This paper studies the impact of shape and local environment (pattern layout) on the ability to detect defects on the
reticle and the extent to which they affect the dimension of the printed image on the wafer. The authors have made
extensive use of design information to perform a thorough evaluation. OPC software was used to generate mask data that
was comparable to product mask data. Defects were placed on the post-OPC layout and OPC software was also used to
simulate the dimension of the defective features as printed on the wafer. "Design Based Metrology" was used to create
accurate metrology recipes to support wafer and mask metrology. Ultimately the procedures described in this paper
allow a direct correlation to be made between reticle inspectability and the impact of the same defects on wafer CD. Data
is presented for the case of the Contact Hole layer of a "65nm" Logic technology, though the methods described in the
paper are applicable to all layers.
Mask manufacturing for the 45nm node for hyper NA lithography requires tight defect and printability control at small
features sizes. The AIMS<sup>TM</sup><sup>1</sup> technology is a well established methodology to analyze printability of mask defects,
repairs and critical features by scanner emulation. With the step towards hyper NA imaging by immersion lithography
the AIMS<sup>TM</sup> technology has been faced with new challenges like vector effects, polarized illumination and tighter specs
for repeatability and tool stability.
These requirements pushed the development of an entirely new AIMS<sup>TM </sup>generation. The AIMS<sup>TM</sup> 45-193i has been
designed and developed by Carl Zeiss to address these challenges. A new mechanical platform with a thermal and
environmental control unit enables high tool stability. Thus a new class of specification becomes available. The 193nm
optical beam path together with an improved beam homogenizer is dedicated to emulate scanners up to 1.4 NA. New
features like polarized illumination and vector effect emulation make the AIMS<sup>TM</sup> 45- 193i a powerful tool for defect
disposition and scanner emulation for 45nm immersion lithography.
In this paper results from one of the first production tools will be presented. Aerial images from phase shifting and
binary masks with different immersion relevant settings will be discussed. Also, data from a long term repeatability
study performed on masks with programmed defects will be shown. This study demonstrates the tool's ability to
perform defect disposition with high repeatability. It is found that the tool will fulfill the 45nm node requirements to
perform mask qualification for production use.
Pushing the limits of optical lithography by immersion technology requires ever smaller feature sizes on the reticle. At the same time the k1-factor will be shifted close to the theoretical limit, e.g. the OPC structures on the reticle become very aggressive. For the mask shop it is essential to manufacture defect free masks. The minimum defect size, which needs to be found reliably, becomes smaller with decreasing feature sizes. Consequently optical inspection of masks for the 45nm node and below will be challenging.
In this paper the limits of existing KLA inspection tools were investigated by systematic inspection of different structures without and with programmed defects. A test mask with isolated and dense lines/space patterns including programmed defects was manufactured, completely characterized by CD-SEM and inspected with state-of-the-art inspection system. AIMS<sup>TM</sup> measurements were used to evaluate the defect printing behavior. The analysis of the measurement data gives an input for requirements of reticle inspection of upcoming 45nm node and beyond.
With decreasing pattern sizes the absolute size of acceptable pattern deviations decreases. For mask-makers a
new technology requires a review, which mask design variations print on the wafer under production illumination
conditions and whether these variations can be found reliably (100%) with the current inspection tools. As
defect dispositioning is performed with an AIMS-tool, the critical AIMS values, above which a defect prints
lithographically significant on the wafer, needs to be determined. In this paper we present a detailed sensitivity
analysis for programmed defects on 2 different KLA 5xx tools employing the pixel P90 at various sensitivity
settings in die-to-die transmitted mode. Comparing the inspection results with the wafer prints of the mask
under disar illumination it could be shown that all critical design variations are reliably detected using a state-of-the-art tool setup. Furthermore, AIMS measurements on defects with increasing defect area of various defect
categories were taken under the same illumination conditions as for the wafer prints. The measurements were
evaluated in terms of AIMS intensity variation (AIV). It could be shown that the AIMS results exhibit a linear
behavior if plotted against the square-root area (SRA) of the defects on the mask as obtained from mask SEM
images. A consistent lower AIV value was derived for all defect categories.
