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Silicon Nitride Etch

Automated On-line Chemical Monitoring and Control for Hot Phos and Tungsten Etch in 3D NAND

Abstract:

In the process of fabricating 3D NAND devices, the complex deposition and etch have been proven to be challenging. Two etch processes: silicon nitride sacrificial removal and W etch-back in the 3D NAND word-line formation have been identified as the two critical steps that significantly impact the 3D NAND product yields. In this paper, we present the results of an automated on-line chemical management system that were specifically developed to enable real-time monitoring and control of both the sacrificial silicon nitride removal and W etch-back processes.

Keywords—Silicon Nitride Etch, Tungsten Etch, 3D NAND, Phosphoric-Acetic-Nitric Acid, PAN

Introduction

One of the key challenges of 3D NAND is scaling stack height for higher bit density. Unlike 2D planar NAND that is constrained by lithography, the bit density of 3D NAND is limited by the complex deposition and etch process steps while stacking the NAND structures in the vertical direction. The process of fabricating 3D NAND begins with multilayered silicon nitride and oxide deposition, followed by high aspect-ratio hole etch for the channel and word-line. The silicon nitride in the word-line is a sacrificial layer that is removed by immersion wet-etch, followed by dielectric (ONO) and tungsten metal gate, deposition and etch-back [1]. In this process flow, the silicon nitride sacrificial removal and W etch-back have been identified as the two critical steps that require accurate real-time metrology and process control.

Critical Wet Etch Processes

A. Sacrificial Silicon Nitride Etch using Hot Phosphoric Acid

The method of using hot phosphoric (Hot Phos) acid to etch silicon nitride is well understood and has been used in semiconductor manufacturing for many years. The control of temperatures and water content in H3PO4 was found critical in controlling the nitride and oxide etch rates. It was also found that seasoning the Hot Phos etching bath with silicate can further reduce the etching rate of SiO2 and improve the etch selectivity. Theoretically, a critically high etch selectivity can be achieved by seasoning the H3PO4 with high concentration of silica. Nevertheless, maintaining a stable etch process with such a high etch selectivity over time has been proven difficult to achieve without real-time monitoring and control, due to the dynamic bath loading behavior and etch by-products. A reliable real-time monitoring and control of Si is also important to prevent process induced defects due to Si precipitation.

At ECI, we have developed a suite of methods [2,3] designed to accurately analyze the components of the Hot Phos etch bath for stable and reliable monitoring and control of the etch process. These methods not only enable a reliable and stable etch process in the life time of the etching solution, but also the feed and bleed and cost savings that extend the lifetime of the etching baths. We have demonstrated that real-time results can be obtained using the methods implemented in our automated on-line system QualiSurf QSF-500 (see figure 1). To ensure the real-time results are accurate, we measured and compared the results with off-line Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) (see Figure 2).

Figure 1. QualiSurf QFS-500 Series
Figure 2. Comparison of Measured Si vs ICP-AES

In the experiments of seasoning and etch, we demonstrated the capability of accurate monitoring and control of the Hot Phos silicon nitride etch process. The results are shown in Figure 2 and Figure 3.

Seasoning Process

Figure 3. Measured Si ppm in Hot Phos Seasoning Process
Figure 4. Monitoring Si during Feed and Bleed, Etch Process

B. PAN Tungsten (W) Etch

For a well-controlled selective etch of aluminum over Si or SiO2, PAN (Phosphoric-Acetic-Nitric acid) is commonly used. PAN is also considered for the W etch-back in the 3D NAND process. Similar to Aluminum etch, W oxidizes in the nitric acid forming a by-product W(NO3)x, which dissolves in the phosphoric acid. Acting as a wetting agent, the acetic acid in PAN facilitates the etch process by removing the H2 by-product.

Over the lifetime of the PAN solution, the concentration of H3PO4 increases due to the evaporation loss of the Nitric/Acetic/H20. To maintain a stable and consistent etch rate of W, the H3PO4 concentration must be controlled. Figure 5 illustrates the consequences of inconsistent etch of W where 3D NAND devices will short when W under-etches. At ECI, we have developed an on-line automated chemical management system that accurately monitors and controls the components of PAN. In a spiking experiment, different concentrations of H3PO4 were added into the bath. The system accurately measured the H3PO4 component concentrations as shown in Figure 6.

