New chemical solutions for the etching of (011) oriented V-grooves in InP(001) for CSBH laser diodes
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I. INTRODUCTION Wet chemical etching is an important manufacturing process for electronic devices, especially semiconductor lasers. In the choice of etchants, the principal criteria are their compatibility with the photoresist materials and their ability to etch the desired profile at an optimal rate with a minimal mask undercut. An aqueous solution of HC1 and H3PO4 was often used to etch V-groove in InP(OOl) for channeled substrate buried heterostructure (CSBH) laser diodes.1"6 This etchant is not desirable because it creates a relatively large V-groove opening, which can lead to a large threshold current and a poor far-field pattern. In our previous papers4'5 we have demonstrated the use of photoresist etch masks to fabricate V-groove in InP(OOl) wafer for CSBH laser applications. The use of photoresist masks has significant advantages over other mask materials, e.g., SiO 2 ,'" 3 ' 7 Si3N4,8 or InGaAsP,910 in terms of the process simplicity. We have discussed that the mask undercut can be minimized by reducing the thickness of native oxide formed on InP surfaces during bake-out and other processing steps. We have also modified the mask preparation process to minimize the thickness of oxide layers. Despite these modifications, the undercut is still too large to permit a high yield production. We have developed two techniques to reduce V-groove width in InP(OOl) wafers: (a) by changing InP surface properties to reduce the etch mask undercutting rate11; (b) by changing the etching solution compositions to increase the (001) depth etching rate. The effectiveness of an etchant can be characterized by the etch factor which is the ratio between the etch depth in the vertical [001] direction and the undercut in the horizontal [110] direction. The conventional V-groove etchant (5HC1: IH3PO4) had an etch factor around 2, and the average mask undercut was measured to be around 1.5 /xm.4 To etch a 4.0 /xm wide V-groove, a photoresist mask opening should be less than 1.0 jam wide (see Appendix I). It is difficult to maintain a high yield production using this small mask opening. If a new etchant can be found to increase the etch factor, then a larger mask opening can be J. Mater. Res., Vol. 4, No. 4, Jul/Aug 1989
permitted. Thus, the discovery of a more effective etchant with a larger etch factor can lead to a significant increase in the production yield. It should also be noted that the need to increase the etch factor is unique in the use of photoresist masks because other nonphotoresist masks usually have a large etch factor and it is not sensitive to the choice of etching solutions. In this paper, we report two new etching solutions: (1) HC1-H 3 PO 4 -H 2 O 2 ; (2) HCl-H 3 PO 4 -HBr. Both are suitable for the V-groove etching in InP(OOl) wafers. These solutions are compatible with commercial photoresist, e.g., HPR-204, and they give good quality V-groove {111} side walls in InP. More significantly, the new etching solution increased the (001) depth etching rate by 17% more than that of the conventional 5HC1:1H3PO4 etchan
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