Improved thin film model for overlay metrology

Tung, Chiou-Kou; Ku, Yi-Sha; Liu, An-Shun; Smith, Nigel

doi:10.1117/12.599274

Cited by 3 publications

(8 citation statements)

References 0 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…If the target is small (1x1-3x3µm target size) -that is with overall dimensions similar to the spatial resolution of the imaging tool -then the image is not fully resolved and its symmetry changes with overlay offset. This change of image symmetry with overlay offset was predicted by our image formation model 6 . This is a simple diffraction-based model, which has been shown to provide useful predictions for front-end layers which are nearly planar -i.e.…”

Section: Image Modelingsupporting

confidence: 52%

“…However, there is a relationship between the asymmetry of the image and overlay error so that overlay error can actually be determined or calculated based on asymmetry 4,5,6 . The symmetry of the image, F, is the smallest value of the root-mean-square difference of intensities at equal distances about any point s within the image (equation 1).…”

Section: The Measurement Algorithmmentioning

confidence: 99%

See 1 more Smart Citation

In-chip overlay metrology for 45nm processes

Pang

Smith³

et al. 2007

SPIE Proceedings

View full text Add to dashboard Cite

The feasibility of measuring overlay using small (between 1x1µm and 3x3µm total size) targets has been demonstrated 4 . The symmetry of the image of isolated test features changes with overlay offset. The targets are small enough to be positioned within active areas of the device, and total measurement uncertainty (TMU) is sufficient to allow these targets to be used in characterizing overlay variations in the active device. In this paper we describe further development of this technique and its application to overlay control in 45nm processes. A simple image model has been used to predict how the target images change with overlay error, and to study the unwanted effects of process variation on the measurement. In order to ensure the accuracy of measurements made using the in-chip targets it is necessary to provide for dynamic calibration of the symmetry-to-overlay response. This calibration can be readily achieved by printing multiple targets close together, with each target having a programmed offset that differs by a small amount. In our tests we have used triplets of targets with programmed offsets of 30nm, 50nm and 70nm. Normal targets are printed with a programmed offset of 50nm.A test reticle was designed for double-pass printing of a range of in-chip targets with different sizes and component dimensions, and with designed overlay offsets of 0nm to 100nm in 4nm steps. Standard bar-in-bar targets were printed with every in-chip target to allow the change of symmetry with overlay to be measured directly. Wafers were printed using a 45nm process poly-to-STI stack. Measurements were made in multiple fields and from multiple wafers using an unmodified Nanometrics Caliper élan overlay tool. The test wafers were printed with extremes of process variation. Correlation of the measured image symmetry to the programmed offset under different process conditions shows that the response is sensitive to film changes, as predicted by the image model.Results from production wafers show that the effect of normal process variation is small enough that calibration is not necessary at every location. Placing a few calibration targets in the scribe lines of the device is more than sufficient, allowing the smallest possible space requirement for measurement inside the active area of the device.Detailed study of the change in overlay with programmed offset along a line of test samples with the same design properties shows short-scale variations of order 1-5nm. According to the 2005 ITRS 1 these variations account for nearly 50% of the overlay budget for a 45nm process. This effect cannot be described by any model where overlay variations are purely a mathematical function of position, and in process control at this node it will become necessary to use characterization of overlay by measurement instead of models.

show abstract

Section: Image Modelingsupporting

confidence: 52%

Section: The Measurement Algorithmmentioning

confidence: 99%

In-chip overlay metrology for 45nm processes

Pang

Smith³

et al. 2007

SPIE Proceedings

View full text Add to dashboard Cite

show abstract

“…If the target is small (1x1-3x3µm target size) -that is with overall dimensions similar to the spatial resolution of the imaging tool -then the image is not fully resolved and its symmetry changes with overlay offset. This change of image symmetry with overlay offset was predicted by our image formation model [4]. This is a simple diffraction-based model, which has been shown to provide useful predictions for front-end layers which are nearly planar -i.e.…”

Section: Image Modelingsupporting

confidence: 51%

“…However, there is a relationship between the asymmetry of the image and overlay error so that overlay error can actually be determined or calculated based on asymmetry [4][5][6]. The symmetry of the image, F, is the smallest value of the root-mean-square difference of intensities at equal distances about any point s within the image(equation 1).…”

Section: A the Measurement Algorithmmentioning

confidence: 99%

Overlay metrology for next generation lithography at CMS

Ku¹,

Tai²,

Chang³

2007

2007 7th IEEE Conference on Nanotechnology (IEEE NANO)

View full text Add to dashboard Cite

The Center for Measurement Standards (CMS) has a research program in optical overlay metrology. The main goal of this work is to improve overlay measurement accuracy. Two main threads are developed to the work -novel in-chip overlay target design, and best algorithm. The former is hoped to prove a capability for measuring overlay error inside the active area of product devices. The later is for extracting the most from image detail.

show abstract

“…If the patterns are not truly isolated then I(x) will contain a significant contribution from the cross-product √I 1 (x)I 2 (x). Experiment 2 shows that the elements of the two patterns must be spaced by several times the resolution limit of the tool for equation (6) to hold well, with a failure appearing as a scale error in the measurements. Newer designs intended for process control improve TMU with greatly increased information density, but are analyzed using algorithms that make the same assumption of pattern independence.…”

Section: Equation (5) Is Valid Only If the Condition In Equationmentioning

confidence: 99%

Overlay metrology at the crossroads

Smith¹,

Binns²,

Plambeck³

et al. 2008

SPIE Proceedings

View full text Add to dashboard Cite

The introduction of new techniques such as double patterning will reduce overlay process tolerance much faster than the rate at which critical feature dimensions are shrinking. In order to control such processes measurements with uncertainties under 0.4nm are desirable today and will become essential within the next few years. This very small error budget leads to questions about the capability of the imaging technology used in overlay tools today and to evaluation of potential replacement techniques. In this paper we will show that while imaging technology is in principle capable of meeting this requirement, the real uncertainty in overlay within devices falls well short of the levels needed. A proper comparison between techniques needs to focus on all of the possible sources of error, and especially those that cannot be simply reduced by calibration or by repeating measurements. On that basis there are more significant problems than the relative capability of different measurement techniques. We will discuss a method by which overlay within the device area can be controlled to the required tolerance.

show abstract

Improved thin film model for overlay metrology

Cited by 3 publications

References 0 publications

In-chip overlay metrology for 45nm processes

In-chip overlay metrology for 45nm processes

Overlay metrology for next generation lithography at CMS

Overlay metrology at the crossroads

Contact Info

Product

Resources

About