Junction isolated IC with thick EPI portion having sides at least 20 degrees from (110) orientations

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described by reference to embodiments illustrated in the accompanying drawings.
An embodiment in which a high breakdown voltage transistor having a breakdown voltage (BV.
sub.
CEO) of 150 V between the collector and emitter and a low breakdown voltage transistor (small signal) of BV.
sub.
CEO =15 V are integrated on one substrate is first described.
Necessary resistivity .
rho.
.
sub.
EP and thickness t.
sub.
EP of the epitaxial layer are first illustrated.
In order to form a high breakdown voltage transistor having the above-mentioned collector-emitter breakdown voltage, if a maximum value of the DC current gain h.
sub.
FE of the transistor is 200, .
rho.
.
sub.
EP should be at least 12.
OMEGA.
-cm, and therefore, it is set at 15.
OMEGA.
-cm to provide an allowance for the deviation or error in the process steps is taken into account.
In this case, the thickness of the epitaxial layer is set at 30 .
mu.
m in view of the depletion layer extending at a base-collector reverse bias voltage of 150 V.
When the thickness of the epitaxial layer on which the low breakdown voltage obtained if the thickness is about 10 .
mu.
m.
Preparation of the structure of the above specification according to the method of the present invention will now be described by reference to FIGS.
5-A to 5-E.
FIG.
5-A is a sectional view showing the structure obtained by forming an oxide film (SiO.
sub.
2 film) 52 on a P type silicon substrate 51 having a surface on the (100) crystal plane and forming on this oxide film 52 is rectangular window having only sides parallel to the <100> crystal axis by the photo-etching process.
In the subsequent Figs.
, these reference numeral represent the same elements.
Referring to FIG.
5-B, a dent 501 having a depth of about 20 .
mu.
m is formed on the substrate by using an anisotropic etching solution.
The etching solution is formed by incorporating in a 20% by weight aqueous solution of KOH isopropyl alcohol and a surface active agent FC-95 for preventing formation of pyramidal projections on the etched surface