A novel ferroelectric based microphone

A novel ferroelectric based microphone
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  Microelectronic Engineering 66 (2003) 683–687 www.elsevier.com/locate/mee Anovel ferroelectric based microphone *Tian-Ling Ren , Lin-Tao Zhang, Jian-She Liu, Li-Tian Liu, Zhi-Jian Li  Institute of Microelectronics ,  Tsinghua University ,  Beijing 100084,  China Abstract A novel ferroelectric based microphone with lead zirconate titanate [Pb(Zr,Ti)O , PZT] coated silicon 3 cantilever has been proposed in this paper. The cantilever structure is composed of a Pt/PZT/Pt/Ti/SiO /  2 Si N /SiO /Si multilayer and is designed using a multimorph model. Optimum fabrication process of the PZT 3 4 2 thin films and the cantilever has been developed. The acoustic outputs of the fabricated microphones have beenmeasured with a standard microphone and a high sensitivity of 40 mV/Pa can be obtained. The frequencyresponse of the microphone is very flat in the audio frequency range. ©  2002 Elsevier Science B.V. All rights reserved. Keywords :   PZT; Cantilever structure; Microphone; Thin films 1. Introduction Normal piezoelectric microphones have the advantages of simple fabrication process and easy to beintegrated into semiconductor devices. The sensitivity of diaphragm-based MEMS acoustic transduc-ers is a function of the diaphragm deflection, which is greatly affected by the residual stress in thediaphragm [1]. Freeing up three edges (thus forming a cantilever) would improve the deflectiongreatly. The cantilever concept for an audio microphone has been proposed by Lee et al. in 1996 [2].A ZnO coated cantilever has been used either as microphone or microspeaker in the audio rangefrequency.Ferroelectric materials, such as PZT, have much larger piezoelectric constant than usual piezoelec-tric material. As a kind of important ferroelectrics, PZT exhibits the strongest piezoelectric activitywhen its composition is close to the morphotrophic phase boundary between rhombohedral andtetragonal phase fields. The piezoelectric constant of PZT materials is known to be dramatically larger * Corresponding author. Tel.:  1 86-10-6278-2712; fax:  1 86-10-6277-1130.  E  - mail address :   rentl@mail.tsinghua.edu.cn (T.-L. Ren).0167-9317/02/$ – see front matter  ©  2002 Elsevier Science B.V. All rights reserved.doi:10.1016/S0167-9317(02)00983-8  684  T  . -  L .  Ren et al .  /   Microelectronic Engineering 66 (2003) 683–687  than that of ZnO. In the case of bulk materials, the PZT(53/47) ceramics possess the transversepiezoelectric coefficient,  d   , with  2 93.5 pC/N, while the ZnO single crystal exhibits the transverse 31 piezoelectric coefficient,  d   , with  2 5.4 pC/N. The use of PZT materials to replace the ZnO 31 materials as the piezoelectric layers of the cantilever structure was proven to be a simple and effectiveway to improve the sensitivity of the cantilever microphone [3].In this paper, a PZT-based cantilever structure used for the integrated microphone is proposed. Thefabrication process including a sol–gel deposition and micromachining is outlined. The characteristicsof the PZT based cantilever are examined. 2. Cantilever structure and fabrication The core structure of the PZT-based piezoelectric microphone is a multilayer cantilever diaphragm.Considering its high piezoelectric constant, PZT thin film has been used as the main functional part of the cantilever. Fig. 1 shows the cantilever structure on silicon substrate.Deflection of the cantilever is an important factor for the microphone. Based on the multimorphmodel of the cantilever structure, the deflection of the cantilever can be obtained [4]: 2 1 d   DA C 312 ]]] ] d   x  5  x s d  S D 2 1 2 2 DA B where,  d   is the piezoelectric coupling constant of the PZT layer, and the matrices  A ,  B ,  C ,  D  are 31 related to the structure parameters of the cantilever according to Ref. [4]. According to therelationship between the deflection and the thickness of each layer, the structure of the cantilever hasbeen optimized.Bulk micromachining technology was applied to sculpt the freestanding micro-cantilever. Thefabrication flow chart is shown in Fig. 2. The processes include lift-off technique, reactive ion etching,wet chemical etching, etc.The process starts with a double side polished P-type (100) silicon wafer. The major fabricationsteps as shown in Fig. 2 are described as follows: Fig. 1. Cross section and top view of the PZT-based cantilever structure.  