Microsyst Technol (2013) 19:757–761 DOI 10.1007/s00542-012-1681-z
TECHNICAL PAPER
Void-free trench isolation based on a new trench design Xiao-Ying Li • Guang-Tao Li • Sen Ren Da-Yong Qiao
•
Received: 16 July 2012 / Accepted: 1 October 2012 / Published online: 16 October 2012 Ó Springer-Verlag Berlin Heidelberg 2012
Abstract In this paper, in order to avoid the voids during deep trench isolation, an inverted trapezium shaped trench is proposed which is beneficial to polysilicon refill since the top opening size is larger than that of the bottom. An optimized micromachining process is used and an inverted trapezium shaped trench is achieved by isotropic etching. On the other hand, for the filling effect, the completely smooth transition curve type trench is better than the trench with sharp corners. Compared with the linear trench, the completely smooth transition curve type trench can improve the strength of mechanical connection. Through combining the novel trench design with the optimization of trench design, a deep trench without voids can be guaranteed. A void-free deep isolation trench is finally realized which enables the electrical isolation between two movable microstructures or between a movable and a fixed microstructure.
1 Introduction In recent years, MEMS technology has attracted widespread attention as a high-tech and practical technique. Compared with conventional devices, MEMS devices have advantages, such as small size, low weight, stable performance, low cost and low power consumption. With a variety of micro-devices and systems emerging in areas including aerospace, military defense, biomedical instrument, energy and electronics, MEMS is now widely applied in these areas, covering almost all the natural and engineering fields. X.-Y. Li (&) G.-T. Li S. Ren D.-Y. Qiao Key Laboratory of Micro/Nano Systems for Aerospace, Ministry of Education, Xi’an 710072, China e-mail:
[email protected]
The microstructure in a MEMS device generally acts as both a mechanical unit and an electric unit. In design and manufacturing processes of MEMS devices, especially in bulk micromachining, mechanical connection and electrical isolation between two microstructures are often simultaneously needed. The structure can be used for flexible or rigid connection between two moveable microstructures or between a fixed structure and a moveable microstructure. Therefore, systematic research about how to realize the mechanical connection and electrical isolation in bulk micromachining is necessary. The deep trench isolation has to be utilized. In the last few years, the processes of deep trench isolation have been published in many papers. A new isolation technology in bulk micromachining using deep reactive ion etching and a polysilicon refill has been presented by Zhang et al. (2001), giving an interpretation about deep trench isolation micromachining process, the mechanical connection was realized with the guarantee of the electrical isolation. However, the unavoidable voids in the middle of the trench after the polysilicon refill will degrade the property of the device. A novel single-chip bulkmicromachined integrated gyroscope has been put forward by Yan et al. (2004), using high aspect ratio (30:1) trench etching technology, deep trench electrical isolation materials refill, and backside etching technologies. A detailed introduction of wafer level packaging of MEMS has been given by Masayoshi (2008), bonding at the interface, such as glass-Si anodic bonding and metal-to-metal bonding. Not only mechanical connection and electrical isolation are needed, but also gas tightness is necessary, which is very important to the vacuum degree of packaging and the quality factor of MEMS devices. The technique of deep trench isolation has got extensively applications in microstructure. This technique could
123
758
Microsyst Technol (2013) 19:757–761
meet the requirement of mechanical connection and electrical isolation between two microstructures. A novel resonantly excited 2D-micro-scanning-mirror has been presented (Schenk et al. 2001). The deep trench isolation technology plays an important role in the manufacture of the device. 90° of corner has been utilized in the trench profile design. But the voids in the trench can’t be avoided after polysilicon refill. All results from these references show that avoiding voids during polysilicon refill process is very significant. The aim of this paper is to provide a method to avoid the voids during the deep trench isolation process by designing a novel trench.
2 Trench forming principle and the traditional technology of deep trench isolation This paper discusses the SF6–C4F8 system, which is an ion-inhibitor process, to explain the mechanism during etching the wafer. In this system, etching process and passivation process is alternated. C4F8 is used to passivate the silicon surface with polymer inhibitor. F ions are generated by the means of ionizing SF6 and it can etch the passivator, which made etching the silicon possible. In order to achieve anisotropy, a passivation layer is produced by using C4F8, which can prevent side wall from etching while etching ions effect down. During etching high aspect-ratio trench, the specific mechanisms which control the profile are important such as ion deflection, ion reflection, and sidewall passivation (Jansen et al. 1996). Ion deflection is caused by the diffraction of ions while entering a trench but mainly by the negative potential of conducting trench walls with respect to the plasma glow resulting in an electrostatic deflection of these ions to the walls. As shown in Fig. 1, ion reflection will occur when ions collide onto the bottom of the trench to a glancing angle. When the incoming ions are highly energetic, the ions will be specular. Ion deflection and reflection can cause a defect of the trench. L2 is larger than L1. Voids will inevitably be generated after polysilicon refill as shown in Fig. 2 (Chen 2005). The traditional deep trench isolation process is shown in Table 1 (Zhang and Najafi 2002). This process has many disadvantages as presented in Fig. 2, voids are obvious in the deep trench which has been refilled. During the etching process, coil power is used to ionize the etching gas, while the plate power is used to accelerate the etching ion for etching to the depth direction. Through controlling the balance between etching and passivation, anisotropy features can be better controlled, thus a deep trench with high aspect ratio (depth to width) can be achieved (Sarajlic et al.
