410 nm has a significantly enhanced resolution for registration and CD measurements in both, transmitted and reflected light. A new laser interferometer with an enhanced resolution of 0.3 nm contributes to the overall improved system performance. The stage is designed to measure on quartz substrates and next generation lithography (NGL) reticles up to 230 mm square in transmitted light as well as in reflected light on 200 mm and 300 mm wafers for stepper qualification.
To keep pace with continuously shrinking design rules for masks and reticles Leica Microsystems has developed two new mask metrology tools. The LWM 250 DUV is designed to measure Critical Dimensions (CD) on mask in transmitted light at 248nm illumination. The LMS IPR02 is designed to measure pattern placement and CDs in transmitted light at I- line (365nm) illumination. System overview and first performance data are presented for both tools. The step to a shorter illumination wavelength leads to a better optical resolution power resulting in an improved edge detection and CD linearity compared to systems using white light or I-line illumination for imaging.
A new mask and wafer metrology tool is under final development at Leica to be launched in 1996. Recent performance data on the LMS IPRO system at Leica's development center shows precision and accuracy performance required for the 0.18 micrometers design rule device generation.
A new mask and wafer metrology tool is under final development at Leica and will be launched in 1996. The new technical concept is discussed. Recent performance data on the LMS IPRO system at Leica's development center shows precision and accuracy data required for the 0.18 micrometer design rule device generation.
Metrology becomes more and more a key function in mask making and development of new technologies. Due to the Sematech strategy a precision performance of less than 9 nm (3(sigma) ) will be mandatory for the pattern placement and CD metrology tools for masks of the 0.25 micrometers device generation. Performance data below 9 nm demonstrate the capability of today's metrology systems for this application. On phase shift masks (PSM) the pattern placement metrology tool should be able to measure the positions of the structures of both layers, the phase shifter and the chromium. Measurement data obtained with the LMS 2020 on embedded attenuated PSMs as well as results on Levenson type PSMs demonstrate the excellent applicability of optical metrology systems in this field. Cost of ownership (COO) of the metrology tool is another important issue to be reviewed. Currently more and more purchasing decisions among competing tools of similar performance are based on the COO comparison.
Kaizen--a continuously improving--is a philosophy lived in Japan which is also becoming more and more important in Western companies. To implement this philosophy in the semiconductor industry, a high performance metrology tool is essential to determine the status of production quality periodically. An important prerequisite for statistical process control is the high stability of the metrology tool over several months or years; the tool-induced shift should be as small as possible. The pattern placement metrology tool Leitz LMS 2000 has been used in a major European mask house for several years now to qualify masks within the tightest specifications and to monitor the MEBES III and its cassettes. The mask shop's internal specification for the long term repeatability of the pattern placement metrology tool is 19 nm instead of 42 nm as specified by the supplier of the tool. Then the process capability of the LMS 2000 over 18 months is represented by an average cpk value of 2.8 for orthogonality, 5.2 for x-scaling, and 3.0 for y-scaling. The process capability of the MEBES III and its cassettes was improved in the past years. For instance, 100% of the masks produced with a process tolerance of +/- 200 nm are now within this limit.
In the evaluation of new manufacturing processes, metrology is a key function, beginning with the first step of process development through the final step of everyday mass production at the fabrication floor level. RIM-type phase shift masks are expected to be the first application of phase shift masks in high volume production, since they provide improved lithography process capability at the expense of only moderate complexity in their manufacturing. Measurements of critical dimension (CD) and pattern position (overlay) on experimental rim-type and chromeless phase shift masks are reported. Pattern placement (registration) was measured using the Leitz LMS 2000. The overall design and important components were already described. The pattern placement of the RIM type phase shift structures on the photomask described above was determined within a tolerance of 25 nm (3s); nominal accuracy was within 45 nm (3s). On the chromeless phase shift mask the measurement results were easily obtained using a wafer intensity algorithm available with the system. The measurement uncertainties were less than 25 nm and 50 nm for precision and nominal accuracy respectively. The measurement results from the Leitz CD 200 using transmitted light were: a CD- distribution of 135 nm (3s) on a typical 6 micrometers structure all over the mask; the 0.9 micrometers RIM structure had a distribution of 43 nm (3s). Typical long term precision performance values for the CD 200 on both chrome and phase shift structures have been less than 15 nm.
In any process that generates or measures pattern-placement (overlay), these parameters need to be
regarded at least as two-dimensional. We show this on our procedure bringing a mask repeater under statistical
process control SPC).
In order to increase the accuracy of the overlay measurement process itself, plate bending has to be
included as a third dimension. By taking the third dimension into account, the LMS 2000 Metrology System
significantly reduces the maximum uncertainity of measurement results.
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