Accurate in-resolution level overlay metrology for multi patterning

Accurate in-resolution level overlay metrology for multi patterning lithography techniques Ilan Englarda, Rich Piecha, Claudio Masiaa, Noam Hillelb, Liraz Gershteinb, Dana Soferb, Ram Peltinovb , Ofer Adanb a

b

Applied Materials Europe, Veldhoven, The Netherlands Applied Materials Israel, PDC, 9 Oppenheimer, Rehovot 76705, Israel ABSTRACT

Multi patterning lithography (MPL) breaks the k1=0.25 barrier to become the main candidate for 32nm device fabrication before 2010. When using MPL, overlay (OVL) becomes an essential part of the overall critical dimension (CD) budget and therefore can no longer be treated as a separate process control measure. Furthermore, the CD measured at each of the two consecutive lithography steps must be combined into one single 32nm process control measure and will require further improvements of CD-SEM precision, resolution and accuracy. The metrology challenges involved in measuring double patterning CD and OVL arise from the fact that across chip pitch variations (ACPV) are determined by the two separate lithographic processes [1]. This aspect makes the control of the process significantly more complex and requires careful measurement of the processes, both individually as well as combined. Meeting the ITRS specifications for CD and localized OVL measurements beyond 32nm half pitch is challenging and will require innovative CDSEM algorithmic solutions. This paper is a follow-up from last year’s paper that introduced SEM metrology for MPL technology. In this paper, we report on the actual implementation of combined CD and OVL metrology solutions for the latest immersion scanner generation. We will describe the latest OVL measurements performed at ASML and demonstrate the robustness of the novel algorithm for accurate separation and recombination of two individual CD populations related to the consecutive MPL steps. Key words: Multi, patterning, Double, Exposure, lithography, overlay, Measurement, identification, pitch, Contact, Hole

1. CD-SEM OVERLAY METROLOGY OVERVIEW 1.1. Double Exposure overlay assessment MPL can be roughly divided into three main categories: Double patterning lithography (DPL), self aligned double patterning (SADP) and Double Exposure (DE) lithography. DPL has two main flavors: Dual Line and Dual Trench Patterning (DLP and DTP). The principle is based on double exposure of the wafer at mask level with an intermediate hard mask etch step in between the two consecutive lithography steps to accomplish pitch reduction. A DTP process of 50nm half pitch for single damascene applications had recently been developed successfully by IMEC [2]. The second option, SADP, uses sacrificial spacer processing technology to double the resolution while the third option, DE, uses the same resist to expose twice and is based on the use of a complimentary mask set. The best known DE method is the so-called Double Dipole Lithography (DDL) process in which horizontal and vertical features are split at mask level and consecutive exposures are done using dipole illumination. One possible way to assess and monitor the double exposure overlay performance, consists of a simple 1:2 line/space ratio module. A pattern, as shown in Figure 1 (SEM top-view image), results from a double litho pass with a half pitch shift between exposure 1 and exposure 2.

Metrology, Inspection, and Process Control for Microlithography XXII, edited by John A. Allgair, Christopher J. Raymond Proc. of SPIE Vol. 6922, 69221D, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.776099

Proc. of SPIE Vol. 6922 69221D-1 2008 SPIE Digital Library -- Subscriber Archive Copy

1.2. Double Exposure overlay measurement technique The described DE overlay measurement could become the method of choice for overlay assessments because of its simplicity and the limited number of processing steps required (other than the lithography related steps). OVL performance of the scanner can be assessed by measuring the gap between the two modules which are exposed separately with a known introduced offset (Fig. 1).

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The gap can be assessed by measuring the centre of gravity (COG) [3] of the two regions of interest within the measurement box. By subtracting the centre of gravity (COG1 - COG2) of the two regions of interest (ROI) and by knowing the half pitch design (HPdesign), the overlay error can then be calculated as: OVL = HPdesign – (COG1 – COG2).

2. MEASURING DOUBLE EXPOSURE OVERLAY 2.1. Design Of Experiment Imaging work has been performed using an ASML TWINSCANTM 1900i scanner. Further experimental details are provided in Table 1.

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Table 1

Summary of settings used on scanner.

Illumination mode Illumination setting Dose (mJ/cm2) Focus (µm)

X Y Polarized Na = 1.2 σ = 0.5/0.8 33 0.03

The DE overlay feasibility test includes measuring precision of five (5) repetitions of COG measurements across a DE wafers with no intentionally introduced overlay error. The relative accuracy is determined by correlation of intentionally introduced overlay errors and actual overlay measurements of the two exposures.

2.2. Experimentation results: overlay precision and relative accuracy The precision results of the DE overlay measured as described above in section 2.1 are summarized in Table 2. Table 2

Overlay x5 repeatability test

A robust repeatability result (such as summarized in Table 2, 0.2nm 3σ) is essential for reliable measurement of overlay uniformity. By measuring the double exposure OVL target on several wafers with different intentionally introduced OVL errors, it is possible to determine the relative accuracy of the COG based methodology. A high correlation of 0.998 was found for a range from -4 nm to + 4 nm induced overlay errors when correlated to the actual measurements (Fig. 2). =

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3. MULTI PATTERNING LAYER IDENTIFICATION MPL requires measurement of two critical dimension populations. For Double Exposure this takes place after the second exposure whereas for DPL this occurs after the metal hard mask (MHM) etch. The main challenge is to distinguish between the two populations which each result from a separate exposure.

