1.  Field of the Invention
This invention relates to the fabrication of microelectronic devices, and more particularly the fabrication of magnetic bubble domain chips having multiple magnetic films thereon which can be provided by a process having only a single critical masking step. 
2.  Description of the Prior Art
In the fabrication of microelectronic devices, such as semiconductor devices and bubble domain devices, it is frequently necessary to form multiple layers of material which must be in accurate alignment with one another.  For instance, in the fabrication of magnetic bubble domain devices, it is often necessary that the conductor layers be precisely aligned with propagation layers used to move magnetic bubble domains in the magnetic material.  For instance, reference is made to an IBM Technical Disclosure Bulletin article, appearing in Volume 15, No.  6, November 1972, at page 1826.  In that article, a plurality of masking steps and alignments are used to provide the final device structure. 
In the bubble domain art, attempts have been made to provide improved fabrication processes which will not require extensive use of high resolution masks and which will not require multiple masking steps where critical alignments must be maintained.  For instance, A. H. Bobeck et al describes such a process in IEEE Transactions on Magnetics, Volume MAG-9, No.  3, September 1973, at page 474.  In the process of Bobeck et al, single level metallurgy for producing bubble domain devices is described.  In particular, a shadow mask is used to protect the sensor area of the bubble domain chip during deposition of the conductor layers which are used for various device functions and for providing current to the sensor.  However, the use of a shadow mask is disadvantageous when full wafer processing and small magnetic bubble domain sensors are utilized. 
Accordingly, the present invention provides an improved process in which all functions of the bubble domain chip can be provided using only a single critical masking step.  These various functions of a complete magnetic chip are described in U.S. Pat.  No.  3,701,125 to Chang et al.  They include reading, writing, propagation (storage), transfer, and/or annihilation.  As can be seen by referring to the Chang et al patent, conductor lines are used for the individual bubble domain devices which perform these functions.  For instance, a conductor line is used to bring current to the sensor element, which is conveniently a magnetoresistive sensor comprised of the same material as the material used to move bubble domains in the magnetic bubble domain film.  A suitable material is NiFe where permalloy (a trademark of Alleghany Ludlum Corp.) is a particularly good example. 
In the present method, the propagation circuitry and the conductor circuitry are deposited at different times, using the same masking layer.  Here "deposition" includes any process for forming a layer of material, and includes such well known processes as electroplating, evaporation, sputtering, etc.  During formation of the conductor layer, the sensor is protected by a coarsely aligned masking layer.  This coarsely aligned masking layer can be removed later, and its alignment is not critical.  Also, the use of such a masking layer provides a fabrication process which eliminates the need for a shadow mask, which is expensive.  Additionally, shadow masks do not last for multiple deposition runs because of material buildup on the mask.  Further, processing techniques using shadow masks do not lend themselves to the processing of large wafers and especially those having very small magnetic bubble domains.  With such large wafers, many hundreds of sensors are scattered all over the wafer rather than being merely located along the edge of the bubble domain wafer.  For such a magnetic chip, full wafer packaging is particularly desirable, and such packaging can be provided with the present invention but cannot be easily provided with processes using shadow masks. 
Accordingly, it is an object of the present invention to provide a technique for fabricating microelectronic structures having multiple metallic layers, where said layers are formed using the same masking layer. 
It is a further object of the present invention to provide an improved process for making magnetic bubble domain chips, using only a single critical masking step. 
It is another object of the present invention to provide an improved process for making magnetic bubble domain chips where multiple resist layers are used, but in which removal of the resist layers does not adversely affect underlying metallization layers. 
It is a still further object of the present invention to provide an improved process for making microelectronic devices in which shadow masks are not required.