Optimization of developing uniformity by resist thickness measurement

As design rule shrinks down continuously, various technology have been developed to extend the resolution limits of lithography. One of those is Double Exposure Technology(DET). This paper is about not only resolution improvement but also Critical Dimension(CD) variation reduction with DET. As the design rule shrinks below 100nm, the core/peripheral area where we used to think we had sufficient margin is becoming the bottle neck for device fabrication. In this paper, in order to compare optimized single exposure (cell focus) and DET (cell, core/peripheral focus) for critical dimension uniformity(CDU) on cell and core/peripheral area, CDU was measured from wafer by use of simulation and measurement. Gate layer of DRAM device was used for the experiment. Exposure condition for the single exposure was set to crosspole and for DET, dipole and conventional respectively. Optical proximity correction(OPC) was done with in-house simulation tool on stiching area of the double exposure experiment. Same exposure tool and same process condition were used for each experiment and only the exposure condition was changed to compare local CDU, intra-field CDU, wafer CDU to find out how much CD variation can be reduced.

Extreme ultraviolet lithography (EUVL) is getting closer to practical use for mass production every year. For applying EUV lithography to manufacturing, productivity improvement is a critical challenge. Throughput and yield are important factors for productivity. EUV source power is steadily advancing year by year, bringing improvements in throughput. Furthermore, yield improvement is necessary for productivity enhancement. In order to improve the yield in EUV lithography processing, further improvement of defectivity and critical dimension (CD) uniformity is required. Reduction of residue and in-film particle defects is very crucial work to enhance the productivity. For residue defects, it is found that the residue defects detected after development inspection (ADI) are perfectly transferred to after etching inspection (AEI), meaning that high defectivity of residue deteriorates yield directly. Furthermore, in-film particle defect counts increase from ADI to AEI because particles included in spin-on-carbon (SOC) and spin-on-glass (SOG) as well as that in resist film are the sources of defects in AEI, while particles mainly included in resist film can be observed in ADI. Figure 1 shows the normalized defectivity comparison between conventional rinse and filtration used and optimizing rinse used and improving filtration efficiency in ADI and AEI. ADI defectivity in which optimized rinse is applied and improving filtration efficiency achieved 0.04, while that in which conventional rinse and filtration are used defined 1.00, meaning the 96 % of improvement. In addition, AEI defectivity by using optimized rinse and improving of filtration efficiency shows 86 % of reduction as compared with that using conventional rinse and filtration. These results reveal that our novel actions for rinse and material supply show excellent performance for defect reduction. For the other aspect of yield improvement, CD uniformity control is one of the crucial factors. CD variations are comprised of several components such as wafer to wafer CD uniformity, field to field CD uniformity. To achieve CD uniformity target for manufacturing, we have optimized developing process with the latest technology. Then, 15 % of field to field CD uniformity improvement and significant improvement of wafer to wafer CD uniformity are achieved.

Extreme ultra violet (EUV) resists have been developed to be able to print sub-30nm L/S features with EUV alpha DEMO tool (ADT) having 0.25NA. However, a lithographic performance of EUV resist is not comparable to that of DUV resist. At same process constant (k1), the imaging capability of EUV resist is poor than that of DUV resists. The most critical issues are line width roughness (LWR) and critical dimension (CD) variation across a field. Although there are many studies to improve the LWR of EUV resist, the issue on CD variation across a field is not much explored, because the problem can be detected at full field exposure. In this paper, sources of the CD variation across a field are mainly investigated, and solutions to improve the CD uniformity are explored. Out of band (OOB) radiation and its reflectivity at REticle MAsking (REMA) unit of scanner or absorber of mask is regarded as one of the sources which aggravates imaging quality of EUV resist. In addition, the optical density of black border at EUV wavelength is also known to have an impact on this CD variation. Although the exact spectrum of OOB radiation is not open, LASER produced plasma (LPP) type source and discharge produced plasma (DPP) type source are believed to have the OOB radiation. Therefore, to improve pattern fidelity and LWR of EUV resist, the mitigation of OOB radiation impact is required. It is found that the resist sensitivity to DUV compared to EUV is important, and this property affects on CD uniformity. Furthermore, new material which can mitigate the OOB radiation impact is developed. This material is applied as an additional layer on conventional EUV resist film, and shows no intermixing. Process window is not changed by applying this layer. The filtering ability of OOB radiation is explored. LWR and pattern fidelity are much improved by applying this material to EUV process.

