Abstract:
A method for fabricating a monolithic chip including multi-sensors that can detect pH, temperature, photo-intensity simultaneously and a readout circuit. As such, as well as the multi-sensors, the readout circuit also has a reduced chip area at low cost since selection switches are used to sequentially read pH, temperature and photo-intensity detecting values, wherein the readout action is completed within a clock cycle. The entire structure is fabricated with standard 0.5 μm CMOS IC, Double Poly Double Metal (DPDM), n-well technology and allows the integration of the on-chip signal conditioning circuitry. The chip fabricated by the method can not only sense the Ph, temperature, photo values but also apply the extended gate field effect transistor (EGFET) on the temperature and light compensation to produce realistic pH values.

Description:
This nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 91124266 filed in TAIWAN, R.O.C. on Oct. 21, 2002, which is herein incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a method for layout of a monolithic chip with multi-sensors, and in particular, to a method for fabricating a monolithic chip with multi-sensors and readout circuit using standard IC process. The chip is capable of sensing pH, temperature and photo-intensity simultaneously. Additionally, the invention can achieve the objects of mass production, high yield and low cost, without additional process but the standard process. 
   2. Description of the Related Art 
   A glass electrode is typically used as an ion-sensing electrode. However, the glass electrode has the disadvantages of non-miniaturizable measurement, easy damage and portable inconvenience. Piet Bergveld introduced an ion sensitive field effect transistor (ISFET) in 1970. The ISFET has the advantages of providing pH measurement, compatible with the standard CMOS process, high input impedance and low output impedance, miniaturization for a minute solution measurement, and fast response. Therefore, the ISFET has been widely studied and applied in pH sensor miniaturization and biomedicine fields. 
   However, existing measurements for ISFET pH sensors have found that temperature and environment light could affect ISFETs in practical applications due to the open-gate design. 
   SUMMARY OF THE INVENTION 
   Accordingly, an object of the present invention is to provide a method for fabricating a monolithic chip including multi-sensors that can detect pH, temperature, photo-intensity simultaneously and a readout circuit using the IC standard process, so as to obtain mass production, high yield, and low cost without additional process. 
   The present invention provides a method for fabricating a monolithic chip including multi-sensors that can detect pH, temperature, photo-intensity simultaneously and a readout circuit using the standard IC process. The method uses the IC standard process to sequentially layout the monolithic chip in horizontal direction as below. First, an ISFET with an extended gate sensitive window is formed and an SnO 2  sensitive membrane is coated on the window using sputtering process, so as to form an extended gate field effect transistor (EGFET) as an ion sensitive sensor (pH sensor). Second, a metal oxide semiconductor field effect transistor (MOSFET) is formed, wherein an integrated readout circuit with selective switches is implemented to sequentially output values sensed by the multi-sensors. Using the ion implanting process, an N-type well is formed over a P-type substrate and then a p-diffusion diffusion region is formed within the N-type well. Thus, a temperature diode consisting of the p-diffusion region with respect to the N-type well is formed such that the temperature diode acts as a temperature sensor to sense temperature in forward bias voltage. Using the ion implanting process, an n-diffusion region is formed within the P-type substrate such that a photodiode consisting of the n-diffusion region with respect to the P-type substrate is produced and acts as a photo-sensor to sense photo-intensity in reverse bias voltage. 