3.1. Lines/Spaces multi patterning identification for DPL MPL Lines and Spaces can be measured for each of the two lithography process steps. In case of DPL it can be measured

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after each exposure and etch step. An anchor, printed during the first or second exposure, can be used to distinguish between the exposures. In order to measure the local variations accurately, the use of a so-called MacroCD (MCD) algorithm [4] is useful. MCD uses averaging of CD within a group of features that is defined within a measurement box (e.g. odd and even lines, pitch, etc.). Both CD populations and the overlay can be measured accurately as can be seen in Figure 3 [3].

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Odd and even 64nm pitch design test structures measured by MacroCD. The overlay error is extracted from the differences between odd and even spaces (for DLP) or lines (for DTP) [3]

3.2. Contact Hole exposure identification for Double Exposure Identification of the individual contact hole (C/H) exposures in a multi-patterning lithography process by the metrology tool could become more challenging than that of lines and spaces upon further shrinkage of designs. Although the CH layout may consist of a simple matrix (see for example Figure 4A) it could also be a more complex structure such as, for example, that in an SRAM design (Figure 4B)[5].

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Figure 4B

An example of an SRAM C/H pattern based on double exposure lithography is shown [5] as well as a possible measurement location. The C/H layout for each of the two lithography steps is shown at the left side for the corresponding SRAM C/H layout shown in the SEM picture at the right side

The overlay error between the exposures and their CD can be extracted by using the MCD algorithm. MCD is an algorithmic solution for high confidence averaging of multiple critical dimension (CD) features. It reduces the random CD variations by averaging a large number of features at once. The metrology challenge for C/H matrices relates to the identification of the correct exposure within a measurement box and enhanced by the lack of a unique measurement anchor that can be used as a point of reference. As can be seen in Figure 4B, the SRAM C/H matrix design lacks unique signature (e.g. odd/even features order) which makes it difficult to distinguish the interlaced exposures by SEM and thus average sufficient amount of C/H efficiently. The challenge is therefore to distinguish between the respective C/H exposures and to determine the average CD related to each exposure as well as the overlay error between them (e.g. a group of 12 – 30 C/Hs, averaged simultaneously by MCD). In the next section, we will propose a novel methodology that provides a possible solution to this challenge.

3.2.1. C/H Ellipticity and orientation manipulation with image tuner As discussed in an SPIE 2007 presentation [5], C/H ellipticity and rotation can be aligned for a given exposure/defocus window by the presence of astigmatism aberration (as Zernike’s Z5). This can be implemented with ASML’s image tuner (see Fig 5). The image tuner enables ASML customers to tune the system according application specific requirements such as, for example, focus, distortion, uni-directional or specific features [8]. The exposure system will also be optimized for the actual field sizes and NAs that are used. The desired image optimization criteria can be selected in the exposure recipe on a per image basis. The system will dynamically optimize itself on a shot by shot basis according to the customized requirements by adjusting lens actuators, focus and wavelength. The product comes with a set of optimized recipes which are beneficial for uni-directional memory, logic applications or multi-directional focus sensitive applications. The key benefits can be summarized as follows, - Lens optimization at actual NA and field size - Features specific optimization capability - Unidirectional optimization capability - Inter-field automatic optimization - Improved CDU and focus control at specific applications - Improved flexibility in utilizing the tool for various applications

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Scanner

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Figure 5

Image Tuner to scanner connectivity and functional flow [8]

In this case study, the image tuner’s zernike correction process was used to tweak the Z5 aberration and modulate the C/H shape. This is in contrast to the original purpose of this application which is correcting Zernike Z5 – Z37 RMS across the slit positions. Figure 6 (see box labeled A) demonstrates that the contact hole ellipticity and orientation can be modulated and aligned for various exposures with very small variability within certain focus range. 80nm Isolated CHs ellipticity with induced Z5 Astigmatism

80nm isolated CHs without induced Z5 Astigmatism

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80nm CH with (left) and without (right) Z5=40nm. The CHs orientation distribution within the measurement box becomes smaller within certain focus range (A)

A feasibility check to find the best working conditions with separable exposure signature was performed by benchmarking the Z5 aberration with various strengths (5, 10, 30nm) and by measuring the C/H CD, ellipticity, and orientation accordingly. The aim was to find the minimum CD process window with a distinguishable C/H signature. This signature can be employed to distinguish between exposures and extract the average CD of each exposure and their

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overlay error.