Critical Dimension (CD) Uniformity is one of the necessary parameters to assure good performance and reliable functionality of any integrated circuit (IC), and towards the advanced technology node 28nm and beyond, corresponding CD Uniformity becomes more and more crucial. It is found that bad mask CD Uniformity is a significant error source at 28nm process. The CD Uniformity on mask, if not controlled well, will badly impact wafer CD performance, and it has been well-studied that CD Uniformity issue from gate line-width in transistors would affect the device performance directly. In this paper we present a novel solution for mask global CD uniformity error correction, which is called as global loading effect correction (GLEC) method and applied nesting in the mask exposure map during the electron beam exposure. There are factors such as global chip layout, writing sequence and chip pattern density distribution (Global Loading), that work on the whole mask CD Uniformity, especially Global Loading is the key factor related to mask global CD error. From our experimental results, different pattern density distribution on mask significantly influenced the final mask CD Uniformity: the mask with undulating pattern density distribution provides much worse CD Uniformity than that with uniform one. Therefore, a GLEC model based on pattern density has been created to compensate the global error during the electron beam exposure, which has been proved to be efficacious to improve mask global CD Uniformity performance. Furthermore, it ’s also revealed that pattern type is another important impact factor, and GLEC coefficient need be modified due to the specific pattern type (e.g. dense line-space only, iso-space only or an average of them) to improve the corresponding mask CD uniformity.

For sub half-micron lithography, control of the Critical Dimension, represents the key to a robust and manufacturable process. The lithographer must understand and tune the exposure system for critical dimension performance using variables such as Numerical Aperture (NA), Partial Coherence (PC), position of the Focal Plane. Exposure tool must be characterized for its lens signature and the tools must be matched for their CD variation within the optical field In a previous study, we demonstrated that the phase-shift focal plane monitor, PSFM accurately measures focal plane variations when appropriate calibrations are employed. That paper also described the development of a model to determine lens classic aberrations such as Coma, Astigmatism and Field Curvature. The publication correlated the results to an approximation of the stepper's Critical Dimension (CD) behavior for a matrix of NA and PC settings. The present study, continues to study CD uniformity through examination of the lens aberrations and field focal signature of optical steppers in a 0.35 um process. We describe a method of measuring the uniformity of numerical aperture (NA) and partial coherence (PC) across the exposure field this new tool is then applied as an aid in optical lens characterization and tool-to-tool matching. The information gathered is thereupon applied to measure lens aberrations and predict CD variation across the field under various settings of the lens focal plane, the predictions are validated by a comparison against CD uniformity as measured by an advanced critical dimension metrology system.

As critical dimension (CD) control requirements increase and process windows decrease, it is now of even higher importance to be able to determine and separate the sources of CD error in an immersion cluster, in order to correct for them. It has already been reported that the CD error contributors can be attributed to two primary lithographic parameters: effective dose and focus. In this paper, we demonstrate a method to extract effective dose and focus, based on diffraction based optical metrology (scatterometry). A physical model is used to describe the CD variations of a target with controlled focus and dose offsets. This calibrated model enables the extraction of effective dose and focus fingerprints across wafer and across scanner exposure field. We will show how to optimize the target design and the process conditions, in order to achieve an accurate and precise de-convolution over a larger range of focus and dose than the expected variation of the cluster. This technique is implemented on an ASML XT:1900Gi scanner interfaced with a Sokudo RF^3S track. The systematic focus and dose fingerprints obtained by this de-convolution technique enable identification of the specific contributions of the track, scanner and reticle. Finally, specific corrections are applied to compensate for these systematic CD variations and a significant improvement in CD uniformity is demonstrated.

As pattern design rule of device shrinks, CD control becomes more critical and important especially for resistance devices. As CD (Critical Dimension) increases, CDU (Critical Dimension Uniformity) becomes worse generally. The question with this relationship is a starting point of our study. Mainly we focused on two points. One is which factor affects CDU. The other is whether CDU degradation with large CD happens at all cases or not. We have analyzed with simulation and experiment results about CDU with splitted mask layout CD under limited conditions such as same equipment, illumination and exposure dose. As a result, we will show the relationship between CD size and CDU.