   
     DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention, reference is made to a detailed description to be read in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a cross-section of a laminated structure used in a chip layout according to the present invention; 
       FIG. 2  is a cross-section of the whole chip structure according to the present invention; 
       FIG. 3  is a layout diagram of the summation chip according to the present invention; 
       FIG. 4  is a schematic diagram of an extended gate field effect transistor (EGFET) circuit of  FIG. 2  according to the present invention; 
       FIG. 5  is a schematic cross-section of the circuit of  FIG. 4  according to the present invention; 
       FIG. 6  is a schematic cross-section of a temperature diode of  FIG. 2  according to the present invention; 
       FIG. 7  is a schematic cross-section of a photodiode of  FIG. 2  according to the present invention; 
       FIG. 8  is a schematic diagram of a readout circuit of the chip according to the present invention; 
       FIG. 9  is a diagram of partial readout circuit of the pH sensor of  FIG. 8  according to the present invention; 
       FIG. 10  is a diagram of partial readout circuit of the temperature diode of  FIG. 8  according to the present invention; 
       FIG. 11  is a diagram of partial readout circuit of the photodiode of  FIG. 8  according to the present invention; 
       FIG. 12  is a schematic diagram of a measurement configuration according to the present invention; 
       FIG. 13  is curves of the sensing current-reference voltage (I DS -V EGREF ) under the different pH buffer solutions for the EGFET pH sensor of  FIG. 12 ; 
       FIG. 14  is a curve of the output voltage-pH (Vout-pH) of  FIG. 13  according to the present invention; 
       FIG. 15  is curves of the sensing current-voltage (I DS -V DS ) under the different pH buffer solutions for the EGFET pH sensor of  FIG. 12 ; 
       FIG. 16  is a curve of the output voltages of the pH sensor of  FIG. 12  at the different time; 
       FIG. 17  is a curve of the output voltages of the temperature sensor of  FIG. 12  at the different time; 
       FIG. 18  is a curve of the output voltage-temperature (Vout-T) of  FIG. 17  according to the invention; 
       FIG. 19  is a curve of the output voltages of the photo-sensor of  FIG. 12  at the different time; and 
       FIG. 20  is a curve of the output voltage to photo-intensity (Vout-PIL) of  FIG. 19  according to the invention, 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows cross-sectional layers fabricated by the standard 0.5 μm complementary metal oxide semiconductor integrated circuit (CMOS IC), Double Poly Double Metal (DPDM), n-well process, In  FIG. 1 , in addition to the deposition and metal layers produced by the standard CMOS IC process, a temperature sensor ( 20 ,  21 ) and a photo-sensor ( 19 ,  22 ) are also produced by the doping step of the standard process. As shown in  FIG. 1 , the layers are characterized as below: passivation layer  10  with 0.7 μm thick; metal layer  11  with 1.1 μm thick; oxide layer  12  with 0.5 μm thick; metal layer  13  with 0.6; oxide layer  14  with 0.5 μm thick; gate poly-Si layer  15  with 0.3 μm thick; gate oxide layer  16  with 135 Å thick; capacitance poly-Si layer  17  with 0.2 μm thick; field-oxide layer  18  with 0.5 μm thick; the N-type metal oxide semiconductor (NMOS), which is included in an extended gate field effect transistor (EGFET) over a silicon substrate  19 , with aspect width/length (W/L) of 600 μm/20 μm, sensitive window of 300 μm×300 μm, temperature diode formed by the P-type diffusion  21  to N-well  20 , photodiode formed by the N-type diffusion  22  to P-type substrate  19 ; and the inventive chip with a total area of 1.8 mm×1.8 mm. 
     FIG. 2  is a whole cross-section of the whole chip structure according to the present invention. In  FIG. 2 , the structure includes an EGFET; a metal oxide semiconductor field effect transistor (MOSFET), a temperature diode TD and a photodiode PD. As shown in  FIG. 2 , the chip includes an ion sensitive field effect transistor (ISFET), formed by an N-type diffusion  221  (applying the N-type diffusion  22  of  FIG. 1 ) and a middle gate  150  (applying the gate poly-Si layer  15  of FIG.  1 ), with extended structure  130  (applying the metal layer  13  of FIG.  1 ), using the standard IC process. Additionally, the ISFET with extended structure  150  has an open sensitive window coated with SnO 2  thin film  110  (applying the metal  110  of  FIG. 1 ) using a sputtering machine to produce required sensitive means and the sensitivity of which is 58 mV/pH in a concentration range between pH 2 and pH 10, which is very close to ideal Nernst response that shows a linear pH sensitive curve. The temperature sensor TD is a diode formed by the P-type diffusion  210  (applying the P-type diffusion  21  of  FIG. 1 ) to N-well  200  (applying the N-well  20  of  FIG. 1 ) and operated in reverse bias voltage. The sensor TD has a sensitivity of −1.83 mV/° C., which has high reliability and linearity. The photo-sensor PD is a diode formed by the N-type diffusion  220  (applying the N-type diffusion  22  of  FIG. 1 ) to P-type substrate  190  (applying the P-type substrate  19  of  FIG. 1 ) and operated in forward bias voltage. The sensor PD has a fast response and a sensitivity of −92.66 μV/lux using a 234 k feedback resistor. The inventive readout circuit is also fabricated by the standard CMOS IC process to layout in the MOSFET (not shown). A field-oxide  180  (applying the field-oxide  18  of  FIG. 1 ) is layered between the devices for electrical isolation. 