Figure 5. Tungsten (W) under and over-etch

 

Figure 6. PAN spiking experiment showing matched results of measured and expected

Conclusion

The demand for a higher bit density in 3D NAND will continue to push the limits of the fabrication process and stack height. The monitoring and control of the process becomes critically important as the number of stacking layers increases. In this paper, we presented the results of real-time on-line automated solutions in accurately monitoring and control Si3N4 and W etch.

References

[1] J.H. Jang, H.S.Kim, W.Cho and W.S.Lee, “Vertical cell array using TCAT(Terabit Cell Array Transistor) technology for ultra-high density NAND flash memory,” IEEE Symposium on VLSI Technology, page 192-193 2009.
[2] ECI Technology, Inc. Press Release, “Quali-Surf Qualifies in Japan and Taiwan Fabs”, Totowa, NJ, Feb 2, 2012.
[3] C. N. Bai, G. Liang, E. Shalyt, “Metrology for High Selective Silicon Nitride Etch”, Solid State Phenomena, Vol. 255, pp. 81-85, 2016.

Metrology for High Selective Si Nitride Etch

Silicon Nitride etch has been a building block of Semiconductor manufacturing for many years. The overall Si etch rate is dominated by the combination of process temperature and %H2O. Selectivity is controlled by Si level. Water content can be monitored through conductivity, refractive index, or the most preferred method, non-contact Near Infrared (NIR) spectroscopy. There is a variety of commercial analyzers designed for this purpose.

The main challenge is measurement of Si. We have previously described an automated method for analysis of Si in traditional Si3N4 etching solution. However, high selectivity processes require new solutions.

H3PO4 and H2O Measurement

H3PO4 and its counterpart H2O are measured by both NIR spectroscopy and conductivity methods. Table 1 summarizes the performance of the two methods.

Table 01

Comparison of H3PO4 results between on-line automated NIR method and off-line ICP-MS:

The on-line results are comparable to those of ICP-MS, but with much better time response (<5min) and automated sampling/feedback (lab analysis by ICP-MS can take several weeks with fab logistics.)

figure 02

 

figure 03

 

Conductivity

Conductivity represents mobility of the ions when under the driving force of an electrical field and is highly sensitive to temperature. Modern temperature control devices enable efficient temperature control so that the effect of temperature is greatly suppressed. The figure above shows a typical conductivity calibration curve with temperature correction, which has a good correlation with H3PO4 concentration.

Si Measurement

Silicon is measured by adding predetermined concentrations of Flouride ions to a predetermined amount of etchant solution, and measuring the potential of a Flouride Ion Specific Electrode (FISE) in this test solution. Under ideal conditions, the potential (E) of a FISE is given by the well known Nernst equation:

E = E0 – (2.303 RT/F) log [F–]

Si Measurement in Low Temperature Etch

Si is measured in a wet bench low temperature hot phosphoric etch process.

figure 04

 

Organosilicate Measurement

  • Reagent with Carboxylic acid added improves the sensitivity.
  • Sensitivity is further studied with various fractions of Acetic acid in the reagent
figure 05

 

figure 06
  • The accuracy of this method with Carboxylic acid in the reagent is evaluated by off-line ICP-MS method. The results from this improved Flouride method match those from ICP-MS.
  • Good stability of Organosilicate results by the same method with Carboxylic acid in the reagent.
  • All measured results have an accuracy of <2%.
figure 07

 

figure 08
figure 08

 

Conclusions

A variety of methods have been developed to measure the Silicon Nitride etch process bath in realtime. Results from these analyses can be used for tight process control to achieve high selectivity for Silicon Nitride removal.

References:

[1]S.J. Baffat, M.S. Lucey, M.R. Yalamanchilli “Hot Phosphoric Acid APC for Silicon Nitride Etch”, Semiconductor International, 8/1/2002
[2]Hong et al. “Compositions for Etching and Methods of Forming a Semiconductor Device Using the Same”, US Patent 9,136,120
[3]Cho et al. “Etching Composition and Method for Fabricating Semiconductor Device Using the Same”, US Patent 8,821,752
[4]Nowling et al. “Low Temperature Etching of Silicon Nitride Structures Using Phosphoric Acid Solutions”, US Patent 8,716,146
[5]Shalyt et al. “Analysis of Silicon Concentration in Phosphoric Acid Etchant Solutions”, US Patent 8,008,087
[6]E. Shalyt, G. Liang, P. Bratin, C. Lin “Real-Time Monitoring for Control of Cleaning and Etching Solutions” Proceeds of SPWCC Conference, USA, 2007
[7]Shalyt et al. “Analysis of Silicon Concentration in Phosphoric Acid Etchant Solutions” US Patent Application 20160018358

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