Abstract:
The present invention relates to a process for implementing a base circuit design with configurable modifications to compensate for variations in component parameters due to material or processing characteristics. During the fabrication process, the base circuit is electrically tested to determine the characteristics of the base circuit given the materials and processes used to implement the circuit. Based on the testing, subsequent processing steps are used to modify the base circuit as necessary to compensate for variations in the circuit or component parameters due to the material or processing characteristics. 
     Preferably, the base circuit includes hetero-junction bipolar transistors and the in-process testing is used to determine an associated beta value for the transistor. Based upon the determined beta value, the circuit is modified during processing to insure proper quiescent currents occur during normal operation given the beta of the transistor. Preferably, a top level mask is used to select the resistance in the base current path to select the proper base current for a desired quiescent collector current.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to semiconductor processing, and in particular, adaptive processing to minimize the effect of wafer and processing variations in the final semiconductor circuit. 
     Natural variations in materials and processing steps cause variations in performance of semiconductor devices. These variations may change drastically from wafer to wafer and lot to lot. In the case of amplifier circuitry incorporating heterojunction bipolar transistors (HBT) formed using a gallium arsenide process, there is significant variation in final circuit performance based on the variations in manufacturing materials. In particular, large variations in beta for each HBT device requires designers to work with similar variations in idle current, gain, power, efficiency and other key parameters bearing directly on performance. Further, large variations in beta result in low manufacturing yields because these variations in beta are often outside of the already excessive tolerances taken into consideration in the circuit design. As such, there is a need for a way to improve manufacturing yields and to compensate for process variations in a cost-effective and time-efficient manner. 
     SUMMARY OF THE INVENTION 
     The present invention provides an adaptive manufacturing technique for measuring device parameters at a point in the manufacturing process and for making adjustments during processing to compensate for variation in these parameters. The preferred process reduces the performance variation resulting from variation in transistor beta in power amplifier circuits made from hetero-junction bipolar transistors (HBT). 
     The process involves using beta values, determined by on-wafer testing at a midpoint in the fabrication process. Typically, testing is done after application of the first interconnect metal layer. The beta value determined from the testing is used to select connection points to a series of resistors used to control the bias currents driving the HBT amplifier circuit. Appropriate connections are made in a subsequent metal layer and optimize bias conditions in the final circuit by compensating for variations in beta. The given device beta and selected bias resistance effectively trims the quiescent current settings and results in much tighter performance tolerances for the final device. Compensating for variations in beta significantly reduces variations in idle current, gain, power, efficiency and other key parameters bearing directly on device performance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS FIGURES 
     FIG. 1 is a flow chart outlining the basic adaptive manufacturing process of the present invention. 
     FIG. 2 is a simplified schematic of an amplifier circuit. 
     FIG. 3 is a simplified schematic of the amplifier circuit of FIG. 2 with variable resistors used to compensate for variations in beta. 
     FIG. 4 is a simplified schematic representation of a modification of the amplifier circuitry of FIG. 2 according to a preferred embodiment of the present invention. 
     FIG. 5 is a representation of semiconductor elements forming the circuit in the schematic of FIG.  4 . 
     FIG. 6 shows a modification of the representation of FIG. 5 according to a first exemplary embodiment. 
     FIG. 7 shows a modification of the semiconductor circuit of FIG. 5 according to a second exemplary embodiment. 
     FIG. 8 is a cross-sectional representation of a semiconductor manufactured according to the preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Variations in wafer processing have a negative impact throughout the design, processing and application of the final device. Variation in certain process parameters can be compensated for by changing the nature of subsequent processes, if the parameter can be measured in time for such compensation to be made. The present invention relates to a process that measures and compensates for variable parameters during the manufacturing process. 
     The basic manufacturing process according to the present invention is outlined in FIG.  1 . Typically, semiconductor circuitry is designed with known process variations in mind. Preferably, a base or primary circuit is designed with one or more modifications configured to compensate for anticipated process variations (block  100 ). For example, assume the base circuit design is the amplifier circuitry  10  shown in FIG.  2 . The circuit includes three hetero-junction bipolar transistors, T 1 , T 2  and T 3 . The amplifier circuitry  10  also includes resistors R 1 , R 2  and R 3 , configured to set the base currents and other quiescent currents for the amplifier circuitry  10 . Importantly, all of the circuitry shown in FIG. 2 may be implemented in the semiconductor device. Once the semiconductor is manufactured, the values for resistors R 1 , R 2  and R 3  are substantially fixed. External components to compensate for variations in device performance are preferably avoided. 