     FIG. 3  is a layout of the chip  30  according to the invention. As shown in  FIG. 3 , when the above-cited process is applied to layout the devices to the chip  30  in an area of 1.8 mm×1.8 mm, the invention has features of easy manufacture, no additional photomask required by the back-end process and adaptive standard CMOS IC process (such as UMC or TSMC). In the back-end process, only an SnO 2  sensitive membrane is coated on the gate window to produce required sensitive means. The devices of EGFET, TD, PD and integrated readout circuit are described in detailed as below, wherein the readout circuit includes amplifiers OP 1 -OP 3 , a bandgap reference circuit BREF. 
     FIG. 4  is a schematic diagram of the EGFET circuit of FIG.  2 . In  FIG. 4 , the EGFET circuit includes an extended gate SnO 2 /Al multi-layer electrode sensor that consisting of an open window Wo and a transistor M 1 . As shown in  FIG. 4 , the configuration produces a linear dependence between the ion sensitive membrane potential and the output voltage. 
     FIG. 5  is a cross-section of the circuit of FIG.  4 . In  FIG. 5 , the right cross-section is an EGFET layout and the left cross-section is an MOSFET layout. As shown in  FIG. 5 , using the standard N-well poly-Si gate semiconductor process with accordance to the structure provided by  FIGS. 1 and 2  layouts the EGFET and MOSFET devices over a P-type substrate, wherein an SiO 2  field-oxide oxide layer  18  is located between two adjacent devices. Next, using A 1  metal A 1 , the gate G of the EGFET is extended to the position of the sensitive window. A layer of glass A 4  is an electrical insulator over the A 1  metal except for the sensitive window. Finally, the sensitive window is coated on a layer of SnO 2  sensitive membrane ISM to complete the structure. Because the readout circuit (described later) layout is integrated into the SiN passivation layer of the MOSFET (not shown) and the entire structure is encapsulated with epoxy resin A 3  except for the sensitive window where provides electrical connection to the external, the readout circuit is not affected by any external factor. Because an NMOSFET has higher mobility than a PMOSFET, the NMOSFET is preferred for the MOSFET. 
     FIG. 6  is a cross-section of the temperature sensor TD of FIG.  2 . As shown in  FIG. 6 , the sensor PD is fabricated by the standard semiconductor process to dope P-type ions to the N-well in the P-type substrate and thus produce the diffusion region p +  to further form a diode structure with p + -N well. The formed diode is active as the desired temperature sensor TD because the active voltage will reduce with increasing temperature in forward bias voltage. 
     FIG. 7  is a cross-section of the photodiode PD of FIG.  2 . As shown in  FIG. 7 , the sensor PD is fabricated by the standard semiconductor process to dope N-type ion to a region between two N-wells in the P-type substrate and thus produce the diffusion region n +  to further form a diode structure with n + -P substrate. The formed diode is active as the photo-sensor PD because the active current caused by charge density will change with photo-intensity change. 
     FIG. 8  is a schematic diagram of the readout circuit. In  FIG. 8 , the readout circuit includes two switches SW 1 , SW 2 , three amplifiers OP 1 -OP 3 , a constant current source Is, a bias voltage, a variable resistor R, a gain resistor Rf, an ion sensor M 1 , a temperature sensor TD, a photo-sensor PD, and an output terminal Out. As shown in  FIG. 8 , the readout circuit sequentially reads a pH signal from the pH sensor M 1 , a temperature signal from the temperature sensor TD, and a photo-intensity signal from the photo-sensor PD within a clock cycle through the SW 1  and SW 2  control. When the switch is on position  1 , the output terminal Out outputs the pH signal. When the switch is on position  2 , the output terminal Out outputs the temperature signal first and the photo-intensity signal later. Accordingly, the above operation is repeated and so all sensitive values from the sensors are read out. This requires only 60% of original area for a conventional layout and further increase cost efficiency. The sensors are now described as below. 
