Structure and method for determining isolation of integrated circuit
BACKGROUND OF THE INVENTION 
     The present invention concerns integrated circuit devices comprising multiple active devices and passive elements on a single die and, in particular, a novel structure and method for determining the degree of isolation between devices and elements. 
     Monolithic integrated circuits, including both analog and digital circuits, include a multiplicity of both active devices and passive elements formed on a single die. For such circuits to be effective, it is essential that certain structures be electrically isolated from each other over a wide frequency range. For example, monolithic microwave integrated circuits, also known as MMICs, may include a number of transistors with isolated collectors formed on a p-type silicon substrate with epitaxial n+ and n-type silicon layers thereon. 
     Isolation in the integrated circuit art is typically achieved by forming vertical grooves, or trenches, which may be one-half to two microns wide and six to eight microns deep. Selective interconnection between isolated structures is then accomplished by technique known in the art as &#34;air bridging&#34;. An alternate interconnection technique that is finding increasing favor in the art because of its compatibility with planar processing and multi-dimensional structures involves &#34;back filling&#34; of trenches with a suitable dielectric material. 
     In any event, the effectiveness of the isolation in the prior art has generally been determined by a DC measurement, which fails to predict behavior either at high frequencies or in digital applications, or more commonly, from an assessment of a circuit&#39;s performance after complete fabrication, often necessitating costly empirical optimization. Therefore, it is desirable for the designer of both the integrated circuit and the fabrication process to have a quantitative measure of isolation. Such a measure of isolation at the frequencies of interest would permit the selection of materials, dimensions and processing steps for isolation more quickly, more effectively and at lower cost than previously. Moreover, such measure should be capable of being incorporated as a wafer scale quality control tool. 
     Accordingly, an object of this invention is structure and method for quantitatively determining the electrical isolation between regions on a semiconductor substrate. 
     SUMMARY OF THE INVENTION 
     The present invention is embodied in at least two electrically isolated devices formed on a semiconductor substrate. Each device includes in monolithic electrical series connection a first contact pad connected to the substrate by a first substrate contacting means, a matching resistor connected to the substrate by a second substrate contact means to a first resistor contact means, a second contact pad connected to a second resistor contact means. The first and second contact pads of the first device are connected to an electrical signal generator and the first and second contact pads of the second device are connected to an electrical signal analyzer. The values of the matching resistors of each device are selected to match the input and output impedances of the signal generator and of the signal analyzer, respectively. 
     According to another aspect of this invention, isolation between the structures is determined by applying a signal of selected frequency to the input of the first structure and analyzing the signal detected at the outputs of the second structure using the method of scatter port analysis. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of an isolation test structure in accordance with the teachings of this invention. 
     FIG. 2 is an equivalent circuit of the isolation test structure in accordance with the teachings of this invention. 
     FIG. 3 is a plan view of the topography of an isolation test structure in accordance with the teachings of this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, an isolation test structure in accord with the teachings of this invention is formed on a conventional substrate comprising p-type silicon wafer 10, n+ buffer layer 16 and n-type epitaxial layer 18. The test structure further comprises at least two devices 11 and 12 separated by isolation trenches 15, which completely surround each device. Although trenches 15 are shown in FIG. 1 as open spaces, the invention contemplates that trenches 15 may be back filled with a dielectric material such as undoped polysilicon, glass, polyimide and the like, whereby such levelized isolations can be quantitatively determined. 
     Referring further to FIG. 1, the device 11 comprises first substrate contact pad 20, which is in electrical contact with n-layer 18 through first substrate metalization 22 and is otherwise insulated from the substrate by insulating oxide 19. Also shown in FIG. 1 is substrate resistor SR1, which symbolically indicates the equivalent resistance of the substrate for later reference in connection with FIG. 2. Second substrate metalization 24, which is spaced apart from 22, makes a second contact to the substrate. First resistor metalization 26 provides electrical connection from the second substrate contact to one end of n-type polysilicon resistor 28. The other end of resistor 28 is contacted by metalization 30 and device is completed by second contact pad 32. Thus, a circuit comprising substrate contact pad 20, first substrate metalization 22, and equivalent substrate resistance SR1, second substrate metalization 24, first resistor metalization 26, polysilicon resistor 28, second resistor metalization 30 and resistor contact pad 32 in electrical series is formed on test device 11. Test device 12, which is formed identically to device 11 on wafer 10 and isolated by trenches 15 in accord with the teachings of this invention comprises a corresponding circuit having in series connection: substrate contact pad 40; substrate metalization 42; equivalent substrate resistance SR2; second substrate metalization 44; first resistor metalization 46; polysilicon resistor 48; second resistor metalization 50 and resistor contact pad 52. 
     Referring to the equivalent circuit of FIG. 2, signal generator SG is connected to device 211 at substrate contact 220 and resistor contact 232, which serve as inputs. Signal analyzer SA is connected to device 212 at its substrate contact 240 and resistor contact 252, which serve as outputs. Resistors 228 and 248, which correspond to resistors 28 and 48 in FIG. 1, are selected to match the output and input impedances of the signal generator and signal analyzer, respectively. In general, a value of 50 ohms, which is standard for high frequency devices is preferred. Other values, such as 75 ohms for cable television applications, may also be used. In any case, isolation between devices is quantitatively determined in accord with the teachings of this invention by applying an input signal from SG at a selected frequency to device 211 and the output at device 212 with SG and applying the known methods of analysis such as two port scatter parameter analysis. In practice, commercially available automatic network analyzers such as the model 8510 by Hewlett Packard can be employed to determine the precise value of isolation when used in conjunction with the structure and method of this invention. 
     FIG. 3 shows the topography of an exemplary implementation of this invention. Test devices 301 and 302 are isolated in this example by single trench 310. Also shown is device 303 which is isolated by a double trench, comprising 310 and 313. Bond pad 320 to device 311 corresponds to substrate contact 20 in FIG. 1. Substrate contacts 322 and 324 correspond to substrate metalizations 22 and 24 of FIG. 1. Resistor contacts 326 and 330, connected to opposite ends of resistor 328, and terminating at resistor bond pad 332, correspond to resistor metalizations 26 and 30 to resistor 28 and resistor contact 32 of FIG. 1. The essential elements of device 312 and device 12 of FIG. 1 correspond in the same way as the aforementioned relations between the elements of 311 in FIG. 3 and of 11 of FIG. 1. Furthermore, application of the teachings of this invention to determining other isolation techniques, such as double trenching as illustrated by device 313 in FIG. 3, should be apparent to those skilled in the art. 
     Moreover, in addition to aiding in design and improvement monolithic integrated circuit devices requiring effective isolation, one skilled in the art of semiconductor device fabrication can utilize the structure and methods of this invention to provide more effective wafer scale quality control in the manufacture of monolithic integrated circuit devices. 
     The structure and methods for quantitatively determining isolation are broadly applicable for the design and fabrication of isolation a variety of monolithic integrated circuit devices operating at frequencies ranging from DC to several GHz and utilizing compound semiconductor materials such as GaAs, InP and alloys thereof and including both analog and digital circuits.