T  . -  L .  Ren et al .  /   Microelectronic Engineering 66 (2003) 683–687   685Fig. 2. Fabrication process of the micromachined PZT cantilever structure. 1. Silicon nitride is deposited on thermal oxidized silicon substrate by low pressure chemical vapordeposition. It will be used as masking layer during the silicon bulk micromachining.2. The bottom electrode is formed by sputtering a thin Ti adhesion layer followed by a platinum layerof 150 nm.3. Sol–gel derived PZT (100)/(001) thin film of 0.5 m m thick was deposited on the bottom electrodewith a 70 nm PbTiO (100) seeding layer [5]. 3 4. Pt top electrodes were then sputtered and structured by lift-off process.5. The PZT capacitor structure was patterned by reactive ion etching. Two separate masks were usedfor the formation of the PZT capacitors. PZT multilayer was etched with the first mask usingHCFC-124, and the Pt/Ti bottom electrode was etched with the second mask using SF and O as 6 2 the reactive gas. After patterning the entire PZT capacitors, the capacitors were annealed at 650 8 Cfor 1 min to avoid the degradation of its quality during the etching process.6. The slit around the cantilever was structured using dry/wet etching of the Si N /SiO /Si films. 3 4 2 7. Backside windows were opened through Si N /SiO using dry/wet etching and double side 3 4 2 lithography.8. Bulk silicon under the Si N film was removed by anisotropic etching in aqueous KOH. In this 3 4 process, the front side of the wafer was properly protected.  686  T  . -  L .  Ren et al .  /   Microelectronic Engineering 66 (2003) 683–687  Fig. 3. Optical photo of two kinds of PZT-based cantilever microphone. 3. Results and discussion Using the fabrication process above, cantilever structures with different sizes and shapes have beenfabricated for microphone applications. The size of the fabricated cantilever microphone ranges from 2 2 200 3 200  m m to 2000 3 2000  m m .Fig. 3 shows the optical photo of two kinds of the fabricated PZT cantilever structures. The top andbottom electrodes have been conducted to the surface of the bulk silicon for electrical measuring.Wafer level measurement was carried out using a Norsonic 840 real time acoustic analyzer with astandard microphone as a reference. The measurement results have shown that the sensitivity rangesfrom 10 to 40 mV/Pa for the cantilever microphones with different sizes and layer thickness. The 40mV/Pa sensitivity is much higher than the usual micro-manufactured microphone as it is known. It isinteresting to point out that the frequency response of the PZT-based cantilever microphone is veryflat in the audio frequency range. This quality is prominent in the existed microphones.As an example, the measuring result of a cantilever microphone is shown in Fig. 4. The sensitivityis about 20 mV/Pa, and the frequency response is flat up to 20 kHz. Fig. 4. Frequency response of a cantilever microphone.  T  . -  L .  Ren et al .  /   Microelectronic Engineering 66 (2003) 683–687   687 It is interesting to point out that this ferroelectric based cantilever structure can also be used as amicrospeaker, and a novel integrated microphone and microspeaker acoustic system can be realized. 4. Conclusions A novel ferroelectric-based microphone with a multilayer PZT cantilever structure has beenrealized. Optimum fabrication process of the PZT thin films and the cantilever has been developed.The sensitivity of the fabricated microphones can be as high as 40 mV/Pa, and the frequency responseis very flat in the audio frequency range. The novel piezoelectric microphone should be verypromising for micro-acoustic devices. Acknowledgements This work is supported by the National Natural Science Foundation (69806007), the ‘‘973’’ Project(G1999033105), and ‘‘985’’ Project of China. References [1] R.P. Ried, E.S. Kim, D.M. Hong, R.S. Muller, J. MEMS 2 (1993) 111.[2] S.S. Lee, R.P. Ried, R.M. White, J. MEMS 5 (1996) 238.[3] T.L. Ren, L.T. Zhang, L.T. Liu, Z.J. Li, Integrated Ferroelectrics 41 (2001) 1753.[4] D.L. Devoe, A.P. Pisano, J. MEMS 6 (1997) 256.[5] T.L. Ren, L.T. Zhang, L.T. Liu, Z.J. Li, Integrated Ferroelectrics 39 (2001) 1165.
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