Extreme ultra violet (EUV) resists have been developed to be able to print sub-30nm L/S features with EUV alpha DEMO tool (ADT), and several resists start to meet the target for 2X node DRAM application in resolution and sensitivity. However, an overall performance of EUV resists is still not comparable to that of DUV resists. At the same process condition having same process constant (k1), the imaging capability of EUV resists is poorer than that of DUV resists. The most critical issues are line width roughness (LWR) and critical dimension (CD) variation across a field. Although there are many studies to improve the LWR of EUV resist, the issue on CD variation across a field is not much explored, because the problem can be detected at full field exposure. In this paper, sources of the CD variation across a field are mainly investigated, and solutions to improve the CD uniformity are explored. Out of band (OOB) radiation and its reflectivity at REticle MAsking (REMA) unit of scanner or absorber of mask is regarded as one of the sources which aggravates imaging quality of EUV resist. In addition, the optical density of black border at EUV wavelength is also known to have an impact on this CD variation. Therefore, to improve pattern fidelity and LWR of EUV resist, the mitigation of OOB radiation impact is required. It is found that the resist sensitivity to DUV compared to EUV is important, and this property affects on CD uniformity. Furthermore, new material which can mitigate the OOB radiation impact is developed. The resolution limit of current EUV resists is also investigated to see the extendibility of it. At an exposure condition which readily resolves 16nm half pitch (HP) resolution, 20nm resolution is hardly achieved even with a champion resist. To overcome this barrier, more efforts on development of alternative resists are required from today.

As optical lithography will be extended to device generations below 150 nm of design rule, the critical dimension (CD) uniformity on a photomask is required to be as small as 13 nm (3(sigma) ). The relationship between development method and dark erosion is discussed in view of CD variations. The temperature variation on a photomask is found to be more than 4 degrees Celsius for a conventional spin-spray development. An optimized process using a 'puddle process' to minimize developer temperature effect and non-uniform spray pressure effect on CD variation. The CD uniformity using the optimized process brings 11 nm in 3(sigma) over 127 mm X 127 mm area on a 6-inch mask.

The significant optical absorption of most currently available commercial single layer 193 nm resists, even at a coating thickness of 0.4 micrometer, implies increased sensitivity to process control fluctuations of the kind that negatively impact critical dimension (CD) uniformity, process latitude, resist sidewall profile, and line edge roughness. These problems, although less severe on reflective substrates, are particularly acute on wafers with bottom anti-reflection coatings (BARCs), which are useful in CD control. With different intensity of light reaching different levels in the resist film on a BARC, a gradient is thus established in the extent of the chemical amplification reactions on which semiconductor lithography is based. The result is the familiar sloped sidewall profile and poor CD uniformity after the resist is developed. Further, with most of the photoacid generators and the polymer resins in the 193 nm resists having very low quantum efficiencies and significant absorption at 193 nm, respectively, most of the absorbed light in the resist is used up in energy dissipative processes, instead of in generating photoacids which catalyze the chemical amplification chemistry of these resists. One approach to overcome this absorption problem is to use significantly thinner resist films, but etch considerations preclude such option as these materials do not have very good etch stability. The purpose of this paper is to quantitatively assess the impact of absorption on the process control of sub- 0.15 micrometer features patterned on a full field ASML 193 nm scanner, interfaced to a TEL MARK-8 track. Optical properties of different resist films/BARC stack combinations are characterized by UV spectroscopic ellipsometry and broad band spectrometry, and sidewall profiling is done by atomic force microscopy.

With the development of the lithography, the demand for critical dimension (CD) and CD uniformity (CDU) has reached a new level, which is harder and harder to achieve. There exists reflection at the interface between photo-resist and substrate during lithography exposure. This reflection has negative impact on CD and CDU control. It is possible to optimize the litho stack and film stack thickness on different lithography conditions. With the optimized stack, the total reflectivity for all incident angles at the interface can be controlled less than 0.5%, ideally 0.1%, which enhances process window (PW) most of the time. The theoretical results are verified by the experiment results from foundry, which helps the foundry achieve the mass production finally.

