Abstract
Low-temperature anodic bonding to stoichiometric silicon nitride surfaces has been performed in the temperature range from 350°C to400°C. It is shown that the bonding is improved considerably if the nitride surfaces are either oxidized or exposed to an oxygen plasmaprior to the bonding.Both bulk and thin-film glasses were used in the bonding experiments. Bond quality was evaluated using a tensiletest on structured dies. The efect of oxygen-based pre-treatments of the nitride surface on the bond quality has been evaluated. Bondstrengths up to 35 N/mm^and yields up to 100% were obtained.
1. Introduction
In the construction of a micro-electro mechanical system (MEMS), design steps toward protecting the deviceagainst its working environment must be considered. It isusually desired to encapsulate the device in some wayand /or to join devices with supplementary functions. Forthese process steps, a reliable wafer scale bonding methodis of vital importance. Several bonding processes for joining silicon wafers have been developed, each imposingspecific restrictions to the device fabrication. Anodic bond.ing has been used extensively for bonding silicon to bulkglass wafers. In recent years, anodic bonding using sputtered glass [1,2l, evaporated glass [3-5] and even spin-onglass [6] has been developed. These methods enable join-ing silicon wafers with only a thin (e.g, 2-10 um) glasslayer, reducing the effect of residual thermal stress andgreatly increasing the application span. When further en-hancing the flexibility of the anodic bonding process, it isobvious to investigate bonding to other surfaces than cleansilicon. In this respect it is interesting to evaluate bondingto thin films commonly used in the production of MEMSThis work evaluates oxygen-based activation methods enabling improved anodic bonding to stoichiometric siliconnitride covered wafers.
Although the advantages of anodic bonding to siliconnitride surfaces are obvious, hardly any literature on thesubject is available. Anodic bonding of glass substrates tonitride coated silicon is reported in Ref. [7] (no details) andin Ref. [8] (very poor quality bond). A more thoroughinvestigation of thin-film bonding to nitride coated wafersusing sputtered glass films is found in Refs. [19]. Bondstrengths between 10 and 20 N/mm2 were reported forbonding temperatures around 400°C.I comparison, weachieve higher values when using the oxygen-treated ni-tride at a lower bonding temperature.
2.Experimental
Stoichiometric silicon nitride with a thickness of 100nm was deposited by LPCVD at 800°C onto polished 4"Si(100) wafers. Prior to nitride deposition the wafers werestructured with a test pattern using anisotropic KOH-etch-ing at 80°C. Each wafer contains 140 test structuresAnother set of polished 4” Si(100) wafers was coveredwith a 1000-A thick dry thermal oxide and a sodium-con-taining boro-silicate glass (Schott 8329) was deposited byelectron-beam evaporation. The average thickness of the glass layer was 5 um deposited at a rate of 20 nm /s. Thethin-film anodic bonding and the evaporation of glass havebeen described extensively in Ref. [3]
A stylus-type surface profiler with a vertical precisionof approximately 5 A was used to measure roughness ofthe deposited silicon nitride and thin film glass. Determination of thickness and refractive index of the depositedsilicon nitride films was performed by ellipsometry.