123
Fig. 1 The schematic of etching ion reflection
Fig. 2 Filling results for the traditional process
2004; Marty et al. 2005; Mcnie et al. 2000; Ayazi and Najafi 2000a, b). LPCVD is a key process during the deep trench isolation. The overall deposition process includes the following individual steps: forced convection, boundary-layer diffusion, surface adsorption, decomposition, surface diffusion and incorporation. All these individual steps can limit the overall deposition process. There are several factors which can affect the deposition process, such as temperature, pressure and the flow rate of silane. If the temperature is too high, the rate of reaction will be faster than the ions rate of remove, which
Microsyst Technol (2013) 19:757–761
759
Table 1 The traditional process Process variant
Content
1.
Lithography
Form graphics
2.
Inductively coupled plasma anisotropic etching
Get deep trench
3.
Removal
Photoresist (wet process)
4.
Thermal isolation oxide
Get silica insulating layer (100 nm)
5.
Low pressure chemical vapor deposition (LPCVD)
Polysilicon refill
6.
Chemical mechanical polishing (CMP)
Remove the polysilicon of wafer surface
affects the uniformity of the deposition film and in development the refill result (Ayazi and Najafi 2000a, b). On the other hand, with the rise of temperature, the crystal grain size is bigger, which will lead to bigger voids. If the temperature is too low, the rate of reaction will be too slow, which will lead to the failure of deposition. The pressure and the flow rate of silane can also influence the reaction and the refill effect. The parameters of deep trench are shown in Fig. 2, the bottom of the trench is U-shaped, the width of the trench is 2.72 lm and the depth is 24.5 lm, the ratio of depth to width is 9:1.
Fig. 3 A new trench design
Table 2 The optimized micromachining process Process variant
Content
1.
Lithography
Form graphics
2.
Inductively coupled plasma anisotropic etching
Achieve deep trench
3.
Removal
Photoresist
4.
Oxygen cleaning
Remove wall passivation layer
5.
Inductively coupled plasma isotropic etching
Get optimized trench
3 A new trench design and optimized micromachining process
6.
Thermal isolation oxide
Get silica insulating layer (100 nm)
7.
LPCVD
Polysilicon refill
The cross section of the trench is inverted trapezium, so the defect caused by ion deflection and reflection can be revised. The width of the trench gets smaller along depth direction, which is good for refill afterwards. In order to acquire the novel trench, an optimized micromachining process should be adopted (Fig. 3). Based on traditional process, the specific steps of optimized process are shown in Table 2. Different from traditional process, the optimized micromachining process including two additional steps: oxygen cleaning and inductively coupled plasma isotropic etching. After the removal of the photoresist, oxygen cleaning is used to remove the sidewall passivation layer, followed which is isotropic etching. As shown in Fig. 4, the passivation layer can’t be removed without oxygen cleaning step. If the passivation layer exists, the etching rate of the opening field and sidewall will be reduced. These etching ions spread down along the deep trench, the bottom is etched larger than the original one. After the polysilicon refill, voids appears inevitably. Isotropic etching is carried out after the removal of photoresist, having an effect on the whole wafer. However,
8.
