Fluorine-enhanced low-temperature wafer bonding of native-oxide covered Si wafers
Applied Physics Letters
The bonding energy of bonded native-oxide-covered silicon wafers treated in the HNO 3 /H 2 O/HF or the HNO 3 / HF solution prior to room-temperature contact is significantly higher than bonded standard RCA1 cleaned wafer pairs after low-temperature annealing. The bonding energy reaches over 2000 mJ/ m 2 after annealing at 100°C. The very slight etching and fluorine in the chemically grown oxide are believed to be the main contributors to the enhanced bonding energy.
... ion-electron-microscopic images have shown that the chemically formed native oxide at bonding interface is embedded with many flake-like cavities. The cavities can absorb the by-products of the interfacial reactions that result in covalent bond formation at low temperatures allowing the strong bond to be retained. Direct bonding of native-oxide-covered silicon wafers (termed bare silicon wafers) has many applications, such as in the fabrication of sensors and actuators. 1 Commercially available silicon wafers are usually covered with a thin native oxide layer of ϳ10 Å in thickness with a smooth surface having a rms roughness of about 1 Å. These wafers are adequate for direct wafer bonding. The silicon wafers are usually cleaned in standard RCA1 ͑NH 4 OH: H 2 O 2 :H 2 O͒ and RCA2 ͑HCl: H 2 O 2 :H 2 O͒, or concentrated HNO 3 or H 2 SO 4 :H 2 O 2 solutions followed by rinsing in de-ionized (DI) water to form Si-OH groups on the surfaces. When two silicon wafers are brought into contact, they are readily bonded via the water molecules that are hydrogen bonded to the polar OH groups on the two mating surfaces. 2 Upon annealing at elevated temperatures, the interface water molecules diffuse out most likely through the native oxide to the bulk Si where they react to form SiO 2 and hydrogen: Then, the polymerization reaction takes place between the silanol groups of the mating surfaces to form siloxane bonds by The bonding energy can reach about 1.2 J / cm 2 after annealing at 150°C. 3 In order to obtain a stronger bond for applications such as leak-free sensors, usually annealing above 1000°C is necessary. However, the high temperature treatment is not compatible with bonding of processed silicon wafers that are thermally sensitive. Therefore, it is highly desirable to develop methods to yield a strong bond at low temperatures. Several different methods have been developed for low temperature bonding of bare silicon wafers. A surface treatment of plasma of oxygen or other gases in reactive ion etch mode prior to room temperature bonding can significantly increase the bonding energy at low temperatures. 4-6 It has been reported that an oxygen plasma treatment of nativeoxide-covered silicon wafers can increase the bonding energy to about 1500 mJ/ m 2 at room temperature. 6 However, the plasma treatment introduces a damage layer in the near surface region that may degrade the device performance. 7 Although a dip in concentrated 70% nitric acid can enhance the bonding energy of bonded bare silicon wafers at low temperatures, 8 many voids developed at the bonding interface upon annealing due to the release of the trapped gaseous nitrogen monoxide in the chemically grown oxide. 9 In this letter, we report the use of the mixtures of an oxidizing agent ͑HNO 3 ͒ and an oxide-etching agent (HF) to treat bare silicon wafers prior to bonding. Bonding energy close to the bulk silicon fracture energy has been obtained after annealing at 100°C. HNO 3 and HF mixtures are well-known solutions for etching silicon wafers through oxidizing silicon by HNO 3 followed by etching the oxide off by HF simultaneously. Previously, it has been reported that low HF concentration (0.025%-0.1%) in concentrated (70%) HNO 3 can effectively clean silicon surfaces, 10 and has been used for wafer cleaning prior to DI water rinse and room temperature bonding. 11 Only a small improvement in bonding energy at low temperatures was observed. 11 In this study, a mixture of waterdiluted HNO 3 and low concentration HF, termed as HNO 3 /H 2 O / HF, was employed to treat native-oxidecovered silicon wafers. For comparison, a solution of concentrated HNO 3 mixed with low concentration HF (termed HNO 3 /HF) was also used. When bare silicon wafers were removed from either mixture, the chemical solution does not stick on the wafer surfaces. It is probably due to the Si-F termination on the chemically formed oxide surfaces. 11, 12 However, the fluorine passivation is immediately lost by rinsing in water, and the surface becomes hydrophilic as a result of substituting Si-F groups with Si-OH groups in water. 13 To preserve the Si-F termination, the bare silicon a)