に沿って延在する
... that extend along

I'd say cover, though "extends beyond" is also reasonable, because this sounds like a CVD (or similar) application, so the metaphor of rain coming down is more appropriate. Here's my stab at the sentence.

このとき、Aℓ層の段差における端面はフォトレジスト層の端面には延在するも、レジスト層の端部がPSG層の端部より突出した「ひさし」状にあるため少なくともフォトレジスト層の「ひさし」状突出部のＰＳＧ層に対向する主面（図における下面）でフォトレジスト層を露出する。
At this point, even if the end face at the Al layer step forms an "eave" that covers the end face of the photoresist layer, the photoresist layer will be exposed at least on the main surface of the projecting photoresist eave that faces the PSG layer (the bottom surface in the drawing) since the end of the photoresist layer forms an eave that projects beyond the end of the PSG layer.

Incidentally, I would just list this under electronics (semiconductors) rather than law, even if it did come from a contract. Sounds like a patent...?

A very rough attempt at the sentence. Fix it up accordingly after looking at the figure.
"At this time, the end of the step of the Al layer extends over the end of the photoresist layer, and since there is an eave-like formation caused by the end of the resist layer projecting over the end of the PSG layer, at least the photoresist layer is exposed at the main end facing the photoresist layer eave-like projection portion of the PSG layer (at bottom of Fig. ___)."