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CRITICAL ASPECT RATIO DEPENDENCE IN DEEP REACTIVE ION ETCHING OF SILICON Junghoon Yeom', Yan Wu', Mark A. Shannon' 'Department of Mechanical Engineering, University of Illinois at Urbana-Champaign, 1206 W. Green, Urbana, Illinois 61801 2Departmentof Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, 405 N.Manews, Urbana, Illinois 61801 Te1.:(217)244-1545 , Fax: (217)244-6534, e-mail:
[email protected]
ABSTRACT Aspect ratio dependent etching and microloading effects are two mechanisms leading to non-uniformities in the etching of silicon using deep reactive ion etching technology. This paper focuses on the apparent presence of a critical aspect ratio when etching three-dimensional MEMS structures. A mask designed to separate the effects of microloading from aspect ratio dependent etching was made, and various sizes of features (3 to 1000 pm) are etched simultaneously for different etching times (10 to 180 min) using a Bosch etching process. Experimental data exhibiting three distinct regimes for the etch rate, the corresponding critical aspect ratios, and the dependence on mciroloading effects are presented. Possible mechanisms governing these regimes are also postulated. INTRODUCTION The advent of deep reactive ion etching (DRIE) of silicon using Inductively Coupled Plasma (ICP) sources and Bosch etching processes has unleashed a wave of new highaspect ratio microdevices [l]. However, in etching of high aspect ratio features, researchers have encountered several problems due to nowuniformity in the etch rate of different structures. One problem is aspect ratio dependent etching (ARDE), which refers to the phenomenon that the etch rate scales not with absolute feature sizes, but with aspect ratio. Increasing aspect ratio usually decreases etch rate. ARDE also includes bottling, bowing, and microtrenching [2]. Another problem is referred to as a microloading effect, which refers to a local dependence of etch rate on pattern area density. The closer those patterns are packed andor the greater the area to be etched in a given region, the slower the etch rate 121. The non-uniform characteristics due to ARDE and microloading effects are extremely problematic in producing three-dimensional MEMS structures when etching features a few millimeters size size in concurrence with features a few micrometer in size. The problem become acute for through-wafer etching, when large feature etches through the wafer first, causing helium gas, which is used to maintain the substrate temperature in ICP-DRIE, to leak through to the chamber, aborting further etching of the smaller featured structures.
The terms ARDE and microloading are sometimes used in ambiguous ways since in both of the phenomena the etch rate is a function of mask geometry. Gottscho[Z] et. al.,. carefully defined ARDE and microloading in their comprehensive overview on microscopic etching nonuniformities of conventional RIE systems. They pointed out that the possible mechanisms responsible for ARDE include ion shadowing, differential charging, neutral shadowing, and Knudsen transport of neutrals. As to the mechanism for microloading, it is equivalent to a macroscale loading effect, where the reactant concentration is depleted as a result of an excessive substrate load. The ARDE in DRIE systems differs from that in conventional RIE systems in that the Bosch process uses a time-multiplexed plasma (with SF6 for etching and C4Fs for sidewall passivation). The deposition of the passivation layer is deliberately segregated and the etching includes not only silicon etching but also the depassivation at the hottom of the etched structures. While characterizing ICP-DRIE etching of Si, Ayon [I] et al. presented experimental data showing the effect of A W E . Flow conductance model proposed by Cobum and Winters [3] was chosen to explain the ARDE in their experiments. In the etch rate vs. aspect ratio plot drawn by Blauw [4] et. al., a tuming point in the etch rate with aspect ratio was observed at relatively high ratios. They suggested that dimimished removal of the passivation layer at the trench bottom was responsible for the turning point. Furthermore, critical aspect ratios from 24 to 30 depending on process parameters were predicted using an ion shadowing model in their paper. More recently, Karthmen [SI et. a1 included microloading in their study of ARDE in a DRIE system. They observed that loading effects played a minor role in long time deep etching, while for a short etch time both microloading and ARDE had a significant influence on the etching rate. The purpose of this current work is to provide additional insight into the etch rate dependence on aspect ratio and microloading in an ICPDRIE system, particularly for critical aspect ratios. The targeted applications of this study are for three-dimensional MEMS devices fabrication. Therefore, instead of dealing with submicron features as most work previously done in the literature, the absolute feature sizes in this study are orders of magnitude larger (3 to 1000 pm) and the range of aspect ratios investigated is also wider (0 to 30).
