Polysilicon sacrificial layer etching using ClF3 for thin film encapsulation of silicon acceleration sensors with high aspect ratio

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Abstract

We present a new thin film encapsulation technique for surface micromachined sensors using a polysilicon multilayer process. The main feature of the encapsulation process is that both the sacrificial layer above the silicon sensor structure and the cap layer consist of epitaxial polysilicon. The sacrificial layer is removed by chlorine trifluoride (ClF3) plasmaless gas-phase etching through vents within the cap layer. The perforated cap membrane is sealed by a nonconformal oxide deposition. The method has been applied to a silicon surface micromachined acceleration sensor with high aspect ratio structures, but is broadly applicable. Capacitance–voltage measurements have been performed to show the electrical functionality of the accelerometer.

Introduction

Microelectromechanical devices with movable structures e.g. acceleration and yaw rate sensors are susceptible to damage or contamination after wafer processing is completed. For example, die sawing and subsequent cleaning will cause damage to the fragile structures if they are not protected. Furthermore, a durable protection is imperative to enable standard handling and packaging or if a vacuum environment is needed. It is highly advantageous if the sensors are encapsulated during wafer processing (zero level packaging). For encapsulation in most cases a bulk micromachined silicon cap wafer is generally used. The cap wafer is commonly bonded to the sensor wafer with a glass frit [1]. Usually a broad sealing frame around the sensor structure is needed to guarantee hermetical sealing, enlarging the die size.

Encapsulation methods using a thin film microshell above the sensor have the potential to significantly reduce the die size and chip thickness, which is attractive for cost reduction and innovative packaging concepts. Several thin film wafer level encapsulation processes have already been demonstrated. Lin et al. applied a thin LPCVD nitride layer as microshell material [2], [3], HF permeable polysilicon has also been used as membrane material [4], [5], [6]. Thereby the focus was on vacuum encapsulation of resonant devices rather on the mechanical stability of the cap. Partridge et al. [7] describes a thin film encapsulation process featuring a thick epitaxial polysilicon cap layer. In this work the polysilicon membrane was designed for mechanical stability in order to withstand the large mechanical stress during standard plastic packaging.

A common feature of all mentioned techniques is the use of a thick sacrificial oxide between the cap layer and the sensor structure in addition to the common sacrificial layer beneath the sensor structure. The movable sensor structure is released by removing the upper and the lower sacrificial oxide with HF [3], [4], [5], [6], [7], [8]. With respect to cost and yield optimization in high volume production oxide refill techniques have disadvantages: (A) the intrinsic stress of the few micron thick refill oxide layer (upper sacrificial layer) has to be carefully adjusted in order to avoid high wafer bow or cracking of the film especially at high temperatures during subsequent processing e.g. membrane deposition [9]. (B) The high oxide thickness and the moderate etch rate leads to a long process time for sacrificial oxide etching. (C) During gas-phase sacrificial etching residues in the form of particles are created that have a negative impact on the sensor quality. The origin of the etch residues is mainly attributed to contaminants within the oxide, e.g. carbon or nitrogen incorporation during PECVD deposition. (D) Due to the limited oxide thickness the distance between the functional sensor structure and the membrane is limited to a few microns in vertical as well as in lateral direction. Thus, the lateral sensor deflection is restricted leading to a reduced design flexibility especially for gyroscope.

In the following an alternative encapsulation technique using a polysilicon refill instead of the upper sacrificial oxide layer is described. The schematic structure of an on-chip encapsulated surface micromachined acceleration sensor used in this study is shown in Fig. 1. In this setup the sensor structure, the upper sacrificial layer and the cap layer consist of epitaxial polysilicon. Thus the functional layers (sensor structure and cap) have to be coated with a protective oxide layer to withstand sacrificial etching of the refill polysilicon. The polysilicon refill technique is advantageous over the oxide refill: (A) the polysilicon refill layer can be deposited with high deposition rates up to a thickness of 20 μm or even more. Intrinsic stress can be controlled in a wide range [10]. (B) Sacrificial polysilicon etching can be performed using ClF3 or XeF2 [11] with high etch rate. (C) The polysilicon refill can be removed with less residues. (D) Sensor designs with large lateral amplitudes can be realized because large gaps in the sensor structure can easily be refilled with polysilicon with a thickness exceeding the thickness of the sensor structure.

