Sherjang Singh

Study on surface integrity in photomask resist strip and final cleaning processes

In recent years, photomask resist strip and cleaning technology development was substantially driven by the industrys need to prevent surface haze formation through the elimination of sulfuric acid from these processes. As a result, ozone water was introduced to the resist strip and cleaning processes as a promising alternative to a Sulfuric - Peroxide Mixture (SPM). However, with the introduction of 193i double patterning, EUVL (Extreme Ultraviolet Lithography) and NanoImprint Lithography (NIL) the demand on CD-linewidth control and surface layer integrity is significantly expanded and the use of ozone water is questionable. Ozone water has been found to cause significant damage to metal based mask surface layers, leading to significant changes in optical properties and CD-linewidth shift. In this paper HamaTech APE demonstrates the use of an alternative acid-free resist strip and cleaning process, which not only overcomes the named drawbacks of conventional ozone water use, but reduces resist strip time by 50% to 75%. The surface materials investigated during this study are; chrome absorber layers on binary masks, MoSi based shifters, chrome hard mask layers on EPSM, and ruthenium capping layers on EUV masks. Surface material integrity and CD-stability results using this new, acid-free approach are presented in the following pages.

Techniques for removal of contamination from EUVL mask without surface damage

Mask defectivity is an acknowledged road block for the introduction of EUV lithography (EUVL) for manufacturing. There are significant challenges to extend the conventional methods of cleaning developed for standard 193nm optical photomask to meet the specific requirements for EUV mask structure and materials. In this work, the use of UV activated media for EUV mask surface cleaning is evaluated and the effects on Ru capping layer integrity are compared against conventional cleaning methods. Ru layer surface is analyzed using roughness measurements (AFM) and reflectivity changes (EUV-R and optical).

Automated imprint mask cleaning for step-and-flash imprint lithography

Step-and-Flash Imprint Lithography (S-FIL) is a promising lithography strategy for semiconductor manufacturing at device nodes below 32nm. The S-FIL 1:1 pattern transfer technology utilizes a field-by-field ink jet dispense of a low viscosity liquid resist to fill the relief pattern of the device layer etched into the glass mask. Compared to other sub 40nm CD lithography methods, the resulting high resolution, high throughput through clustering, 3D patterning capability, low process complexity, and low cost of ownership (CoO) of S-FIL makes it a widely accepted technology for patterned media as well as a promising mainstream option for future CMOS applications. Preservation of mask cleanliness is essential to avoid risk of repeated printing of defects. The development of mask cleaning processes capable of removing particles adhered to the mask surface without damaging the mask is critical to meet high volume manufacturing requirements. In this paper we have presented various methods of residual (cross-linked) resist removal and final imprint mask cleaning demonstrated on the HamaTech MaskTrack automated mask cleaning system. Conventional and non-conventional (acid free) methods of particle removal have been compared and the effect of mask cleaning on pattern damage and CD integrity is also studied.

Megasonic cleaning: possible solutions for 22nm node and beyond

Megasonic energy transfer to the photomask surface is indirectly controlled by process parameters that provide an effective handle to physical force distribution on the photomask surface. A better understanding of the influence of these parameters on the physical force distribution and their effect on pattern damage of fragile mask features can help optimize megasonic energy transfer as well as assist in extending this cleaning technology beyond the 22nm node. In this paper we have specifically studied the effect of higher megasonic frequencies (3 & 4MHz) and media gasification on pattern damage; the effect of cleaning chemistry, media volume flow rate, process time, and nozzle distance to the mask surface during the dispense is also discussed. Megasonic energy characterization is performed by measuring the acoustic energy as well as cavitation created by megasonic energy through sonoluminescence measurements.

Effective EUVL mask cleaning technology solutions for mask manufacturing and in-fab mask maintenance

Extreme Ultraviolet Lithography (EUVL) is considered the leading lithography technology choice for semiconductor devices at 16nm HP node and beyond. However, before EUV Lithography can enter into High Volume Manufacturing (HVM) of advanced semiconductor devices, the ability to guarantee mask integrity at point-of-exposure must be established. Highly efficient, damage free mask cleaning plays a critical role during the mask manufacturing cycle and throughout the life of the mask, where the absence of a pellicle to protect the EUV mask increases the risk of contamination during storage, handling and use. In this paper, we will present effective EUVL mask cleaning technology solutions for mask manufacturing and in-fab mask maintenance, which employs an intelligent, holistic approach to maximize Mean Time Between Cleans (MBTC) and extend the useful life span of the reticle. The data presented will demonstrate the protection of the capping and absorber layers, preservation of pattern integrity as well as optical and mechanical properties to avoid unpredictable CD-linewidth and overlay shifts. Experiments were performed on EUV blanks and pattern masks using various process conditions. Conditions showing high particle removal efficiency (PRE) and minimum surface layer impact were then selected for durability studies. Surface layer impact was evaluated over multiple cleaning cycles by means of UV reflectivity metrology XPS analysis and wafer prints. Experimental results were compared to computational models. Mask life time predictions where made using the same computational models. The paper will provide a generic overview of the cleaning sequence which yielded best results, but will also provide recommendations for an efficient in-fab mask maintenance scheme, addressing handling, storage, cleaning and inspection.

