FOUP material influence on HF contamination during queue-time

Transcription

1 Solid State Phenomena Online: ISSN: , Vol. 219, pp doi: / Trans Tech Publications, Switzerland FOUP material influence on HF contamination during queue-time Julien Bounouar 1, Thi-Quynh Nguyen 1, Olivier Le-Barillec 1, Arnaud Favre 1, Emmanuelle Veran 1, Ingo Stassen 2 and Astrid Gettel 2 1 adixen Vacuum Products: 98, Avenue de Brogny - BP 2069, Annecy Cedex - France 2 GlobalFoundries: Wilschdorfer Landstrasse 101, Dresden - Germany Keywords: AMC, HF, FOUP, contamination, cleaning, low absorbing material, monitoring, queuetime, outgassing, acid, desorption, adsorption. Introduction Airborne Molecular Contamination (AMC) concentrations become critical during queue-time (between two successive process steps) when wafers are degassing inside Front Opening Unified Pod (FOUP), a confined environment [1]. In that case, AMC concentrations can reach maximum level. In order to limit AMC effect, focus has first been given to moisture control between process steps, leading to the development of a new FOUP generation, made of low absorbing material. This material has better performances compared to standard polycarbonate (PC) [2]. But, as today s major AMC concern is Hydrofluoric acid (HF), new developments are needed to assess the benefit of such material with respect to HF acid challenge. First studies based on HF contamination control have been insufficient to fully understand queue-time behavior [3]: FOUPs were intentionally contaminated by a limited HF source (10µL liquid droplet) that is not representative of real wafer outgassing dynamics. Moreover, HF measurement was based on ion chromatography requiring a longer time for sampling and analysis. In this paper, we will present a new experimental protocol designed to contaminate the FOUP in a controlled way. Then based on this protocol, we have studied the variations of HF concentration depending on: 1. FOUP type, 2. FOUP material memory effect, 3. Wafer storage time. Finally, wet cleaning effect on different types of FOUP has been evaluated. Experimental protocol Four different FOUPs have been investigated: FOUP_STD1 made of standard material n 1 FOUP_STD2 made of standard material n 2 FOUP_LOW1 made of low absorbing material n 1 FOUP_LOW2 made of low absorbing material n 2 Etched Si wafers have been stored inside FOUPs in order to contaminate them with HF. Prior to this, the Si wafers (300 mm) were etched by CF 4 plasma (100 sscm CF 4, 100 sscm O 2, 100 mt, 400 W, 60 sec.) using Lam 2300 Exelan Flex Chamber tool. Fastest and more accurate measurements have been done using dedicated AMC monitoring tool (adixen Vacuum Products APA302) equipped with HF analyzer option. This analyzer is based on Cavity RingDown Spectroscopy and can detect HF from 0.5 to 1000 ppbv after only 2 minutes of measurement. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-10/05/16,12:43:09)

2 252 Ultra Clean Processing of Semiconductor Surfaces XII FOUP memory effect: wafers were removed from the FOUP after a short time (1 hr) or a long time (24 hrs) of contamination. Then, HF was monitored 2 hours after wafer removal in order to evaluate FOUP outgassing (fig 1). FOUP cleaning effect: FOUPs in production are usually cleaned before starting a new production loop. This test consists of measuring residual contamination after cleaning, depending on FOUP material and wafer storage time (fig 2). FOUPs were cleaned by spraying DIW at 60 C during 20 minutes, followed by 15 minutes spin drying (Ilios FOUP - Cleaner by Lotus Systems). Queue-time calculation is started as soon as the FOUP door is closed (after etch, wafer removal or FOUP cleaning). It corresponds to HF accumulation time inside FOUP. Results Protocol validation: HF concentration is not influenced by etch time: HF concentration increase is proportional to the quantity of wafers (tab.1). Protocol validation was done by repeatable contamination tests on 10 FOUP_STD1. Average HF concentration inside the FOUP 1h after wafer storage is ± ppbv (1σ). Wafers outgas very fast and strong during the first storage hour (fig. 3). FOUP material influence on HF concentration during wafer storage: Fig. 3 describes HF concentration inside the FOUP as a function of queue-time for 4 types of FOUP during at least 18 hours. HF concentration decreases as queue-time increases for all types of FOUP. The conclusion discussed hereafter is that diffusion inside the FOUP material has to be considered even for low absorbing material. FOUPs with low absorbing material present the strongest contamination. FOUP memory effect after wafer storage: HF concentration inside empty FOUP 2 hours after wafers removal is summarized in tab. 2, which demonstrates a dependency on the FOUP material. Low absorbing material always leads to higher contamination in the FOUP atmosphere. Regarding wafer storage time effect, HF levels are similar after short wafer storage time (1 hr) and after long wafer storage time (24 hrs). DIW cleaning evaluation: HF is still detected inside the FOUP after DIW cleaning. FOUP which have experienced a long contamination display a higher HF residual concentration for both types of FOUP (fig. 4). For short term contamination, no difference by material is observed, while long term contamination shows higher concentration with low absorbing material. Discussion The theory of FOUP contamination is described in the fig. 5, it can be decomposed into four successive steps: FOUP atmosphere is contaminated by wafer outgassing HF is accumulated on polymer surface by adsorption; in the case of low HF partial pressure, HF concentration in polymer surface is proportional to HF concentration in FOUP atmosphere according to Henry law: C s = S.C g (i) S is solubility of HF in polymer C s is HF concentration on polymer surface C g is HF concentration in FOUP atmosphere 2 C( x, t) C Diffusion of HF inside the polymer characterized by Fick law: D (ii) 2 t x D is diffusion coefficient C(x,t) is the local gas concentration at a position x inside material thickness and at time t

