Hyuk-Joo Kwon

Modeling and correction of global CD uniformity caused by fogging and loading effects in 90-nm-node CAR process

As critical dimensions (CDs) continue to approach the 90 nm node, it is inevitable that the industry has employed the use of chemically amplified resist (CAR) with 50 kV e-beam writing tool. However, the fogging effect by re-scattered incident electron at a high acceleration e-beam writer and the loading effect at dry etching step due to pattern density are critical issues since these effects make the variation of CD mean to target (MTT) and the degradation of CD uniformity. Tracking the CD error sources in CD uniformity and minimizing the error are very important task for high technology node mask production. In this paper, we focus on finding the source of the radial error in CD uniformity for each process step since the radial error occupy the main part of total CD uniformity. Also we present the radial error modeling using convolution equation between Gaussian CD error distributions with pattern densities. Finally, we describe the radial error correction method by phantom exposure with rectangle representing local pattern density. Fogging effect at writing process is one of the main sources of the radial error in global CD uniformity. The error by fogging effect is linearly proportional to mask pattern density, whereas loading effect at dry etch process increases the radial error in the case of the higher pattern density. The correction method using defocused beam based on our CD uniformity model effectively reduces the radial error and total error to 50% of their original value.

Dry etching of Cr layer and its loading effect

The Cr etch rate is affected by Cr density to be etched at the photomask and the Cr loading effect has become main obstacles to overcome for the next generation photomask process. Different mask-to-mask and within-mask pattern densities have made difficult to control the final CD (critical dimension). We have tested loading effect using binary Cr mask with ZEP7000 (3000 angstroms) photoresist. The loading effect was evaluated for the masks fabricated at the same process condition with the different mask-to-mask Cr loading and different within-mask Cr loading. The CD variations of dark field and clear field were observed and the phenomenological approach was proposed for the loading effect by some simplified equations.

Applications of MICP source for next-generation photomask process

As critical dimensions of photomask extends into submicron range, critical dimension uniformity, edge roughness, macro loading effect, and pattern slope become tighter than before. Fabrication of photomask relies on the ability to pattern features with anisotropic profile. To improve critical dimension uniformity, dry etcher is one of the solution and inductively coupled plasma (ICP) sources have become one of promising high density plasma sources for dry etcher. In this paper, we have utilized dry etcher system with multi-pole ICP source for Cr etch and MoSi etch and have investigated critical dimension uniformity, slope, and defects. We will present dry etch process data by process optimization of newly designed dry etcher system. The designed pattern area is 132 by 132 mm2 with 23 by 23 matrix test patterns. 3 (sigma) of critical dimension uniformity is below 12 nm at 0.8 - 3.0 micrometers . In most cases, we can obtain zero defect masks which is operated by face- down loading.

Loading effect parameters of dry etcher system and their analysis in mask-to-mask loading and within-mask loading

The Cr etch rate is affected by Cr density to be etched at the photomask. Different mask-to-mask and within-mask pattern densities have made difficult to control the final CD. We have tested loading effect using binary Cr mask with ZEP7000 photoresist. The loading effect was evaluated for the masks fabricated at the various dry etch conditions with different within-mask Cr loading. The Cr etch rate and selectivity was observed at various process conditions and relations between parameters of dry etch process and Cr loading were evaluated. The horizontal and the vertical Cr etch rates were investigated and the process parameter dependence on the Cr loading was analyzed. The horizontal and the vertical photoresist etch rates were evaluated for the photoresist loading effect. The cause of mask-to-mask loading and within-mask loading are mainly from Cr loading and photoresists loading, respectively. The Cr loading is mainly affected by source power, pressure, and Cl2/O2 ratio. In our system, within-mask Cr loading is strongly dependent on the process parameters when the selectivity of Cr to PR is below 1. If uniformity and selectivity are acceptable, high DC bias, high ga flow, low pressure, and high Cl2/O2 ratio are recommended to reduce loading effect.

Dry Etching Technology of Cr and MoSi Layers Using High-Density Plasma Source

We have manufactured dry etcher system for photomask process utilized the new plasma source and process optimizations have been done for CD (critical dimension) uniformity and loading effects. The 3 ? of CD uniformity(final CD - develop CD, point by point subtraction) of Cr pattern, with 132 x 132 mm2 area and 11 x 11 pattern arrays, was obtained below 10 nm, where the target CD is 0.8 um clear pattern. Cr and MoSi slopes are 88° ~ 90° , which shows highly anisotropic etch. The selectivity of PR to Cr was over 1.6 at the clear area ratios of < 50 % and the selectivity was mainly affected by oxygen partial pressure and clear area ratio. Phase uniformity for PSM was 180 ± 1° and transmittance uniformity is within 6.3 ± 0.02 %. Validity and probability of dry etcher system to produce next generation photomask were discussed.