Investigation of EUVL reticle capping layer peeling under wet cleaning

In the absence of a pellicle, an EUVL reticle is expected to withstand up to 100 cleaning cycles. EUVL reticles constitute a complex multi-layer structure with extremely sensitive materials which are prone to damage during cleaning. The 2.5 nm thin Ru capping layer has been reported to be most sensitive to repeated cleaning, especially when exposed to aggressive dry etch or strip chemicals [1]. Such a Ru film exhibits multiple modes of failure under wet cleaning processes. In this study we investigated the Ru peeling effect. IR-induced thermo-stress in the multilayer and photochemical-induced radical attack on the surface are investigated as the two most dominant contributors to Ru damage in cleaning. Results of this investigation are presented and corrective actions are proposed.

Preserving the mask integrity for the lithography process

The optical performance stability of a photomask is one of the most critical factors in the photolithography process and stringent specifications create greater challenges with each advancing technology node. Throughout its lifetime, a photomask is exposed to a variety of cleaning cycles. It is essential that the integrity of the mask is preserved throughout each of these processes. Standard mask cleaning treatments include surface preparation with 172nm VUV for better wetting, organic resist/particle removal with aqueous ozone (DIO3) and residual ion removal for haze control. However, high energy radiations from 172nm VUV have been reported to cause overlay shift and wet oxidizing chemistries adversely affect mask CD and optical properties, ultimately influencing lithography performance. Previously, HamaTech APE successfully demonstrated an advanced cleaning method using photolyzed DIO3 with minimal metal layer damage. In this paper, performance of different media under the UV photolysis effect is explored for various steps in the cleaning process. Photolyzed DI water based surface preparation of photomask under atmospheric conditions without any overlay shift is demonstrated. Alternative chemicals with higher photolysis rates are explored for resist stripping applications. Phase/Transmission and CD change on a PSM (Phase Shift Mask) are compared between regular and modified processes. Potential improvements in residual ion removal using combination of radiation and hot DI water are also presented.

Cleaning of step-and-flash imprint masks with damage-free nonacid technology

Step-and-flash imprint lithography S-FIL ® is a promising li- thography strategy for semiconductor manufacturing at device nodes be- low 32 nm. The S-FIL 1:1 pattern transfer technology utilizes a field-by- field ink jet dispense of a low-viscosity liquid resist to fill the relief pattern of the device layer etched into the glass mask. Compared to other sub-40-nm critical dimension CD lithography methods, the resulting high resolution, high throughput through clustering, 3-D patterning capa- bility, low process complexity, and low cost of ownership of S-FIL makes it a widely accepted technology for patterned media as well as a prom- ising mainstream option for future CMOS applications. Preservation of mask cleanliness is essential to avoid the risk of repeated printing of defects. The development of mask cleaning processes capable of re- moving particles adhered to the mask surface without damaging the mask is critical to meet high-volume manufacturing requirements. We present various methods of residual cross-linked resist removal and final imprint mask cleaning. Conventional and nonconventional acid- free methods of particle removal are compared and the effect of mask cleaning on pattern damage and CD integrity is also studied.

Effect of cleaning chemistry on MegaSonic damage

Acoustic energy applied through the cleaning media results into two kinds of cavitation effects; namely stable and transient cavitation. A uniformly pulsating bubble transforms into stable cavitation behavior whereas a bubble implosion implies transient cavitation. Pattern damage of sensitive features on advanced masks as well as Ru pitting on EUVL reticles is mostly the result of transient cavitation. Stable cavitation on the other hand produces a very narrowly controlled energy distribution which allows cleaning without damage. Stable cavitation can be achieved by suitably tailoring physical, chemical and thermodynamic properties of the liquid and gas media. In this paper we investigate a new cleaning chemistry that has favorable physical and thermodynamic properties to produce stable MegaSonic cavitation. The cavitation created in this chemistry is characterized by measuring acoustic energy as well as by pattern damage and particle removal efficiency on mask level. The chemical properties (pH and zeta potential) of this chemistry are compared with conventional cleaning chemistries. Its effects on CD shift as well as phase and transmission loss are also studied.

Cleaning induced imprint template erosion

Abstract. We studied the erosion and feature stability of fused silica patterns under different template cleaning conditions. The conventional sulfuric acid and hydrogen peroxide mixture (SPM) cleaning is compared with an advanced nonacid process. Spectroscopic ellipsometry optical critical dimension measurements were used to characterize the changes in pattern profile with good sensitivity. This study confirmed the erosion of the silica patterns in the traditional acid-based SPM cleaning mixture (H2SO4+H2O2) at a rate of ∼0.1  nm per cleaning cycle. However, the advanced nonacid cleaning process only showed critical dimension shift of ∼0.01  nm per cleaning. Contamination removal and pattern integrity of sensitive 20-nm features under MegaSonic assisted cleaning was also demonstrated.