3 Solid State Phenomena Vol Desorption of HF at outer atmosphere The diffusion of HF inside polymer is proportional to diffusion coefficient and to gradient concentration of gas (equation ii). The solubility and the diffusion coefficient of gas in low absorbing polymer are lower than those in polycarbonate [2]. Thus, PC surface has stronger adsorption with respect to HF and faster migration of contaminant inside PC depth. Therefore, HF diffusion in PC could be deeper than in low absorbing material, resulting in lower HF concentration on polymer surface (fig. 6). This behavior was demonstrated by simulation [1, 4]. This leads to higher HF concentration in low absorbing FOUP atmosphere during wafer storage (equation i). In the second part of the study, FOUPs were opened during wafer removal; HF concentration in the FOUP atmosphere decreased (may go down to 0 clean room air). FOUP is closed back again. 2 hours after wafer removal, HF concentration in the FOUP ambient is significantly high (tab. 2), meaning that the FOUP polymer is outgassing. HF levels are always higher in low absorbing FOUPs because HF concentration on polymer surface is stronger. Tab. 2 shows that polymer outgassing levels in the FOUP atmosphere are almost same whatever short term (1 hr) or long term contamination (24 hrs). That means HF concentration on polymer surface could be similar after short or long contamination time. But diffusion depth should be higher in the case of long contamination (fig. 7) as observed by simulation [1, 4]. That is why, cleaning FOUPs after long contamination is more difficult than after short contamination (fig. 4), leading to higher HF level in FOUP atmosphere 12 hours post FOUP clean. Cleaning is a desorption phenomenon (reverse absorption): the faster the absorption, the faster the desorption (except for chemical absorption). Therefore, HF can be removed more quickly from PC FOUP. Detailed information about FOUP material is needed in order to better understand FOUP material behavior. Conclusion An improved qualification method has been developed with real wafer outgassing condition, enabling fast, repeatable and realistic source of contamination. Instant AMC values could be monitored using real-time measurement capacity of adixen Pod Analyzer. This method is compatible with production condition. This work confirms that FOUP material influence has to be considered not only for humidity but also for AMC reasons. FOUPs with low absorbing material always display higher HF concentration in the FOUP atmosphere, compared to Polycarbonate FOUPs, during the wafer storage step, during the empty FOUP outgassing step and after the FOUP cleaning. As the AMC level in FOUP atmosphere is related to AMC concentration on wafer surface, subsequent studies will evaluate the reverse effect of FOUP material on wafer surface contamination. Furthermore, test after DIW cleaning showed that efficiency has to be improved. Investigation may be driven towards changing DIW cleaning condition or using other decontamination method. Acknowledgment This work was performed with the framework of the ENIAC program PLACES2BE. We would like to acknowledge the ENIAC organization and the French government for their financial support

4 HF (ppbv) 254 Ultra Clean Processing of Semiconductor Surfaces XII Table 1: HF level inside FOUP with wafers Table 2: HF concentration outgassed by FOUP Q-time HF Wafer Etch time per FOUP type (hh:mm) (ppbv) quantity wafer (min) FOUP_STD1 1:02 48,3 5 2 FOUP_STD1 1: FOUP_STD1 1: FOUP_STD1 1: FOUP_STD1 1:02 94, FOUP_STD1 1: FOUP STD1 FOUP LOW1 Short contamination HF (ppbv) Long contamination 21,1 19,9 62,1 62,7 Contamination with wafers Short (1 hr) Long (24 hrs) wafer removal 2 hrs HF measurement Figure 1: FOUP contamination procedure Contamination with wafers Short (1 hr) Long (24 hrs) wafer removal 7 hrs FOUP cleaning 12 hrs HF measurement Figure 2: FOUP cleaning procedure FOUP_STD1 Short term contamination FOUP_LOW1 Long term contamination Figure 4: HF level inside FOUP post cleaning Figure 3: Kinetic of wafer outgassing inside FOUP HF concentration Figure 5: Schematic of gas transfer in polymer HF concentration Low absorbing material Short contamination Polycarbonate Long contamination Polymer thickness Figure 6: HF profile (speculative) in PC and low absorbing material Polymer thickness Figure 7: HF profile (speculative) in polymer after a short and a long contamination

5 Solid State Phenomena Vol Reference [1] TQ. Nguyen, Thesis, 2012 [2] S. Moon et al, Proceeding of ASMC 2012 [3] P. Gonzales et al, Microelectronics Engineering 2013 [4] N. Santatriniaina et al, International Journal of Applied Mathematical Research, 2014

6 Ultra Clean Processing of Semiconductor Surfaces XII / FOUP Material Influence on HF Contamination during Queue-Time /