Memory effect in silicon nitride deposition using ICPCVD technique
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In this study, a plasma-based low-temperature, low-pressure SiN film deposition is investigated for device applications. Ammonia, nitrogen and silane are being used for optimization of the quality of SiN film for device passivation by ICPCVD. Characterization of SiN film is done using elastic recoil detection analysis, AFM, FTIR and ellipsometry. The effect of previous process parameters on subsequent process is called memory effect, which has been investigated by all the characterization techniques. During deposition, this effect has been observed for the same parameters that are used to maintain the stoichiometry of the film. It has been observed that some of the residues of gases used for SiN deposition remain present even after the deposition in the chamber and are carried over for the next deposition process and alter the film property, though parameters such as flow rate, temperature, pressure and time remain fixed. This memory effect alters the film surface roughness and stoichiometry thus affecting device characteristics after passivation.
KeywordsSilicon Nitride (SiN) ICPCVD PECVD ERDA HEMT Current collapse
The silicon nitride dielectric films are being used widely for electrical isolation, mask fabrication and as protection layers in semiconductor device technology . The important and crucial application of these plasma-deposited silicon nitride dielectric films is for passivation of devices . These films can be deposited with different composition and by different techniques as per the requirement. Silicon nitride (Si3N4) is being used in GaAs HEMTs (high-electron mobility transistors) and GaN HEMTs-based MMICs (microwave monolithic integrated circuits) for device passivation. These films reduce the surface trap density of HEMT structure by reducing the surface states at the surface, which is the source of current collapse or knee walkout problems [3, 4]. Silicon nitride (SiN) film composition and quality affect the performance of the device . There are a number of studies reported in the literature for GaN HEMT gate passivation model using PECVD SiN to reduce reverse leakage current [6, 7]. There are very few studies that report results of ICPCVD passivation effect. Chao et al.  have reported recently the impact of stress in SiN film deposition by ICPCVD. Memory effect is being very crucial for repeatability and reproducibility of plasma process, but no report is yet published of ICPCVD passivation to the best of our knowledge. The present study reports the detailed systematic study of memory effect of ICPCVD process for SiN passivation [9, 10]. The main aim of the present study is to develop a process free from memory effect to mitigate the problem of current collapse/knee walkout in in-house fabricated GaN HEMTs.
The characterization of deposited silicon nitride is carried out using ellipsometric measurement for refractive index and thickness, FTIR for bonding strength and AFM for roughness of the deposited film, ERDA (elastic recoil detection analysis) for investigating quantitatively the proportion of silicon, nitrogen and hydrogen present in the deposited films [14, 15].
Elastic recoil detection analysis (ERDA) is a technique specially suited for depth profiling of light elements, which overcomes the limitations of Rutherford backscattering (RBS) technique .
High-energy heavy ions can induce structural and compositional changes in the material through which they pass. The study of high-energy ions induced compositional changes in materials can be carried out using ERDA. In our case, a telescope detector was used to identify different recoils originating from a sample. Mostly, it comprises two detectors named as ∆E and Erest sub-detectors and therefore called as ∆E−E detector telescope. The recoils entering the detector lose a fraction of their energy in ∆E detector and the rest of it in Erest detector. Energy lost in the first detector was found to be proportional to MZ2, where M and Z are the mass and atomic number of the recoil particle, respectively, which allows for the identification of elements.
Result and discussion
Refractive index measurements show variation in index for the first sample to third sample, which was grown one after another under the same conditions. The refractive index was found to vary from ~ 1.89 to ~ 2.0, while thickness was almost ~ 100 nm. The effect on the refractive index is attributed to the foreign species like carbon and oxygen present in the chamber when earlier depositions have been carried out.
The refractive index and thickness are measured by laser ellipsometer using laser radiation of 632 nm wavelength. In our case, the refractive index and the thickness of Si3N4 thin films deposited on GaAs wafers are measured. For sample A, the measured values for refractive index and thickness are ~ 1.89 and ~ 1050 Å, respectively. For the next two samples, all the deposition parameters are kept the same and clean GaAs wafers are used for silicon nitride deposition. The variation in refractive index and thickness is determined. Sample B is found to have refractive index ~ 1.95, whereas sample C had refractive index ~ 2.0. Slight change in thickness is also observed with 1150 Å for sample B and 1100 Å for sample C.
We have presented a systematic experimental analysis of silicon nitride films deposited by ICPCVD technique. The study shows variation in the composition and quality of the films even with the same deposition parameters. This may be attributed to the residue remained in the deposition chamber and which affect the very quality of the film until stabilized. Elliposemetric measurements have suggested about the film refractive index and thickness, which can be clearly related to density of the film. Lower refractive index films are found to have higher etch rate compared to high refractive index films. ERDA measurement and calculations confirmed the percentage of hydrogen content in the films, and the variation in the refractive index was verified by ellipsometric measurements results. This hydrogen content is initially high, whose incorporation may be attributed to the presence of residuals in the chamber during deposition. Hence, conditioning and stabilization of silicon nitride films are necessary before it can be used for any device application.
The authors acknowledge the characterization and MMIC teams from SSPL for their help. We also are thankful to Inter University Accelerator Center (IUAC), Delhi, for allowing us to carry out ERDA experiments.
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