Assessment Of Cl 2 /CHF 3 Mixture For Plasma Etching Process On Barc And Tin Layer For 0.21 μm Metal Line: Silterra Case Study

Article History : Received :11 January 2021; Accepted: 27 February 2021; Published online: 5 April 2021 ABSTRACT :In wafer fabrication manufacturing, aluminum etching process is a dry plasma etching process used as main process for construction of aluminum (Al) interconnects structures. As customer requirement changed for faster, more reliable and lower cost chips, chip manufacturers have learned to reduce the size of component on a chip in order to achieve those requirements(Ibrahim, Chik, & Hashim, 2016). As the geometry of the chip getting smaller, the width of Al line wiring specification also shrinking. To print the smaller geometry pattern requirement, the thickness in masking process also has to be reduced for better resolution. Such a thinner resist will create a challenge during plasma etching to ensure a minimal resist loss process which required new type of equipment but this research insist to sustain similar equipment. The use of oxide film as a hard mask has been evaluated by other researchers but alternative approach still needed to suit specific requirement of semiconductor factory installation base. This approach does require a process integration change and require a full technology qualification and easily take a lengthy qualification procedures especially when to qualify the existing products. It is worth trying at the situation of no other solution available. The challenge of insufficient margin for the metal line etching process for 0.2 μm width has caused the deformed metal pattern formation. This chemistry study of Cl2/CHF3 as a replacement gas to existing Cl2/O2 to address Organic backside anti refractive coating (OBARC) was evaluated and proven novelty where detail discussed in the following content.


INTRODUCTION
The patterning of aluminum metal interconnect structures is complicated by the multilayer of metallization scheme adopted by industry for IC production. In advanced metallization scheme (Pramanik & Saxena, 1983;Wilson, Tracy, & Freeman, 1993), the aluminum film which is often alloyed with copper serves as the conductor and is sandwiched by the barrier layers of titanium nitride (TiN) layer on the top and by TiN and titanium (Ti) layers at the bottom (Filippi et al., 2001). The purpose of this barrier layers are to avoid the inter-diffusion of aluminum and silicon ("Spiking"). The spiking occurs when the silicon becomes soluble into aluminum film as the temperature increases. The main reason for the complexity is to enhance the electromigration resistance in the device (Hosaka, Kouno, Hayakawa, Niwa, & Yamada, 1998; M. H. Lee et al., 2011). Etching of these films require multi step etching process with different mixture of chemistry (Vignes & Baléo, 1997) combined with optimum power (Kim, Jung, Choi, Kim, & Boo, 2005) and pressure conditions (Christie, 1994;Kunz, 2007;Okumura, 2010) to ensure the metal etching profile meeting the process requirements. The very fundamental requirement in plasma etching method adopted in fabricating of an IC pattern is the photoresist remaining margin. The pre-defined photoresist pattern on the metal substrate during photolithography process comes together with the organic backside antirefractive coating film to improve reflection control and light absorption during photolithography (Boumerzoug, 2014;Huang & Weigand, 2008;Zhuang et al., 2006). BARC layer minimizes thin film interference effects by reducing reflected light.
In the patterning transfer, during BARC opening process, high etch selectivity of resist to BARC process is required to minimize the resist loss to allow further substrate etching (Armacost et al., 1999). BARC also must etch quickly to prevent line width change. The selectivity of BARC to resist depend mainly on the carbon content and also its composition and structure responses towards oxidizing gas, reducing gas and plasma bombardment. As the geometry of the chip getting smaller, the width of Al line wiring specification also shrinking. To print the smaller geometry pattern requirement, the thickness in masking process also has to be reduced for better resolution. Such a thinner resist will create an inconsistency of the pattern width and unable to produce straight profile requirement if Assessment Of Cl 2 /Chf 3 Mixture For Plasma Etching Process On Barc And Tin Layer For 0.21 µm Metal Line: Silterra Case Study having insufficient of resist remaining process during plasma etching (H. J. Lee et al., 2008). This failure will lead to device electrical. Therefore, an optimized matching metal etching process is required. Further assessment in the following content will discuss finding of Cl2/CHF3 mixture to etch BARC and top TiN layer of metallization film having minimum metal line geometry of 0.21µm line and space. Figure 1 explainsworkflow of the methodology adopted in this assessment of Cl2/CHF3 in BARC and TiN Etching of AlCumetallization interconnect.

Figure 1: Methodology approach
The assessment was divided into two parts. The first part of the evaluation was done on the blanket photoresist wafers to determine the photoresist etch rate at different combinations of Cl2 and CHF3 flow rates combinations as tabulated in Table 1. The evaluation was done on the blanket photoresist wafers coated on bare silicon wafers with thickness of 14000Å. A ThermawaveOptiprobe metrology tool was used to measure the film thickness of photoresist before and after the plasma etching process. Each wafer was etched in inductively coupled plasma (ICP) metal etcher system at the cathode temperature of 40C. The remaining process parameters and equipment parameters were remained as constant. The photoresist etch rate were determined for each of the conditions and analyzed. In the second part of the assessment, the evaluation was done in the same metal etcher tool using pattern wafer instead of blanket photoresist wafers. The metal film of 0.40 µm stack (TiN/AlCu/TiN/Ti) was deposited after the dieletric layer formation on bare Si wafer. AlCu alloy where the Cu concentration is 0.5 wt.% and the AlCu is deposited at 350 ͦ C. The wafers were patterned with 7750Å DUV photoresist on 700Å of organic BARC film. The reticle mask used in this experiment from the customer reticle of metal-2 layer having 0.2 µm minimum design rules of line and space. Total of 8 wafers were prepare. The samples ware etched in ICP metal etcher tool using multiple etching step method as summarized in Table 2 below. The metal-2 etching recipes consist of six (6) etching steps. Step Etch process description   Step 3 of Metal-2 etching recipe All the completed etching wafers were processed in solvent cleaning tool for polymer removal steps. The SEM inspection was done on all samples at center die and edge die of the wafers. The inspection point covered the isolated and dense metal line structures with 2µm field of view (FOV) SEM image captured. The observed residue defects in each SEM images were counted and tabulated for further statistical analysis