The Semiconductor Industry Association (SIA) has placed stringent requirements on Next GenerationLithography mask critical dimension (CD) control. A new chemically amplified (CA) positive resist,Shipley XP2040D was evaluated for mask making application. This resist exhibited an extraordinary postexposure bake (PEB) sensitivity, less than 0.6 nm/ C, along with a sub-50 nm resolution. The PEB stabilityin ambient was larger than 4.5 hours and the line-edge-roughness (LER) was less than 2.9 nm. 1. INTRODUCTION Critical dimension (CD) control and uniformity for a SCALPEL mask must be kept within 7 nm (3 ) inorder to meet the SIA Roadmap requirement for 70 nm NGL mask fabrication [1]. Chemically amplified(CA) resists have been considered for NGL mask patterning because of their high resolution, high electronexposure sensitivity and etch selectivity. However, CD variation due to the PEB sensitivity of CA resistcomplicates the mask making process and needs to be minimized.Previous Finite Element Analysis modeling indicates there are PEB temperature gradients across a 200 mmSCALPEL mask and within each membrane of the mask. The thermal variations are due to the combinedcomplex three-dimensional mask structure coupled with the thermal gradients associated with the bakechamber environment. These gradients were experimentally confirmed through the measurement of criticalfeatures across various membranes on the mask array. A CD uniformity of 8-10 nm for inter-membrane and4-7 nm for intra-membrane was observed using a CA negative-tone resist, NEB22[2]. This CD variationwas mainly attributed to the large PEB sensitivity of the resist ( ~8 nm/ C).In order to meet the 7 nm SIA mask budget, CD non-uniformity due to thermal variations must be reduced.One way to minimize the influence of this thermal effect is to use low PEB sensitivity resists. This paperreports on the performance of Shipley XP2040D positive CA resist for 200 mm SCALPEL maskfabrication using the Leica VB6HR 100 KeV e-beam exposure system.

Reticle critical dimension uniformity (CDU) is an important criterion for the qualification of mask layer processes. Normally, the smaller the three sigma value of reticle CDU is, the better is the reticle CDU performance. For qualification of mask processes, the mask layers to be qualified should have a comparable reticle CDU compared to the process of record (POR) mask layers. Because the reticle critical dimension (CD) measurement is based on algorithms like “middle side lobe measurement”, evaluation of the reticle CD-values can not reflect aspects like the sidewall angle of the reticle and variation in corner rounding which may be critical for 45nm technology nodes (and below). All involved tools and processes contribute to the wafer intra-field CDU (scanner, track, reticle, metrology). Normally, the reticle contribution to the wafer CDU should be as small as possible. In order to reduce the process contributions to the wafer intra-field CDU during the mask qualification process, the same toolset (exposure tool, metrology tool) should be applied as for the POR. Out of the results of these investigations the correlation between wafer measurement to target (MTT) and reticle MTT can be obtained in order to accurately qualify the CDU performance of the mask processes. We will demonstrate the correlation between reticle MTT and wafer MTT by use of multiple mask processes and alternative mask blank materials. We will investigate the results of four process-layers looking at advanced binary maskblank material from two different suppliers (moreover the results of a 2X-via layer as an example for a phase-shift maskblank is discussed). Objective of this article is to demonstrate the distribution between reticle MTT and wafer MTT as a qualification criterion for mask processes. The correlation between wafer CD-difference and reticle CD-difference of these mask processes are demonstrated by having performed investigations of dense features of different 28nmtechnology process layers (poly-, active-, contact-, 1X-metal-, 2X-via layers). Referring to the correlation between wafer and reticle MTT, the contribution of the reticle CD-difference to the wafer CD-difference can be used as an evaluation method for the transfer-process of different mask sites.

This paper describes the effect of the mask errors such as mask critical dimension (CD) variation, phase and transmission error of attenuated phase shifting mask (att-PSM) on wafer CD in ArF lithography and also analyzes these errors quantitatively. Mask CD requirement using ELF and MEF is estimated firstly and mask CD should be controlled within about 7nm assuming O.7ONA ArF system with 1% illumination uniformity. Transmission error induces larger CD variation than phase error. However, phase error should be considered otherwise in that it reduces depth of focus (DOF). To control DOF degradation less than 10% in case of O.l4um and O.l6um isolated contact hole(C/H), the phase should be controlled within the range of Considering O.l4um isolated contact hole, transmission error of occupies 10% of CD tolerance. Finally, the budget of these factors are calculated in view of total wafer CD variation quantitatively except lens aberration, resist process, and etc. To reduce wafer CD variation, we should control mask CD more tightly.

We studied the mask error enhancement factor (MEF) for four 0.15-micrometers patterns, isolated lines, line and space (L and S), isolated holes, and dense holes, for various process conditions. The MEF for isolated lines was the smallest of all. The MEF for L and S was not as small as that of the isolated lines. We obtained a less than 15-nm wafer critical dimension (CD) variation, when we reduced the mask CD variation to 20 nm. For the isolated-hole patterns using an attenuated phase shift mask (PSM) with large mask biasing can reduce the wafer CD variation. On the other hand, it is very difficult to reduce the MEF and the wafer CD variation for the dense-hole patterns. The alternating PSM was the best of the evaluated process, but it was not good enough to reduce the mask CD variation.