3.2.2. Design Of Experiment and results Imaging work has been performed using an ASML TWINSCANTM 1450 scanner and a PAR823 resist on 1c5d. The mask used for exposure has an attenuation level of 6.4%. Three Focus Exposure Matrix (FEM) wafers were exposed with 5, 10 and 30nm induced astigmatism using the Image Tuner. Further experimental details are provided in Table 3. Table 3

Summary of settings used on scanner

Parameter Illumination mode Illumination setting Dose (mJ/cm2) Focus steps (µm)

Dense CH Quasar 20 Na = 0.93 σ = 0.97/0.77 65 with steps of 2 0.02

Accurate, MCD based, vectorial ellipticity averaging calculations can be made, taking into consideration the C/H orientation which is aberration sensitive. Ranging from negative to positive defocus, the contact hole starts of with an elliptical shape oriented along the y-axis, then becomes circular, and finally ends up elliptically but now oriented along the x-axis. These different stages can be easily related to the tangential focus, circle of least confusion, and sagittal focus of an astigmatic lens [6]. MCD benchmark test results of induced Z5 aberration at various magnitudes based on FEM wafers are summarized in Figure 7.

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Bench mark test of Zernike Z5 aberrations (astigmatism), induced by ASML’s ImageTuner. Left to right: 5nm, 10nm and 30nm astigmatism

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The results indicate that modulation of C/Hs works from a certain aberration threshold and onwards with an impact on the signature such as the C/H ellipticity maximum threshold level, orientation range, and variability. Only Z5 = 30nm provides the minimum required process window (40nm) for an ellipticity specification between 2 to 8% (4% for best F/E) with a rotational range from 45 – 90º (80º for best F/E). The orientation variability for the best exposure through focus was found to be as low as 4nm versus above 25nm for Z5 = 5 and 10nm. This small variability, related to the optimal settings, would allow separating accurately the two exposure populations for FEM and CDU evaluations.

3.2.3. Results summary

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The image tuner Z5 benchmark demonstrated the feasibility of the novel metrology approach to measure CHs separately on double exposure wafers. We tested the latest MCD vectorial C/H measurements to identify the best C/H focus and exposure settings for ellipticity and orientation in addition to traditional CD measurements. This approach would work for FEM and CDU and allow users to extract the C/H CD and Overlay error in one single measurement (Fig. 8) which simplifies the creation of the CDSEM recipe targets and improve throughput.

The combined measurement for best focus/exposure of the C/H array. The average CD, average CD of each separate exposure and average overlay error between the exposures can be extracted using ellipticity as a differentiator. This method could eliminate the need for a unique anchor to position the measurement box accurately

4. SUMMARY The CD-SEM, traditionally used to measure various CDs of interest, is now also used for local overlay measurements. The basic accuracy and precision measurements of 1D and 2D features have always been the foundation of CD uniformity (CDU) assessment. With MPL, CDU performance is impacted by OVL errors related to the multi-exposure approach. For that reason, the CD-SEM needs to be able to measure overlay with extremely high precision and accuracy similar to CD. As mentioned in the ITRS roadmap report [7], the overlay specification is ≤ 1nm at the 32nm node which requires a CD-SEM precision of 0.2nm. The latest DE OVL results obtained at ASML by using Applied Materials’ VeritySEM 2 CD-SEM confirmed this number and guarantee accurate and robust OVL assessment. Localized multi layer measurement identification capability will become valuable for large line/space arrays and nonunique CH matrices to determine OVL and CD. A novel approach to contact hole layer identification was shown by using ASML’s ImageTuner and the latest Applied Materials MCD vectorial C/H CD-SEM algorithm. This methodology could improves productivity significantly, shortens the CDSEM recipe creation and setup time and maximizes measurement throughput.

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ACKNOWLEDGEMENT The authors would like to thank the ASML team: Ingrid Minnaert-Janssen, Jo Finders, Paul Luehrmann, Peter Vanoppen, Frank Duray, Gert-Jan Janssen, Frensly Vlijt and Guido Schiffelers. Special thanks to Robert Schreutelkamp from Applied Materials, for his valuable editorial contributions.

REFERENCES [1]

Ilan Englard et al, “Novel CD-SEM overlay method improves dual trench patterning CDU ”, Semiconductor International Magazine, (Feb 2008).

[2]

Maaike Op de Beeck et al, “Manufacturability issues with Double Patterning for 50nm half pitch single damascene applications, using RELACS® shrink and corresponding OPC” SPIE 6520-18 (2007)

[3]

Ilan Englard et al, “Metrology challenges for advanced lithography techniques”, Proc. SPIE 6518-50 (2007).

[4] 16,

B.J. Bunday et al. “CDSEM Metrology: Macro CD technology – Beyond the average”, Proc. SPIE 5752, p 1 – (2005)

[5]

Jo Finders et al, “Random 65nm..45nm C/H printing using optimized illumination source and CD sizing by post processing”, Proc. SPIE 6924-106, (2008).

[6]

Ilan Englard et al, “MacroCD contact ellipticity measurement for lithography tool qualification ”, Proc. SPIE 6518-116, (2007).

[7]

ITRS 2006 Lithography update, pages 3 – 8

[8]

ASML’s Image Tuner brochure

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