ISOTROPIC ETCHING OF THE SILICON NITRIDE AFTER FIELD OXIDATION.

Transcription

1 ISOTROPIC ETCHING OF THE SILICON NITRIDE AFTER FIELD OXIDATION. A.J. BALLONI - Fundação Centro Tecnológico para Informática/ Instituto de Microeletrônica Laboratório de Litografia C.P Campinas/S.P. ABSTRACT. The goal is to present an isotropic Si3N4 dry etch process developed to get a very high selectivity towards thermal oxide. The dry process was developed in the Matrix Downstream Isotropic Etcher. As test wafers, Si wafer covered with 1000 A of thermal oxide and 1500 A Si3N4, and Si wafers covered with A of thermal oxide were used. The average etch rate and uniformity are 1450 A/min and 1.5 and 75 A/min 12.0 for silicon nitride and thermal oxide, respectively. A third Si test wafer, half wafer with 1500 A Si3N4 and half with 5500 A field oxide was also used in order to get an optimum time for the breakthrough step. As methodology, the RSM was used and a selectivity (Si3N4/SiO2) higher than 20 was obtained. Key words: isotropic128?isotropic silicon nitride129?silicon nitride field oxidation130?field oxidation plasma etchin This work has been realised at the IMEC, vzw-kapeldreef, 75/B Leuven BELGIUM as part of a 9 months research program (focused on metal, polysilicon, silicon nitride resist strip dry etching steps as well all concerned methodology: Design of Experiments and Response Surface Methods) required to the development of a Double Level Metal (DLM) process at CTI/IM-BRAZIL. This work was supported by RHAE,/CNPq and IM/CTI. I - INTRODUCTION. Silicon nitride films are amorphous insulating materials that find three main applications in VLSI fabrication: 1) as final passivation and mechanical protective layers for integrated circuits, especially for parts encapsulated in plastic packages; 2) as mask for the selective oxidation of silicon (as presented in this work); and 3) as a gate dielectric material in NMOS devices [0 1 ]. Silicon nitride (Si3N4) is suitable as masking layer for selective oxidation (LOCOS - LOCal Oxidation of Silicon/field oxide), since it is a good barrier material against oxygen diffusion [02] The experimental set-up utilised a was Matrix Downstream Isotropic Etcher, and a very high selectivity Si3N4/SiO2, using an NF3/He plasma, was got. In this system, the wafer is submitted to the afterglow of NF3/02 plasma [03]. The etch process followed by monitoring a suitable end point trace with OMA (Optical Multichannel Analyser), showed that the best emission lines was NO-emission (etch product) at 379nm [03]. Other regions of the optical spectrum were also investigated. Finally, as methodology, the simulator Project and Experiment & Response Surface Methods (RSM) [04] was used and Si3N4:SiO2 selectivity higher than 20:1 was got.