Defect disposition and qualification with stepper simulating AIMS tools on advanced masks of the 90nm node and below is key to match the customer's expectations for "defect free" masks, i.e. masks containing only non-printing design variations. The recently available AIMS tools allow for a large degree of automated measurements enhancing the throughput of masks and hence reducing cycle time - up to 50 images can be recorded per hour. However, this amount of data still has to be evaluated by hand which is not only time-consuming but also error prone and exhibits a variability depending on the person doing the evaluation which adds to the tool intrinsic variability and decreases the reliability of the evaluation. In this paper we present the results of an MatLAB based algorithm which automatically evaluates AIMS images. We investigate its capabilities regarding throughput, reliability and matching with handmade evaluation for a large variety of dark and clear defects and discuss the limitations of an automated AIMS evaluation algorithm.
With decreasing structure sizes on masks also the acceptable CD variation corridor for printing on the wafer
and therefore, the maximum allowed defect size is decreasing. This has not only implications to the accuracy
and repeatability of front-end processes such as writers, etchers, etc. but also challenges defect inspection and
qualification. Defect qualification is usually done by an AIMSTM tool which optically simulates the aerial image
of the structures by applying the same illumination conditions as the wafer fabs' scanners. As lithographers
continue to produce smaller and smaller structures, the as well decreasing acceptable design variation pushes the
AIMS<sup>TM</sup> evaluation step by step towards a metrology method. Thus, an advanced measurement capability of
the AIMS<sup>TM</sup> tool is mandatory to reliably disposition defects within these small margins. It is influenced by the
performance of illumination, imaging homogeneity, and stability. A possible measure for the tool's capability
is the (long term) repeatability, i.e. the 3σ-variance of the tool by evaluating the same defect with a certain
frequency over several weeks. The AIMS<sup>TM</sup> fab 193i platform takes into account the tightened requirements
with respect to homogeneity and stability by improved optics such as a new beam homogenizer module, new
energy monitoring and vibration isolation concept. In this paper we show data on the long term repeatability
compared between the first generation AIMS<sup>TM</sup> fab 193SE and the new AIMS<sup>TM</sup> fab 193i platform and discuss
the implications on the measurement capabilities of the two platforms.
For the successful reduction of chip production costs, the usage of more advanced designs with lower area consumption by manufacturing angled line structures is one possibility. The usage of conventional vector shaped electron beam writers does only allow writing Manhattan-like structures as well as 45 degree angled structures. There are several approximation possibilities for writing any angled lines, e.g. they could be approximated by writing only small rectangles or small rectangles in combination with small 45 degree triangles. This method introduces a very pronounced line edge roughness due to the written uneven edges. The critical dimension uniformity on the mask and the printing behavior are directly influenced by this synthesized line edge roughness. This paper addresses the investigation of critical dimension of the angled mask structures as well as the influence on the printing behavior. The different masks used in the experiment were patterned at the Advanced Mask Technology Center (AMTC). Measurements of pattern line widths were performed by using scanning electron microscopy techniques. The printing behavior of different structures was investigated by running AIMS measurements and performing exposure experiments. Comparing the mask structures and the final printed wafer structures, estimations on the transfer function of the synthesized line edge roughness could be performed.