The fabrication process is explained in detail in Section 2. In Section 3 electrical results on demonstrator chips are presented.

Section snippets

Fabrication

The fabrication sequence of the thin film encapsulation technique is shown in Fig. 2. Starting form a standard 150 mm silicon substrate, a thick thermal oxide is grown as shown in Fig. 2a. The oxide acts as an insulator on the one hand and as a sacrificial layer to be partially removed for structure release on the other hand. A lithography step defines contact holes through the oxide layer. For simplicity the buried interconnecting layer as described in Ref. [12] are not shown in Fig. 2.

In the

Results from electrical measurements

Electrical measurements have been performed on wafer level to show the functionality of the encapsulated accelerometer.

The electrical isolation between the contact pillars and the surrounding epipoly layers has been investigated by means of current–voltage measurements. A high isolation resistance of >600 MΩ is measured at room temperature. The series resistance of the contact pillars is about 25 Ω.

In capacitance–voltage measurements static C/V properties of the sensor have been investigated. The

Conclusion

A silicon surface micromachined accelerometer with high aspect ratio structures has been successfully encapsulated using a polysilicon multilayer process. One remarkable feature of the technology is the use of polysilicon for functional and sacrificial layers. The sacrificial layer has been removed by plasmaless gas-phase etching with chlorine trifluoride (ClF3). The structure has been sealed by a nonconformal oxide deposition in order to enclose a partial vacuum inside. The functionality of

Acknowledgements

The authors would like to thank their colleagues at the Sensor Technology Center, Robert Bosch GmbH, Reutlingen (Germany) for their technical assistance and helpful discussions. They would like to thank the cleanroom staff for their cooperation. Further, they thank Franz Lärmer and Klaus Breitschwerdt of the Robert Bosch Research Laboratory for providing the ClF3 etch process. The authors are also grateful for the support within the SUMICAP project by the European Commission (IST-1999-10620).

Lars Metzger was born in Albstadt Ebingen, Germany, in 1972. He received his Master of Electrical Engineering at the University of Stuttgart, Germany in 1999. He is currently working towards his PhD in the Sensor Technology Center at Robert Bosch GmbH in Reutlingen, Germany in cooperation with the University of Technology Aachen, Germany. His research interests focus on gas-phase silicon etching with fluorine-containing etchants for MEMS applications.

References (19)

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Lars Metzger was born in Albstadt Ebingen, Germany, in 1972. He received his Master of Electrical Engineering at the University of Stuttgart, Germany in 1999. He is currently working towards his PhD in the Sensor Technology Center at Robert Bosch GmbH in Reutlingen, Germany in cooperation with the University of Technology Aachen, Germany. His research interests focus on gas-phase silicon etching with fluorine-containing etchants for MEMS applications.

Frank Fischer was born in 1968. He received his Master of Material Sciences at the University of Erlangen-Nürnberg in 1994. In 1998 he finished his PhD on short wavelength semiconductor laser diodes in experimental physics at the University of Würzburg. In 1998 he joined the sensor technology center of Robert Bosch GmbH in Reutlingen where he is responsible for the development of alternative processes for surface micromachining technology.

Wilfried Mokwa was born in 1951. He obtained the Diploma degree in physics from the Rheinisch-Westfälische Technische Hochschule (RWTH) Aachen in 1977 and the Dr. rer. Nat. degree in 1981 at the same place. From 1981 he worked at the two Phys. Institute RWTH Aachen on catalytic reaction on gas sensor surfaces and joined the Fraunhofer Institute of Microelectronic Circuits and Systems in Duisburg in 1985. There he managed a group working on integrated silicon sensor technology. In 1996 he became a full professor in Electrical Engineering at the RWTH Aachen where he is head of chair 1 of the Institute of Materials for Electrical Engineering with special interests in the field of integration of microsystems.

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