Amitabh Sabharwal

A novel process of etching EUV masks for future generation technology

Studies on pattern transfer of next generation lithographic (EUV) photomask were carried out. Based on current absorber layer material candidates, thermodynamic calculations were performed and plasma etch gas system and composition were investigated. The gas systems have the advantage of all etch products being in volatile condition. This is helpful to keep the etch process and etch chamber clean. For etch CD bias challenge in EUV photomask etch, self-mask concept was investigated, which makes anti-reflective (AR) sub-layer of the absorber layer function as a hard mask for the bulk absorber layer beneath. It significantly reduces etch CD bias and improves pattern transfer fidelity. For common candidates of EUV mask absorber layers such as TaBO/TaBN and TaSiON/TaSi, reactive gas systems were proposed according to thermodynamic calculations with all products volatile. AR sub-layers were etched in one gas composition with volatiles. Once the AR sub-layer is etched through, gas composition was changed so that the bulk absorber sub-layer beneath is etched selectively with volatile products. Excellent results in profiles, CD bias, CD uniformity, and underneath buffer/capping layer impact have been demonstrated.

Advances in Etching High-Density Embedded FRAM Structures

The development of ferroelectric random access memories (FRAM) has been a sustained endeavor among semiconductor manufacturers over the past decade in response to the growing communications and consumer electronics demand for nonvolatile memories capable of high-speed/low-voltage operation. Process technology and materials performance improvements have refined cell structure, device density, operating voltage, endurance, and data retention. Similarly, advances are being made in etch technology to enable the patterning of true high-density, electrically sound FRAM capacitor arrays. This paper addresses enhancements in chamber design and etch chemistry that have significantly improved electrical performance of the capacitors and achieved sustained stability of the etch process.

Model-based etch profile simulation of PSM films

For advanced binary and PSM mask etch, final profile control is critically important for achieving desired mask specifications. As an aid to attain profile control, an etch profile simulation method has been developed. The method starts with an initial photoresist profile and incorporates etch rate and directionality information to predict the final etch profile. In this paper, simulated results are compared to measured etch profiles for PSM substrates. The results highlight the importance and implications of incoming resist profile and etch selectivity on final profile.

Extreme ultraviolet lithography mask etch study and overview

Abstract. An overview of extreme ultraviolet lithography (EUVL) mask etch is presented and a EUVL mask etch study was carried out. Today, EUVL implementation has three critical challenges that hinder its adoption: extreme ultraviolet (EUV) source power, resist resolution-line width roughness-sensitivity, and a qualified EUVL mask. The EUVL mask defect challenges result from defects generated during blank preparation, absorber and multilayer deposition processes, as well as patterning, etching and wet clean processes. Stringent control on several performance criteria including critical dimension (CD) uniformity, etch bias, micro-loading, profile control, defect control, and high etch selectivity requirement to capping layer is required during the resist pattern duplication on the underlying absorber layer. EUVL mask absorbers comprise of mainly tantalum-based materials rather than chrome- or MoSi-based materials used in standard optical masks. Compared to the conventional chrome-based absorbers and phase shift materials, tantalum-based absorbers need high ion energy to obtain moderate etch rates. However, high ion energy may lower resist selectivity, and could introduce defects. Current EUVL mask consists of an anti-reflective layer on top of the bulk absorber. Recent studies indicate that a native oxide layer would suffice as an anti-reflective coating layer during the electron beam inspection. The absorber thickness and the material properties are optimized based on optical density targets for the mask as well as electromagnetic field effects and optics requirements of the patterning tools. EUVL mask etch processes are modified according to the structure of the absorber, its material, and thickness. However, etch product volatility is the fundamental requirement. Overlapping lithographic exposure near chip border may require etching through the multilayer, resulting in challenges in profile control and etch selectivity. Optical proximity correction is applied to further enhance the resolution. Other resolution enhancement techniques, such as phase shifting, are also in consideration for EUVL. Phase-shifting will involve partial etching of the multilayer. The trend to use shorter EUV wavelength (e.g., 6.7 nm) for enhancing resolution will use new multilayer and absorber compositions, and will require new etch process development efforts. TaBO/TaBN absorber layers (features down to 40 nm) were etched with vertical profiles, low etch CD bias, and 1.7 nm etch CD uniformity (3σ). In the light shed application, Mo/Si multilayer etching yielded vertical profiles and high etch selectivity.

Photomask etch: addressing the resist challenges for advanced phase-shift and binary photomasks

As lithography requirements mandate ever-thinner resist thickness, the need for in-situ monitoring has become more urgent. In this paper we present an in-situ optical methodology-based system to determine residual photoresist thickness during advanced photomask etch with < 1000 Å photoresist. Several types of phase-shift masks and photoresists were examined. A series of masks were etched to demonstrate the performance of the system. Results show an average accuracy of better than 2%, with a maximum absolute range of all tests within 8%.

Dry etching performance of advanced EUV mask blanks

Mask defectivity is often highlighted as one of the barriers to a manufacturable EUV solution. As EUV lithography matures, other components of mask making also emerge as key focus areas in the industry: critical dimension (CD) control, film variability, selectivity, and profile tolerance. Mask materials and specifications continue to evolve to meet the unique challenges of EUV lithography, creating the need for etch capabilities that can keep pace with the latest developments. In this study, the performance of a new EUV mask etch system will be evaluated using a variety of mask blanks to determine the relative performance of each blank type. Etch contributions to mean to target (MTT), CDU, linearity, selectivity, capping layer uniformity, line edge roughness (LER), and profile quality will be characterized to determine tool performance. The new system will also be used to demonstrate multilayer etching capabilities, important for opaque frame and alternating phase shift applications. A comprehensive summary of the etch performance of various EUV films and the readiness for manufacturing applications will be provided.