通过原子氢表面清洁实现GaAs晶圆键合

A method of large-area wafer bonding of GaAs is proposed. The bonding procedure was carried outin an ultrahigh vacuum. The wafer surfaces were cleaned at 400 and 500 °C by application of atomichydrogen produced by thermal cracking. The wafers were brought into contact either immediatelafter the cleaning, or at temperatures as low as 150°C, without application of a load, ancsuccessfully bonded over the whole area. High-resolution transmission electron microscopyrevealed that the wafers could be directly bonded without any crystalline damage or intermediatelayer. From a mechanical test, the fracture surface energy was estimated to be 0.7-1.0 J/m’, whichis comparable to that of the bulk fracture. Furthermore, this bonding method needs no wet chemicaltreatment and has no limits to wafer diameter. Moreover, it is suitable for low temperature bonding.

INTRODUCTION

Gallium arsenide is a M-V compound semiconductormaterial of the most importance in opto- and high-speedelectronics. The ability to join two GaAs wafers with eachother or with other materials would represent an additionaldegree of freedom in the design of opto-electronic systemsand enhance the flexibility of fabrication procedures. Because “wafer direct bonding’ i does not depend on a thirdmaterial acting as a glue, it may be seen as the joining technique of choice. So far, direct bonding of GaAs has oftenbeen carried out in inert or reducing atmospheres, at rela-tively high temperatures between 400 and 975°C for a fewup to 20 h, often under a compressive load of up to 40kg/cm', and for small pieces of approximately 1 cm’ area.2-4The postbonding high-temperature annealing is intendedto increase the adhesion between the sample, and to removeany enclosed surface adsorbates. This approach, however.often compromises the quality of the interface and of thebonded materials: interlayers of gallium or arsenic oxidesmay be enclosed or bubbles may form because of thermaldecomposition of surfaces adsorbates. Moreover, the highannealing temperatures are not adequate for bonding dissimi-lar materials due to mismatches in thermal expansion behav-ior. In addition, the application of a mechanical load easilyinduces structural damage and is particularly difficult to ex-tend to whole wafers. Large-area wafer bonding’ is, how-ever, a necessity for virtually all practical applications.

Therefore, more moderate bonding conditions are neces-sary, which would enable bonding at lower temperatures andwithout applied pressures. In fact, in the case of wafer bond-ing of Si, it was shown that, if atomically clean surfaces areprepared, the covalent bonding can occur even at roomtemperature.6,7 A H-terminated Si wafer pair was heated todesorb the hydrogen, and thus to obtain the clean Si surface.As long as this clean surface can be maintained, e.g., in anUHV, the wafers can be brought into contact to bond at any desired temperature, even down to room temperature. In thiscase, practically no pressure was applied to bond whole 4 in.wafers by forming an atomically abrupt interface. Alternatively, an ion beam was applied for surface cleaning andactivation, to bond not only Si but also dissimilar Ill-V com-pounds without any heating process,8, but this procedure caninvolve some ion beam damage left at the bond interface.

Thus, the preparation of a clean and damage-free wafersurface was considered to be inevitable to achieve our pur-pose. The cleaning procedure of GaAs surfaces is more com-plex than that of Si, due to the difficulty in removing theoxide of Ga and As congruently. GaAs surfaces exposed toair consist of native oxides, carbon contaminants, and absorbed water. Water can be thermally removed at relativelylow temperatures. Then native oxides are desorbed by heating up to 580 °C. However, it has been reported that thermalcleaning cannot remove carbon contaminants completelyloand leads to surface roughness!l,12 and accumulation ofimpurities,13,14 even if the oxide layer can be removed.As a cleaning method, electron cyclotron resonance(ECR) plasma has been reported to be effective.15-18 It isactually atomic hydrogen in ECR plasma that has the clean-ing effect. Atomic hydrogen, or hydrogen radical (H· ), caneffectively be generated by thermal decomposition of mo-lecular hydrogen flowing through a hot tube.19 It was foundout that the thermally generated atomic hydrogen causes lessdamage due to the less momentum of the formed H·, thecleaning mechanism has been studied in detail.