     In order to compensate for changes in beta, one would theoretically like to manufacture the device and adjust one or more of the resistors to adjust the base current for the transistors within the amplifier circuitry  10 . The variations in beta directly affect the gain of the amplifier circuitry  10 , thus, the collector quiescent current I CQ  may vary drastically between devices. Without being able to vary one or more of the resistors controlling base currents, such as making resistors R 1  and R 2  variable as shown in FIG. 3, designers and manufacturers are significantly limited by the excessively wide tolerances of certain semiconductor devices, especially for gallium arsenide hetero-junction bipolar transistors. 
     Once the base circuit is designed with modifications in anticipation of process variations, the base circuit is fabricated during a semiconductor process to allow for modifications as necessary in subsequent processing (block  102 ). Continuing with our example amplifier circuitry  10 , a representation of a base circuit designed to be modified during processing is shown in FIG.  4 . As shown, resistor R 1  has been segmented into three resistive elements, R 1 ′, R 1 ″, and R 1 ′″. Similarly, resistor R 2  is replaced with three resistive elements, R 2 ′, R 2 ″, and R 2 ″ . Electrical taps are placed at the ends and between the resistive elements. The taps associated with R 1 ′, R 1 ″ and R 1 ′″ are TP 1  through TP 4 . The four taps corresponding to R 2 ′, R 2 ″ and R 2 ′″ are TP 5  through TP 8 . The base circuit is designed to include each of the resistive elements in series. Modifications to the base circuit are made by selectively connecting the taps associated with the resistive elements to control the final base resistance, and thus, base current during operation of the device. Preferably, taps TP 1  through TP 4  and TP 5  through TP 8  are used to short resistive elements out of the circuit or to connect a resistive element to another part of the circuit. 
     During the manufacturing process, once the base circuit design is implemented, an electrical test is conducted during the fabrication process to measure a component or overall circuit parameter. In the present example, connection points P 1  through P 4  are used to conduct the electrical test. Connection point P 1  may be used to introduce an input signal, such as a normalized bias current. Connection point P 2  may be connected to a power supply having a voltage V CC . Connection points P 3  and P 4  may be connected to ground, and connection point P 5  may be connected to a current source capable of measuring the collector&#39;s quiescent current I CQ . Based on the measured currents, the beta for the device and circuitry can be determined (block  104 ). 
     Continuing with the example, assume that the anticipated variations in beta can be categorized into three ranges associated with a low beta wafer, medium beta wafer and high beta wafer. For each of these three classifications, the resistances for R 1  and R 2  are selected by modifying the circuit or component during a subsequent processing step to compensate for the variation in the measured parameter (block  106 ). In particular, the determined beta from the test measurement in the example determines which taps associated with the resistors R 1  and R 2  are shorted together. For a low beta wafer, higher base currents are necessary to effect a desired collector quiescent current I CQ . As such, connections to the taps TP 1  through TP 4  and TP 5  through TP 8  are configured to provide low resistance values for R 1  and R 2 . In contrast, for a high beta wafer, lower base currents will result in the desired collector quiescent current I CQ  . For the medium beta wafer, resistance values for R 1  and R 2  are selected near a midpoint between those necessary for the low beta and high beta wafers. Taps TP 1  through TP 4  and TP 5  through TP 8  are preferably interconnected to short any or all of the resistive elements, R 1 ′, R 1 ″, R 1 ′″, R 2 ′, R 2 ″ and R 2 ′″, to set the resistance values for resistors R 1  and R 2   
     FIG. 5 represents the circuitry of FIG. 4 as formed on a semiconductor wafer. Notably, the resistive elements vary in length depending on the desired resistance. Transistors T 1 , T 2  and T 3  are shown in block form and metal traces connecting the various elements are shown in darkened lines. The taps TP 1  through TP 8  are shown as metal posts or pads connecting the various resistive elements to one another and to the metal traces as necessary. For a high beta wafer, none of the taps TP 1  through TP 4  or TP 5  through TP 8  are shorted in order to provide maximum resistance for R 1  and R 2 . For a medium beta wafer, the configuration of FIG. 7 is preferable, wherein taps TP 1  and TP 2  are shorted, and TP 7  and TP 8  are shorted with a top metal mask (TOPM). In this configuration, the effective value for R 1  is the sum of the resistances of resistive elements R 1 ′ and R 1 ″. R 1 ′″ is shorted out of the circuit. Likewise, the effective resistance value for resistor R 2  is the sum of resistive elements R 2 ′ and R 2 ″, wherein R 2 ′″ is shorted out of the circuit. 
