Книги2 / 1988 Kit Man Cham, Soo-Young Oh, John L. Moll, Keunmyung Lee, Paul Vande Voorde, Daeje Chin (auth.) Computer-Aided Design and VLSI Device Development 1988

11.3 Modified WCOS

One way to reduce the oxide encroachment (bird's beak) is to grow a thick field oxide and then etch back to a thinner thickness. The idea can be understood by looking at a field oxide profile generated by a two dimensional oxidation program SUPRA, as shown in Fig. 11.2. It can be seen from the profile that the bird's beak is long but the initial slope is quite flat. Etching away 200 nm to 300 nm of field oxide can hence reduce the bird's beak significantly. In this section, an optimum design of this etch back LOCOS process using one-dimensional process simulation program SUPREM and twodimensional simulation program SUPRA will be discussed. The goal is to minimize both the boron and oxide encroachments.

The original process uses a stress-relief oxide thickness of 40 nm, a nitride thickness of 200 nm and a boron field implant dose of 5E13 cm-2 to obtain a final field threshold voltage of about 15 V. 800 nm of field oxide is grown at 950 deg C in steam for 300 minutes and then etched back to 500 nm. Fig. 11.8 shows the simulated final field oxide and doping profile from SUPRA. The vertical line indicates the position of the bird's beak before etch back. The oxide encroachment is reduced by about 0.28 microns and the active area is about the same as originally defined by the nitride. However, the boron encroachment effect is quite significant. The dotted line in Fig. 11.8 shows that due to field implant the boron concentration line of 1E17 cm-3 is quite a distance into the active area. Simulation of the active area boron concentration due to threshold and punchthrough control implant is plotted as a function of distance into the silicon substrate in Fig. 11.9. The surface boron concentration is about 8E16. Therefore, a significant part of the active area has a higher boron concentration due to the lateral diffusion of the boron field implant. In this case, the boron encroachment actually dominates over the oxide encroachment. The effective channel width is not much improved by this etch back process.

In order to solve the boron encroachment problem the process is modified and simulated again. A thinner field oxide is grown so that both the thermal cycle and the field implant boron dose can be reduced. In the modified process, 600 nm of field oxide is grown and then etched back to about

500 om. SUPREM is used to find the oxidation cycle for 600 om of field oxide and the boron dose needed to give the same field threshold voltage of 15 V. It is found that the field implant dose can be reduced from 5E13 to 3E13 cm-2 and the thermal cycle can be reduced to 200 minutes at 950"C. SUPRA is then used to simulate the entire new process. The resulting field oxide and doping profile is shown in Fig. 11.10. The field oxide encroachment in this case is about 0.15/Am more than in the previous process. However, the boron encroachment is about 0.13 /Am less, using the 1E17 cm-3 boron line as a reference. Overall, there is a gain of 0.1 /Am over the previous process. Hence, in the design of an optimum LOCOS process, both the oxide and boron encroachment need to be carefully taken into account.

0.7

(micron)

Fig. 11.10. Oxide and impurity profile for modified LOCOS.

11.4 Side Wall Masked Isolation (SWAMI)

The problems of oxide and boron encroachments associated with LOCOS must be minimized in VLSI. New isolation techniques have to be designed such that the considerations discussed in the introduction are satisfied in order to achieve optimum circuit performance. A new isolation process, SWAMI (Side Wall Masked Isolation), has been developed at Hewlett-Packard Laboratories [11.8]. This process effectively eliminates the bird's beak during field oxidation while maintaining a desirable field oxide thickness.

Fig. 11.11 shows the sequence of steps in the fabrication process used to form the SWAMI isolation structure. For more detailed descriptions, the reader is referred to the reference article. The objectives of the process are to prevent field oxide encroachment into the active area by using a second layer of thin nitride on the side wall and to reduce stress by using a sloped side wall. The sloped side wall is used to prevent defect formation due to volume expansion induced stress during field oxidation. SEM cross-sectional view of the isolation structure did not reveal any bird's beak formation in the active area. The sloped side wall is produced by C2F6 plasma etching [11.8], which introduces a loss of island width of about 0.1 p,m per side. Another factor that might contribute to the channel width loss is the boron encroachment, since a field implant step is necessary to eliminate the problem of field inversion, especially at the sloped side wall. In this section, simulations will be used to compare the performance of narrow width devices using an ideal vertical isolation, the SWAMI, and LOCOS. The effect of boron encroachment for the SWAMI process is also simulated.

To simulate the SWAMI oxide structure, a program with the capability of simulating generalized isolation structures is needed. This can be done by the combination of SUPRA and SOAP. The SOAP program is used to generate the isolation structure. The output data file is then loaded into the SUPRA file for the impurity profile simulations. Fig. 11.12 shows the isolation structure generated by SOAP for the SWAMI process. The sloped silicon side wall and the nitride layer covering the side wall have significantly reduced the oxide encroachment into the active area, although there is still a bird's beak formed at the island edge. Since SEM pictures have shown experimentally that there is essentially no bird's beak at the island edge, the oxide growth near that region is overestimated by the SOAP program. One possibility is the effect of stress in that region which might have reduced the oxygen diffusivity [11.17].

The SOAP program also simulates the stress along the oxide/silicon interface as a function of time during oxidation. This is useful if one is concerned about the stress induced by the isolation structure. In the case of SWAMI, stress may be a concern at the island edge covered by the nitride, where volume expansion induced stress occurs during field oxidation. Fig. 11.13 shows the stress calculations using the SOAP program for the

SWAMI process. Here positive stress means compression and negative stress dilation. The stress calculated is after 0.5 p.m of field oxide is grown. The horizontal axis corresponds to the horizontal distance from the left edge of the structure. The results show that stress occurs along the sloped side wall covered by the thin nitride. As oxide is grown, stress is induced by volume expansion. The results also show that stress can be reduced by using higher oxide growth temperature. This is due to the lower viscosity of the oxide, hence more flow is possible to reduce the stress.

In order to see if boron encroachment is a problem in SWAMI, the impurity distribution has to be simulated. This can be done using the SUPRA program. The program accepts the data file from the SOAP program which defines the final oxide structure. The SUPRA program then calculates the impurity distribution associated with the oxide boundary conditions and the temperature cycle of the field oxidation. Then the channel implants and subsequent process steps can follow. Fig. 11.14 shows the simulated structure with the impurity contours without the channel implants. This shows the extent of the boron distribution due to field implant. From this figure, it can be seen that the boron concentration under the active area is not high, in comparison with typical channel implants, especially for submicron devices (Fig. 12.19). The effect on the threshold voltage and channel width narrowing should be only slight.

Since the SOAP program overestimates the oxide encroachment, the device performance simulated by using this structure will not be accurate. The electrical effects of the oxide shape can be approximated by the GEMINI program. If the depletion region of the device does not extend too deeply into the substrate, then it is only necessary to approximate the side wall of the oxide structure. The impurity profile of the structure can be first simulated by SOAP-SUPRA combination, then copied into the GEMINI input file by using a Gaussian profile approximation. This procedure is somewhat lengthy, but should provide better agreement with experiments.

Fig. 11.15 shows the simulation of the SWAMI structure for a MOSFET with channel width of 2 J.'m. Only half of the device needs to be simulated due to symmetry. The GEMINI simulation provides a good representation of the upper part of the oxide structure, which determines most of the narrow width