Decrease of chrome residue on MoSiON in embedded attenuated-PSM processing

In Embedded Attenuated PSMs(Phase Shift Masks), chrome residues on MoSiON, especially at the edge of a pattern, should be decreasing the phase-shift effect and it must be also causing CD(critical dimension) variations in a wafer-process. Chrome residues on MoSiON are well known being generated at second level lithography or according to performance of cleaning process before it. In this paper, we investgated the influence of treatment on Cr surface during MoSiON etch process using CF4 plasma and proposed the optimum treatment procedure to reduce the Cr residues originated form re-deposition of carbon-contained polymers in CF4 plasma.

Comparative evaluation of positive and negative chemically amplified resist characteristics for 90-nm-node photomask production

For the latest photomask fabrication, better critical dimension (CD) control and pattern fidelity to design size are required. According to the latest ITRS roadmap, masks for the 90 nm technology node should have CD uniformity of 6~8nm (3σ). Moreover, CD control is particularly critical for isolated opaque lines, such as those found in gate layers, whose loading is primarily clear field. The high acceleration voltage electron beam (EB) systems that employ variable shaped beams (VSBs) are used for mask writing due to their high throughput. To minimize write time and fogging effects, and to control mean CD and improve CD uniformity for mask production, it is well known that negative tone resists enable better VSB mask writing system performance. In these circumstances, positive and negative tone chemically amplified resists (CARs), FEP171 (Fuji Films) and FEN270 (Fuji-Films), were evaluated empirically for mask making. We investigated and compared resolution, sensitivity, resist profiles, CD variation vs. exposure dose, proximity effect correction (PEC), fogging effect, pattern fidelity, and so on. Furthermore, write tool data volume and throughput, defect trends, and other process parameters on the positive and negative tone resists were evaluated and compared by applying test patterns.

A study of post-exposure baking effect for CAR process in photomask fabrication

As the requirement of specification on photomask continues to be tightening with advanced logic and memory devices, the combined process of chemically amplified resist (CAR) and high acceleration voltage e-beam writing tool is widely used to meet the resolution and throughput for advance photomask fabrication. It is well known that the post exposure baking (PEB) condition makes serious effect on the characteristic of CAR due to its de-protection reaction with thermal acid catalyzation. In this paper, we present the PEB temperature effect on pattern resolution such as line edge roughness (LER) and proximity effect correction (PEC) latitude that is practical limitation in the combined process of 50 kV writng tool and CAR resist. Our results show that LER and PEC lattitude are strongly dependent on PEB temperature due to resist contrast variation. At higher PEB temperature, increasing the contrast value can reduce the LER and it can increase the optimum PEC latitude.

Dehydration bake effects with UV/O3 treatment for 130-nm node PSM processing

As feature sizes of phase shift mask (PSM) have dropped below half-micron, resist adhesion have become a more critical issue, especially during second level lithography. Second writing process requires special consideration, because the resists mechanical strength of resists on patterned chrome and patterned glass is smaller in comparison to that on the un-patterned chrome blank. If the adhesion strength is not sufficient to withstand the stress during subsequent processes, patterns will be damaged during second level lithography. Resists stress at pattern edges that subsequent processes, pattern will be damaged during second level lithography. Resist stress at pattern edges that weaken its adhesive property, together with the low mechanical strength of resists on glass, creates ample probability for the unwanted phenomenon in PSM process. In this paper, we investigate the effects of property and adhesive strength of resists on surfaces at different treatment before resist coating process, and observe the defects generation after different treatment.

Highly anisotropic etching of phase-shift masks using ICP of CF4-SF6-CHF3 gas mixtures

There is considerable interest in phase shift masks as a route to extending the resolution, contrast, and depth of focus of lithographic tools beyond what is achievable with the normal chrome mask technology. A problem that has so far hindered the introduction of phase shift masks has been the difficulty of phase and transmittance control when a phase shift mask is applied to practical use. Also, to apply phase shift layer (MoSiON), it remains that effects several critical mask parameters including sidewall slope, surface roughness, and critical dimension. For these reasons, this process requires a high degree of control of the etch process of shift layer. So in this paper, we described a technique for the fabrication of phase shift masks by etch rate of a MoSiON layer. Etching experiments of MoSiON were performed using different fluorinated gas mixtures. Four of them, CF4/O2/He, SF6/O2/He, CHF3/O2/He and Cl2/CF4/O2/He were chosen for high etch rate, sidewall slope, and surface morphology. Each added gases had a unique property on the etch rate, anisotropy, surface roughness and sidewall morphology. Result indicates that vertical slope and smooth surface are obtained using the Cl2/ CF4/O2/He and SF6/O2/He mixture. With increasing O2 flow rate to the SF6/O2/He Plasma and added Cl2 gas to the CF4/O2/He Plasma, the MoSiON etching profile becomes anisotropic without undercutting and trench profile. It is probably due to both increasing etch rate and sidewall passivation of Cl2 ion flux. When Cl2 gas was added to the CF4/O2/He Plasma, the small addition of chlorine was enough to protect the exposed sidewall of the undercutting, therefore, higher flow rate of chlorine had to be added to protect the sidewall of the undercutting by forming a sidewall passivation layer. These results show that both increasing O2 flow rate to the SF6/O2/He Plasma and the addition of Cl2 to the CF4/O2/He plasma are necessary in order to achieve a vertical profile and a smooth surface morphology.