2 II - HIGH SELECTIVITY NITRIDE. Using the Matrix 303 Downstream Isotropic Etcher and, as methodology, the Simulator Project and Experiment - Response Surface Methods (RSM) [04], a process to etch silicon nitride with very high selectivity to the thermal oxide was developed. Three different 5 (five) inch silicon test wafers were used to carry out the experiment : 1. One Si wafer covered with 1000A of Thermal Oxide and over it 1500A silicon nitride, 2. other with 10000A of thermal oxide. With these two the bulk etch (PROC1) with high selectivity was developed. 3. The third Si wafer: half wafer with 1500A of Si3N4 and half with 5500 A of field oxide, was used to develop the breakthrough step. Note that in this wafer the half Si3N4 was exposed to the field oxide growth, therefore there is a thin alloy SiO2/Si3N4 (SixNy) in it. The best process (PROC1.: P=800, NF3/55, 02/45, P=45W and T=50C) was found with RSM simulation. Before applying the process PROC1. we must use a stabilisation time and breakthrough etch [04]. The results, presented in figures 1 and 2 show a very good fit of the experimental results. Further the chuck temperature was decreased to 20 C to improve the selectivity, c.f. figure 3. PROC1.: BULK ETCH. The process PROC1. was developed with electrode and chamber temperature T=50 C. The figure 2 shows the main trends obtained with the RSM simulation. The results are presented for both T=20 C and T=50 C. T = 20 C (electrode and chamber): average Si3N4 etch rate = 1400 A/min average Si3N4 uniformity = 2.3 average SiO2 etch rate = 53 A/min average SiO2 uniformity = 15.5 average Si3N4/SiO2 selectivity = 26. T = 50 C (electrode and chamber): average Si3N4 etch rate = 1460 A/min average Si3N4 uniformity = 1.5 average SiO2 etch rate = 75 A/min average SiO2 uniformity = 12 average Si3N4/SiO2 selectivity = 20. III - CONCLUSION. Selectivity Si3N4/SiO2 higher than 20 was obtained. Working at pressures higher than 850, the process window changes completely, losing its high selectivity, therefore, the results presented in figure 2 shows a process window very well defined. Finally, the main typical process trends got with step 3 are:.increasing the pressure increases the selectivity, c.f. figure 1 and figure 2..increasing the NF3 flow increases the Si3N4 etch rate, cf. figure 1..Increasing the 02 flow decreases the SiO2 etch rate. The Si3N4 etch rate remain constant, figure 2..Increasing the power increases the etch rate, but decreases the selectivity [03]..Increasing the electrode temperature decreases the selectivity, cf. figure 3.

3 REFERENCES. [01] - S. Wolf and R.N. Tauber, "Silicon Processing for V-LSI ERA", V. 1, Lattice Press, pag 191 and 556 (1987) and, - Daniel L. Flamm, "Introduction to Plasma chemistry", Lattice Press, pag.165 (1987). [02] - P.W. Bohn et all, "A multiresponse Factorial Study of reactor parameters in PECVD growth of amorphous Silicon Nitride" J. Electrochem. Soc. 132,1981(1985). [03] - IX Brazilian Microelectronic Congress - SBu /R.J. -RJ, pag.258 august (1994) [04] - Richard Booth and Luc Dupas "DOE - design of experiments and RSM - Response Surface Methods", Internal publication, IMEC, June (1992).

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6 NF3 - [20, 60] sccm Uniformidade do Si3N4 - [1, 35] lâminas O2 - [30, 70] sccm uniformidade do SiO2 - [7, 40] P - [30, 70] Watts Seletividade SiO2/Si3N4 - [8, 13] p - [650, 850] NF3 - [30, 70] sccm Uniformidade do Si3N4 - [2, 20] lâminas O2 - [40, 50] sccm uniformidade do SiO2 - [6, 40] P - [30, 70] Watts Seletividade SiO2/Si3N4 - [10, 14] p - [650, 850] NF3 - [45, 65] sccm Uniformidade do Si3N4 - [1.4, 2] lâminas O2-40 sccm uniformidade do SiO2 - [8, 11] sem janela de P - 40 Watts Seletividade SiO2/Si3N4 - [14, 18] processo p - [650, 850] NF3 - [35, 55] sccm Uniformidade do Si3N4 - [1, 2] lâminas O2-40 sccm uniformidade do SiO2 - [7, 9] sem janela de P - 40 Watts Seletividade SiO2/Si3N4 - [15, 19] processo p - [550, 650] NF3-55 sccm Uniformidade do Si3N4 - [1, 1.7] lâminas O2 - [30, 50] sccm uniformidade do SiO2 - [7, 15] com janela de P - 40 Watts Seletividade SiO2/Si3N4 - [15, 20] processo p - [650, 850] NF3-55 sccm Uniformidade do Si3N4 - [1, 10] lâminas O2 - [30, 40] sccm uniformidade do SiO2 - [9, 11]

7 sem janela de P - 40 Watts Seletividade SiO2/Si3N4 - [14, 17] processo p - [750, 850]