RESULTS AND DISCUSSION
The result of photoresist etch rate obtained from the experimental run was tabulated in Table 4 below. The sample mean etch rate for each run is tabulated and its respective confidence interval (CI) is determined at 95% confidence level. The statistical analysis is done based on the upper CI value.  Table 4: Sample mean of photoresist etch rate for each run Figure 2 shows the Y-axis of PRERactual value from upper CI and X-axis is the PRER Predicted value obtained by the proposed model derived from the fit model analysis. The suggested model of predicted PRER obtained in this experiment analysis is statistically significant as the RSq is 0.96 and the Pvalue is 0.0152 as tabulated in Table 5 and  Table 6 respectively.     shows the leverage plots for the Cl2 and CHF3 effect respectively. The Y-axis is the predicated response of photoresist etch rate when the magnitude of factor in X-axis is varied. The leverage plot for Cl2 having steeper slope than for CHF3 suggesting that CHF3 factor does effect PRER however it is less significant as compared to Cl2 factor. Therefore the use of CHF3 in BARC etching step is able to preserve the photoresist margin. This is an advantage to process engineer as the factor may be considered as the nob to control the lateral etching of the metal width dimension (FICD) without significantly degrade the photoresist margin. Moreover, CHF3 is one of the polymerizing chemistry that generates polymeric material on wafer surface when used in RF plasma condition (Astell-Burt et al., 1986). Saito et. al reported in his study that CHF3 used in etching aluminum film able to control the metal width dimension by suppressing side wall etching of the aluminum due to the passivation layer deposited on the sidewall of the resist and aluminum pattern. In this application, we propose to use the CHF3 gas in BARC etching to generate polymer film and deposited on the sidewall of the photo resist pattern with the expectation to minimize lateral etching to the resist and able to minimize the width dimension shift of the mask pattern. Based on this qualitative study, the use of CHF3 in the BARC step will reduce the shift in photo resist width dimension during BARC opening step and able to control CD as required. The flow margin of CHF3 has to be determined to ensure the BARC film is not overly polymerized by the carbon based species and no micromasking defect in the underneath TiN ARC film is formed.   Table 5 shows the result of the second part of experiment evaluated on patterned wafers using Metal-2 reticle mask. Total defect counts was counted based on the SEM micrograph image captured at center die and edge die of each sample as shown by Figure 5 and 6 below. Total of 65 counts of defects were recorded for the Run#7 SEM micrograph images inspected at isolated (MPISO) and dense (MPDense) region structures on both die locations.   Table 9 shows all the 3 experimental terms (Cl2, CHF3 and Cl2*CHF3) analyzed in this model found to be significant on the residue defect count response with pvalue of less than 0.05.   Table 9: Parameter estimates of each term with P-value <0.05 indicates its significant in the model. Figure 8 shows the interaction profile between Cl2 and CHF3. Based on plot profile, the sensitivity of the CHF3 flow to the defect residue is minimal when the Cl2 flow is operating at higher set point.  1997). However, too high flow of CHF3 will cause the density of passivation species more than the density of the neutral species. The increase in generation of passivation species may lead to thicker polymerization layer in the etching area and slowing down the reaction of the neutral species with the material to etch. Since the same chemistry mixture was used in BARC and TiN etching steps, we believed the residue defect observed could originate either from BARC or TiN etching step. The unclear BARC or TiN material has transformed into micro-masking defect and locally slow down the etching rate during the subsequent etching steps and remained as residue defect formation.
In this work, we have successfully identified the safe process window of Cl2 and CHF3 flow required in metal etching process for BARC and TiN etching steps to ensure zero residue defects.

CONCLUSION
The replacement of Cl2/O2chemistry mixture to Cl2/CHF3 mixture has enabled us to reduce the photoresistetch rate during the OBARC etchingstep. Our experiment model has suggestedthat CHF3 flow rate in the mixture has least effect on the photoresistetch rate. The findingis in ageementwithotherresearcher(Kwon, Kim, Lee, Lee, & Park, 2009) investigation that the loweretchrateis due to the formation of CFxpolimerization film.
The samechemistrywasalsoevaluated to etch the TiN ARC layer of the metal-2 film stacks. The characterization on the metalresiduedefectsdonebased on the SEM image micrographanalysisdisclosed the findingthat the low flow of Cl2wouldinduced the defect formation in the presence of CHF3species in the mixture. The new alternative solution has been implemented in the real semiconductorprocess application.