CMP
Remove the polysilicon of wafer surface
due to the short effecting time, the etching on the whole wafer could be ignored. The gas for isotropic is SF6. The power of plate is set to be zero. When the etching ions enter into the deep trench, the concentration of the etching ions will decrease along depth direction, which leads to the etching rate decreases gradually according to the etching depth. The etching rate of opening field is maximum and the etching rate of bottom is minimum. Next the inverted trapezoidal trench is formed as shown in Fig. 3. In the optimization process, the parameters of deep trench after refill are shown in Fig. 5, the bottom of the trench is U-shaped, the opening width of trench is 4.21 lm, the middle width of trench is 2.86 lm and the bottom width of trench is 2.26 lm, the depth of the trench is 20.03 lm. The thickness of polysilicon deposition is 3.03 lm. After isotropic etching, the opening width of trench is enlarged from 2.72 to 4.21 lm. The depth of the trench is smaller than the original one. Experimental parameters are
123
760
Microsyst Technol (2013) 19:757–761
4 Refill effect and optimization of trench design
Fig. 4 The etching result of the bottom of the trench for lack of oxygen cleaning step
Refill effect is tested afterwards, as shown in Fig. 6. The trench could be linear or curvilinear, as long as the design can meet the need of mechanical connection and electrical isolation. However linear trench is not practical. It has bad mechanical connection intensity and low reliability. The curvilinear trench can get better mechanical connection intensity because of longer curve length. To figure out the effect of refill after trench design optimization, two trench designs are compared with each other. The first design is a curvilinear trench with sharp corner transition area, as shown in Fig. 6. The other one is a curvilinear trench with smoother arc corner transition area, as shown in Fig. 7. In order to study the influence of corner design on refill results, the curvilinear trench with sharp corner transition area is divided into two categories according to the corner: original corner, arc corner. It can be seen from the refill effects that the trench profile design has considerable influence on refill. The curvilinear trench with smoother arc corner transition area performs better than the curvilinear trench with sharp corner, as shown in Fig. 7. We can draw conclusion that the curvilinear trench with smooth arc corner transition area can get best refill effect and better mechanical connection intensity. The curvilinear trench with smooth arc corner transition area has a characteristic that the trench width of any position on the trench is the same. The width of curvilinear trench with sharp corner is not the same,
Fig. 5 Refill results for the optimized micromachining process
consistent, the difference is caused by the experimental errors. So the mean ratio of depth to width is smaller than the original one. After the optimization process, a deep trench with an inverted trapezium shaped cross-section is achieved. Width is getting smaller along the depth direction. Then thermal isolation oxide process is carried out. The thickness of silicon dioxide is around 100 nm. The silicon dioxide layer is acted as insulating layer. The final polysilicon refill result is illustrated in Fig. 5.
123
Fig. 6 Refill results for trench with sharp corner
Microsyst Technol (2013) 19:757–761
761
the advantages of straight lines and curves. Not only the void-free can be guaranteed, but also mechanical connection and electrical isolation can be realized. Therefore when the electrical isolation between two movable microstructures or between a movable and a fixed microstructure is needed, this optimization of trench design can be used. Acknowledgments This work was sponsored by the National Defense Science and Industry Bureau of China (Grant No. A2720120001).
References
Fig. 7 Refill results for trench with smooth circular arc shape
especially in the corner position. Voids will probably appear, as shown in Fig. 6. A sharper corner can result in bigger weak voids. As can be seen from the Fig. 7, the trench width of straight line and the trench width of curve are different, which is due to the edge effect caused by the etching errors. Through the optimization of trench design, the voids can be avoided completely.
5 Conclusion The process optimizations for avoiding the voids after polysilicon refill is proposed in this paper. By using isotropic etching, the cross-section of deep trench is optimized to an inverted trapezium shape. The refill effect is tested and voids will still appear in the corner. In order to avoid the voids in the corner, an optimization of trench design is used, which can keep the trench width equal. Compared with the linear trench, completely smooth transition curve type trench can improve the strength of mechanical connection. Through adopting the smooth circular arc shape, we can combine
Ayazi F, Najafi K (2000a) High aspect-ratio combined poly and single-crystal silicon (HARPSS) MEMS technology. J Microelectromech Syst 9:288–294 Ayazi F, Najafi K (2000b) High aspect-ratio polysilicon micromachining technology. Sens Actuators 87:46–51 Chen J (2005) Trench profile control of silicon DRIE process on ICP tools. China Mech Eng 16:476–478 Jansen H, de Boer M, Elwenspoek M (1996) The black silicon method VI: High aspect ratio trench etching for MEMS applications. 9th IEEE Annual International Workshop, pp 250–257 Marty F, Rousseau L, Saadany B et al (2005) Advanced etching of silicon based on deep reactive ion etching for silicon high aspect ratio microstructures and three-dimensional micro- and nanostructures. J Microelectron 36:673–677 Masayoshi E (2008) Wafer level packaging of MEMS. J Micromech Microeng 18:073001 Mcnie M, King D, Vizard C, Holmes A, Lee KW (2000) High aspect ratio micromachining (HARM) technologies for microinertial devices. J Microsyst Technol 6:184–188 Sarajlic´ E, de Boer MJ, Jansen HV et al (2004) Advanced plasma processing combined with trench isolation technology for fabrication and fast prototyping of high aspect ratio MEMS in standard silicon wafers. J Micromech Microeng 14:S70–S75 Schenk H, Du¨rr P et al (2001) A resonantly excited 2D-microscanning-mirror with large deflection. Sens Actuators 89:104– 111 Yan GZ, Zhu Y, Wang CW et al. (2004) Integrated bulk-micromachined gyroscope using deep trench isolation technology. Micro Electro Mechanical Systems, 17th IEEE International Conference, pp 605–608 Zhang CB, Najafi K (2002) Fabrication of thick silicon dioxide layers using DRIE, oxidation and trench refill. 15th IEEE International Conference, pp 160–163 Zhang DC, Li ZH, Li T, Wu GY (2001) A novel isolation technology in bulk micromachining using deep reactive ion etching and a polysilicon refill. J Micromech Microeng 11:13–19
123