Defect disposition and qualification with stepper simulating AIMS<sup>TM</sup> tools on advanced masks of the 90 nm node and below is key to match the customer's expectations for "defect free" masks, i.e. masks containing only nonprinting design variations. For defect dispositioning usually printability studies are carried out using the same illumination settings at the AIMS<sup>TM</sup> tool as later on at the steppers in the wafer fab. These studies then establish
an AIMS<sup>TM</sup> criterion (e.g., CD variation or transmission deviation) a structure deviation must not exceed. For ever more advanced technologies the accessible process window gets smaller and thus more and more complex apertures have to be used to allow for a still suitable contrast and reliable printing of the patterns. This results in more time-consuming printability studies and tighter AIMS<sup>TM</sup> specs. Simulations of the printing of mask defects could potentially help to decrease the amount of time for printability studies and also the time for defect disposition in the production. However, usually simulations in their first approximation do not account for effects such as flare, aberrations or illumination inhomogeneities of the AIMS<sup>TM</sup> tool. This makes it difficult to derive the AIMS<sup>TM</sup> criterion by simulations. In this paper we show that a homogeneous aperture illumination is crucial for the image contrast and the defect disposition. We present a method to characterize the pupil illumination and investigate the impact of illumination inhomogeneities on various structures and their orientation employing two different aperture types. The experimental results are compared to simulations with both homogeneous illumination and the real illumination distribution. It turns out that for correct simulation predictions on experimental results it is important to provide the correct illumination distribution to the simulations.
For leading mask technologies the mask inspection for finding critical defects is always a difficult task. With the introduction of chrome-less, high-transmission and alternating mask types, new absorber material and the possibility of quartz defects the defect inspection and -classification becomes even more challenging. To decide whether a defect is critical or a repair is successful, the Zeiss AIMS tool is used to classify defects. For conventional imaging the optical settings are usually chosen such that resolution is maximized, for example a dipole illumination is used for imaging a dense line-space array at an optimum contrast. In this paper we will do the opposite and reduce the optical resolution, such that we can filter out the array pattern and study the resulting defect image. This technique allows using a simple threshold detector to find and classify defects.
The "AIMS fab 193" tool is an aerial image measurement system for ArF-lithography emulation and is in operation worldwide. By adjustment of numerical aperture, illumination type and partial coherence parameter to match the conditions in 193nm steppers or scanners, it can emulate lithographic exposure tools for any type of reticles such as binary masks, OPC and phase shift structures, down to the 65nm node. The AIMS<sup>TM</sup> fab 193 allows the rapid prediction of wafer printability of critical features, such as dense patterns or contacts, defects or repairs on masks without the need to prepare real wafer prints using the stepper or scanner. Recently, a high resolution mode has been introduced based on a sophisticated microscope objective, characterized by a high numerical aperture (NA) and large working distance that allows working with pellicle mounted mask. With this lens system a high contrast image with resolution down to 150 nm lines and spaces (L/S) on mask has been demonstrated. In addition to the AIMS<sup>TM</sup> through-focus mode for printability which is optically equivalent to the latent image in the photo resist of a wafer, the high resolution mode allows the imaging of mask structures in focus and at printing wavelength to review defects or repairs. Such viewing capability is also helpful at the binary stage of a first writing step in the mask manufacturing process. In this work we will present application results for defects and critical features using both, aerial imaging and high resolution mode.
This paper presents first results of a defect printability study for the 70nm and 90nm technology. Two 6% halftone test masks with dense line/space (l/s) and contact hole (CH) structures, containing programmed defects were exposed at different production illumination conditions. The resultant data was compared with respect to the mask defect sizes, the Aerial Image Measurement System (AIMS) values and the mask defect inspection sensitivity. As expected over-and under-sized features exhibity the highest printability and AIMS value intensity deviation. No difference was found in the lithographic behavior of dark and clear extension.
Additionally (to the determination of the print critical AIMS values) the programmed defect masks were used for the evaluation of a KLA 52x inspection system. The performances of two detection pixels named P125 and P90 in combination with two inspection modes named die-to-die transmission (d2dT) and die-to-die reflective (d2dR) were investigated on 90nm and 70nm dense l/s and contact hole areas with respect to the print results. Over and under-sized small dense structures as well as dark and clear defects centered in a clear or dark structure are challenging for the new inspection tool. For dense contact hole arrays d2dR shows a better performance than d2dT.