TRANSDUCERS '03 The 12th International Conler0nce on Solid Stale Sensors, Actuators and Microsystems,Boston. June 8-12,2003
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Fig I d. 2 0 0 p spaced panema etched for 2 hrs
A’
B’
C’
D’
Fig IC. 100 pm spaced panems etched for 3 hm
View of mask layout and its relative position with respect to pump port in the etching chamber. Test structure A, B, C, and D each has 10 mm long and fiom 3 pm to 1000 pm width line trenches with a spacing between the lines of 25 p,50 pm, 100 pm and 200 pm respectively. Various Si loading on edge pattems: A’=80%, B’=50%, C’=30%, D=10%.
Figure 1.
EXPERIMENT The authors attempted to isolate ARDE from other factors affecting etch rates, especially the microloading effect, through the design of the etch mask. As shown in Fig. 1, a wafer was divided up into four quadrants. Each quadrant has two similar sets of patterns: one near the center and the other near the edge of the wafer. Four patterns located at the center of the wafer (Type A, B, C, D) were designed to include 10 mm long and varying (3 pm to 1000 pm) width lines. In each pattern, lines were separated equally with a spacing parameter to manifest the microloading effect due to pattern density. Spacings between the lines in the die A, B, C, D are 25, 50, 100 and 200 pm respectively. At each corner of the wafer, the same patterns (Type A, B, C, D) were laid but surrounded by different area of bare silicon to vary the area loading of each pattern. Figure 1 also illustrates the relative areas of bare silicon in each pattern to the same size of dies. The dark region is bare silicon. The largest dummy loading is around die A, which is denoted as A’,. and approximately 80% of the die area is covered with bare silicon including the pattern itself. B’ has 50% loading, C’ 25%, and finally D’ has no dummy region, making 11% loading of the pattem. By making D’ and D the same, one can investigate whether the closeness to the center of the wafer has an effect on the etch rate. Finally, two identical patterns (Type C, 100 um line spacing) are put on the each side of the wafer to distinguish the effects of tool design such as the location of pump port respect to the patterns on a wafer.
A layer of 1000 A thick aluminum film was sputtered on an n-type lightly doped silicon wafer to serve as etching mask. The designed pattern was transferred to aluminum by lift-off. The wafer was then etched in an ICP etcher using a time-multiplexed Bosch process with SF, for etching and C4F8 for sidewall passivation. Wafers made of the same pattem were etched for different etching times from 10 to 180 min. The processing parameters used throughout this work are listed in Table 1. The dimensions of etched features were measured with Hitachi S-4700 high resolution scanning electron microscopy (SEM). Each etched wafer was cleaved along the crystallographic line to reveal the cross sectional views of etched patterns. Deep etched patterns (over 400 pm deep) were glued to glass plate to support the structures, and then the structure was ground and polished to the center of the cross-sections of features. The actual values of line pattems deviated from the nominal due to the limitations of TobIe 1. Process parameters for ICP-DRIE
Parameters
Deaosition
Etching
Process time SF6 flow rate C,Fs flow rate Ar flow rate Electrode power Coil power Chamber pressure
5 sec I sccm 40 sccm
7sec 100 sccm 4 sccm 40 sccm
0
8W
850 W
850 W
22 mTom
24 mTorr
70 sccm
TRANSDUCERS ‘03 The 12th International Conferencean Solid Stale Sensors, Actuators and Microsystems. Boston, June 8-12. 2003
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__x X
I
Reaction limited
I 0
5
10
15
Aspect
20
25
30
35
Ratio
Figure 3. Normalized ER vs. AR plot for Type A, B , C, D patterns in^ the center (3 hours etching) Experimental data
Figure 2. ER vs. AR plot for Type C panem (100 pm spacing) etched for 4 different times. Three distinct regions shown: reaction rate limited, transport rate limited, and rollover
pattem transfer using a contact aligner. Therefore, after etching in DRIE system, widths and depths of all features were measured, and their actual values were used. Since feature sizes in this experiment vary from a few microns to a few hundred of microns, different magnifications were used for different feature sizes with calibration for each. Due to the destructive nature of these measurements, several wafers Born the same batch were used to investigate the .time dependence on the average etch rate.
RESULTS AND DISCUSSION The SEM pictures of the etched trench profiles for 3 hours of etching are shown in Fig. 1 (a) - (d). The etch rate dependence on the aspect ratio (AR)can be clearly seen. As AR increases, sidewalls become tapered. When the sidewalls taper to a tip, the etching completely stops. The etch rate versus aspect ratio curve for the 100 pm spacing pattem (Type C ) at the center of the wafer is shown in Fig. 2. Three distinct regimes are found in Fig. 2, corresponding to two aspect ratios. In the very low aspect ratio regime (O
