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An improved sputter-etching process
Microelectronics and Reliability
Pergamon Press 1973. Vol. 12, p. 175.
Printed in Great Britain
RESEARCH NOTES AN IMPROVED SPUTTER-ETCHING PROCESS THE ETCHING of patterns into thin films plays an important part in the manufacture of integrated circuits. T o this end the film to be etched is provided photolithographically with a lacquer mask, through the openings of which the film below is removed. T h e frequently employed wet-chemical etching method, in which etching baths act upon the exposed areas of the film, possesses certain fundamental disadvantages, such as under-etching and, in certain circumstances, poor compatibility of the etching solution with parts of the substrate to be etched. For this reason, sputter-etching has lately found increasing application. In this, the masked substrate is placed on the target of a cathode sputtering device and eroded by ion b o m b a r d m e n t (see Fig. 1). Since the mask as well as the film is eroded in this process, it is desirable that the mask-etching rate is low. T h e target material predominantly used hitherto is copper or stainless steel. With these materials, however, the etching rate of the mask is found to be relatively high, erratic and strongly affected by the pressure of reactive gases (such as oxygen and hydrogen) in the cathode sputtering system. Research in the above laboratory has shown that a low and well-reproducible mask-etching rate can be obtained in sputter-etching if a target with strong
gettering power, preferably titanium, is used as substrate holder. T h e titanium which is sputtered around the substrate during etching ensures highly intensive gettering of the reactive gases, in consequence of which the gases cannot attack the mask. A condition for the effectiveness of the method is that the target surface is not entirely covered by the substrate to be etched. T h e influence of the target material on the etching of a photolacquer mask is represented in Fig. 2. T h i s shows the ratio R of the etching rates of a conventional lacquer mask and an underlying SiO= film as a function of the oxygen pressure in the discharge chamber adjusted prior to etching. Curve 1 refers to a copper and curve 2 to a titanium target. Whereas with a copper target, ratio R increases rapidly at increasing oxygen pressure, the effect of a titanium target is that R remains nearly constant at first and increases rapidly only above a specific oxygen pressure, which is determined by the sputtering rate of the titanium. Accordingly, etching results of far better reproducibility can be obtained through use of a titanium target. Research into the effectiveness of other target materials is in course of preparation. T h e results described refer to laboratory experiments; they do not necessarily imply a follow-up in production or marketing.
5 4
!
! ,
i oo
7 10-6
FIG. 1. Schematic representation of the sputter-etching process: (1) target; (2) substrate (e.g. Si); (3) SiOs layer into which a pattern has to be etched; (4) lacquer mask; (5) bombarding ions; (6)material removed by the ion bombardment.
P02
-
10-4.
FIo. 2. Improved sputter-etching process. R is the ratio of the etching rates of a conventional lacquer mask and underlying SiO= film, and PO= is the oxygen pressure in the discharge chamber. Curve 1 : copper target. Curve 2: titanium target. 175
Pergamon Press 1973. Vol. 12, p. 175.
Printed in Great Britain
RESEARCH NOTES AN IMPROVED SPUTTER-ETCHING PROCESS THE ETCHING of patterns into thin films plays an important part in the manufacture of integrated circuits. T o this end the film to be etched is provided photolithographically with a lacquer mask, through the openings of which the film below is removed. T h e frequently employed wet-chemical etching method, in which etching baths act upon the exposed areas of the film, possesses certain fundamental disadvantages, such as under-etching and, in certain circumstances, poor compatibility of the etching solution with parts of the substrate to be etched. For this reason, sputter-etching has lately found increasing application. In this, the masked substrate is placed on the target of a cathode sputtering device and eroded by ion b o m b a r d m e n t (see Fig. 1). Since the mask as well as the film is eroded in this process, it is desirable that the mask-etching rate is low. T h e target material predominantly used hitherto is copper or stainless steel. With these materials, however, the etching rate of the mask is found to be relatively high, erratic and strongly affected by the pressure of reactive gases (such as oxygen and hydrogen) in the cathode sputtering system. Research in the above laboratory has shown that a low and well-reproducible mask-etching rate can be obtained in sputter-etching if a target with strong
gettering power, preferably titanium, is used as substrate holder. T h e titanium which is sputtered around the substrate during etching ensures highly intensive gettering of the reactive gases, in consequence of which the gases cannot attack the mask. A condition for the effectiveness of the method is that the target surface is not entirely covered by the substrate to be etched. T h e influence of the target material on the etching of a photolacquer mask is represented in Fig. 2. T h i s shows the ratio R of the etching rates of a conventional lacquer mask and an underlying SiO= film as a function of the oxygen pressure in the discharge chamber adjusted prior to etching. Curve 1 refers to a copper and curve 2 to a titanium target. Whereas with a copper target, ratio R increases rapidly at increasing oxygen pressure, the effect of a titanium target is that R remains nearly constant at first and increases rapidly only above a specific oxygen pressure, which is determined by the sputtering rate of the titanium. Accordingly, etching results of far better reproducibility can be obtained through use of a titanium target. Research into the effectiveness of other target materials is in course of preparation. T h e results described refer to laboratory experiments; they do not necessarily imply a follow-up in production or marketing.
5 4
!
! ,
i oo
7 10-6
FIG. 1. Schematic representation of the sputter-etching process: (1) target; (2) substrate (e.g. Si); (3) SiOs layer into which a pattern has to be etched; (4) lacquer mask; (5) bombarding ions; (6)material removed by the ion bombardment.
P02
-
10-4.
FIo. 2. Improved sputter-etching process. R is the ratio of the etching rates of a conventional lacquer mask and underlying SiO= film, and PO= is the oxygen pressure in the discharge chamber. Curve 1 : copper target. Curve 2: titanium target. 175