198-201. 13] G.T. Mulhern, D.S. Soane and R.T. Howe, Supercritical carbon dioxide drying of ... A.W. Adamson, Physical Chemistry of Surfaces, John Wiley, New.
SE"SgRs
ACTWORS A
ELSEVIER
Sensors and Actuators A 52 (1996) 145-150
PHYSICAL
Polysilicon surface-modification technique to reduce sticking of microstructures Youngjoo Yee a,., Kukjin Chun a, Jong Duk Lee a, Chang-Jin Kim b Department of Electronics Engineering and Inter-University Semiconductor Research Center, Seoul National University, San 56-1 Shinlim-Dong, Kwanak-Ku, Seou1151-742, South Korea h Department of Mechanical Engineering, University of CaliJbrnia Berkeley, Berkeley, CA 94720, USA
Abstract A new and simple surface-modification technique is proposed to reduce sticking of microstructures fabricated by surface micromachining. This technique realizes a very rugged surface at the polysilicon substrate, resulting in reduced sticking through a decrease of real contact area. The surface, which consists of honeycomb-shaped grain holes at the polysilicon substrate layer, is defined by a two-step dry etch without an additional masking step for photolithography or deposition of thin films. By varying the time for etching the grain holes of the polysilicon substrate, controlled surface roughness can be obtained. Test structures, including polysilicon cantilever beams of various lengths, fabricated by surface micromachining with the proposed surface modification show a doubled detachment length without sticking to the substrate. Keywords: Sticking; Surface micromachining; Polysilicon; Grain holes; Surface modification
1. Introduction The production yield and the reliability of microelectromechanical systems (MEMS) fabricated by surface micromachining are reduced by the irreversible sticking of freestanding microstructures to the substrate. The capillary force of rinsing liquid between released microstructures and the underlying substrate causes mechanical contact with each other during the post-release dry. After the liquid is completely dried, the surface tension of the two solids at the contact interface area brings about permanent sticking of the microstructures [ 1,2]. Various anti-sticking techniques have been proposed by many researchers and these techniques can be classified into two categories. One is an effort to prevent the sticking by eliminating the capillary force of the rinsing liquid with supercritical CO2 drying [ 3 ] or by sublimation of the frozen rinsing liquid. The other is to alleviate the surface energy of the interface between the contacted solids by reducing the real surface contact area. This can be done by introducing antisticking dimple (s) or mesas [ 4 ], and by increasing the surface roughness of the substrate plane with a texturization technique [5]. * Corresponding author. Phone: 82-2-878-5293. Fax: 82-2-887-6575. Email: yyj @ssel.snu.ac.kr. 0924-4247/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSD10924-4247 ( 9 5 ) 01 1 0 7 - 5
In this paper, a new technique is proposed to increase the surface roughness of the polysilicon substrate without any additional photolithographic masking step. No change in conventional silicon processing for IC fabrication is needed for this technique. This technique was initially proposed to enlarge the capacitance of a node capacitor for high-density dynamic random access memory (DRAM) cells [6]. Polysilicon grain-sized trenches are formed at the polysilicon substrate by a two-step dry etch. These trenches were named 'grain holes'. After the formation of grain holes at the polysilicon substrate, the area density of surface energy of this layer beneath the released microstructures decreases significantly through the reduction of the real contact area.
2. Formation of grain holes The oxidation rate of polysilicon differs markedly at grains and grain boundaries when the polysilicon is doped heavily with an n-type dopant, especially phosphorus [7]. When polysilicon is doped heavily with phosphorus, phosphorus tends to be preferably segregated to the grain boundary from adjacent grains during thermal cycles, such as during an annealing or drive-in process. In oxidizing the heavily phosphorus-doped polysilicon, the phosphorus-enhanced oxidation is considerable in the vicinity of grain boundaries rather
146
Y. Yee et al. / Sensors and Actuators A 52 (1996) 145-150
than over the grain itself, resulting in a thicker oxide at the grain boundaries. With the thicker oxide at the grain boundaries as the masking material for etching polysilicon, grain holes are formed without an additional photolithographic masking step. The process used to fabricate grain-holed polysilicon is schematically depicted in Fig. 1. After the deposition of polysilicon on an oxidized silicon wafer, the polysilicon is doped with phosphorus above a concentration of 1020cm- 3, followed by thermal oxidation (Fig. l(a) and (b)). Only the thicker oxide in the vicinity of the grain boundaries remains, while the oxide over the grains is completely etched back (Fig. 1(c)). This etch-back of oxide consists of the first step of a two-step dry etch to form polysilicon grain holes. As the second etch step, polysilicon is etched to complete the grain holes using the remaining oxide as a mask material with reactive ion etching (RIE) (Fig. 1 (d)). The depth of the grain holes can be controlled by varying the time of the polysilicon etch. With 5000 ,~ thick undoped polysilicon deposited by LPCVD, grain-holed polysilicon substrates with different depths were fabricated by doping and two-step dry etch. The average depth of the grain holes Grain ~- boundary
Oxide
Oxide
(b)
Fig. 3. SEM images of surfaces of undoped polysilicon (a) and heavily phosphorus-doped polysilicon (b) modified by a two-step dry etch to form grain holes after oxidation.
according to the different lengths of polysilicon etch was measured with scanning electron microscopy (SEM) and the measured data show good linear dependency of the depth on the time of etching (Fig. 2). For comparison, oxidized by undoped polysilicon was etched under the same conditions to form grain holes, but it does not show any noticeable increase in surface roughness (Fig. 3(a)). Fig. 3(b) is an SEM image of a grain-holed polysilicon surface. In this Figure, grain holes surrounded by side walls of polysilicon are clearly seen. From these two SEM images, it is confirmed that phosphorus doping is important to produce the different profiles of oxide thickness over the grains and along the grain boundaries in order to make grain holes.
3. Sticking model
Oxide
Oxide
(c)
(d)
Fig. 1. Process sequence to form grain holes at polysilicon.
~