     FIG. 6 depicts a configuration for a medium beta wafer, wherein resistive elements R 1 ″ and R 1 ′″ are shorted to provide an overall resistance value for R 1  equal to that of resistive element R 1 ′. The overall resistance value for R 2  is equal to the resistive element value R 2 ′ because resistive elements R 2 ″ and R 2 ′″ are shorted out of the circuit. Those skilled in the art should recognize that any combination of these resistive elements may be used to further define ranges in addition to the three discussed in the exemplary embodiment. Once the circuit is modified by connecting the taps as described above (block  106 ), the semiconductor processing is finalized (block  108 ) to provide a semiconductor device that is compensated for beta variation. 
     Turning now to FIG. 8, a cross-sectional representation of a semiconductor configured according to the present invention is shown. The cross-section depicts the semiconductor substrate  20 , upon which is layered a field silicon nitride layer  22 , upon which is formed a resistive layer  24  formed of nickel chromium (NiCr). Taps  26 A through  26 D are placed on the resistive layer  24  in the first interconnect layer. These taps could correspond to any one of the taps TP 1  through TP 4  or TP 5  through TP 8  as shown in FIG.  4 . Between these taps  26 A through  26 D is deposited an interlayer dielectric between the taps of the first interconnect layer (FIC) and above the resistive layer  24 . Resistive elements are formed between taps  26 A through  26 D, which may be connected to each other or to other elements on the semiconductor using a top metal mask  28 . 
     As depicted, the top metal mask entering the left of the diagram is connected to the far left tap  26 A. Additional top metal is used to short tap  26 A to tap  26 B to provide a short between these taps. The top metal mask may be configured as necessary to short any one or more of the taps  26 A through  26 D to one another or to other circuit elements. Thus, depending on the measured beta, a corresponding top metal mask is selected to connect the various taps in order to configure the circuit to compensate for variations in beta. Preferably, each of the low, medium and high beta wafers have corresponding top metal masks configured to make circuit connections to compensate for beta and to adjust resistor values to keep bias currents, gain, and other performance parameters within a predefined range. 
     Although the preferred embodiment is directed to compensating for beta in gallium arsenide hetero-junction bipolar transistor devices, those of ordinary skill in the art will quickly recognize the application of the inventive process in a variety of semiconductor design and fabrication processes. Virtually any variable caused by the materials used to make the semiconductor or by the process in which the semiconductor is made may be compensated for with the adaptive manufacturing process of the present invention. 
     Based on in-process measurements, the final semiconductor design implemented in the semiconductor device may be modified during processing to adjust resistance, inductance and capacitance values alone or in combination with one another to insure the device operates according to predefined performance requirements. Characteristics such as gain, linearity, efficiency and impedance can be modified by simply implementing a base portion of the design, electrically testing the design in-process, and modifying the design in a subsequent processing step to compensate for any material or processing variables or deficiencies. The invention is applicable to any semiconductive fabrication process, such as silicon and gallium arsenide processes. The process is especially applicable in the manufacture of semiconductors incorporating highly variable materials. Further, the amount of material that would normally be unacceptable may be compensated for and used to manufacture semiconductors within the desired performance requirements. 
     Those of ordinary skill in the art will recognize various modifications to the above process. These variations and modifications are considered within the scope of the disclosure and the claims that follow.