     FIG. 9  is the readout circuit for the pH sensor of FIG.  8 . As shown in  FIG. 9 , the pH sensor is shown as the switches SW 1  and SW 2  are on position  1 . The readout circuit for the pH sensor is constant current constant voltage configuration. Inside the readout circuit, the amplifier OP 1  provides a stable voltage to the EGFET M 1  through the input of a bandgap reference circuit BREF to obtain a sensing voltage V DS  between the drain D to source S of M 1 . Next, the amplifier OP 2  provides a stable current, equal to a sensing current I DS  between the drain D to source S of M 1 , to the pH sensor M 1 . The current I DS  with respect to the voltage V DS  forms a feedback of voltage between the gate E GREF  to source S and thus the output of the current I DS  is changed with the pH change of the solution used. Therefore, the pH value is detected by the extended gate E GREF . After the readout circuit for the pH sensor is configured, the amplifier OP 1  has a positive input terminal, a negative input terminal and an output terminal. The positive input terminal is connected to an external bandgap reference circuit BREF to provide the stable voltage. The negative input terminal is connected to a control terminal  0  of a second selective switch SW 2  and the output terminal. The amplifier OP 2  has a positive input terminal, a negative input terminal and an output terminal. The positive input terminal is connected to a reference bias circuit. The negative input terminal is connected to a control terminal  0  of a first selective switch SW 1  and a constant current Is connected to an external negative voltage source Vss. The output terminal is connected to an external output terminal Out. The pH sensor M 1  has the gate E GREF , the drain D and the source S. The gate E GREF  is connected to the output terminal of the amplifier OP 2 , the source S is connected to a first selective terminal  1  of the first switch SW 1  and the drain D is connected to a first selective terminal  1  of the second switch SW 2 . As such, the readout circuit configuration with constant current constant voltage implementation is complete. 
     FIG. 10  is the readout circuit for the temperature sensor of FIG.  8 . As shown in  FIG. 10 , the temperature sensor is shown as the switches SW 1  and SW 2  are on position  2 . The amplifier OP 2  modulates the temperature diode to obtain a constant current using a connected constant current Is of the negative voltage source Vss and reference of the variable resistor. Thus, the sensing output will vary with temperature changes as the active voltage varies with different temperature. Accordingly, the purpose of temperature detection is achieved, After the readout circuit for the temperature sensor is configured, the amplifier OP 2  has the configuration as cited above. The anode of the sensor TD is connected to the output terminal of the amplifier OP 2  and the cathode is connected to a second selective terminal  2  of the SW 1 , to complete the readout circuit for temperature sensor implementation. The reference bias circuit consists of a cascade variable resistor R and grounding bias source Vb. 
     FIG. 11  is the readout circuit for the photo-sensor of FIG.  8 . As shown in  FIG. 11 , the photo-sensor is shown as the switches SW 1  and SW 2  are on position  2 . The amplifier OP 1  provides photodiodes a stable voltage as an operating voltage. The amplifier OP 3  applies a feedback to the photo-current of the photodiodes to produce a voltage output and complete photo-intensity detection. In the configuration of readout circuit for the photo-sensor, the amplifier OP 1  is configured as the above. The amplifier OP 3  has a positive terminal, a negative terminal and an output terminal such that the positive terminal is connected to the grounding bias source Vb of the amplifier OP 1 , the negative terminal is connected to the anode of the photo-sensor PD and an end of a feedback resistor Rf, and the output terminal is connected to another end of the feedback resistor Rf and the external output terminal Out of the amplifier OP 1  at the same aide. As well, the cathode of the photo-sensor is connected to a second selective terminal  2  of the second selective switch SW 2  to complete the readout circuit of the photo-sensor. 
     FIG. 12  is a schematic diagram of a measurement configuration able to read the desired sensing values according to the present invention. As shown in  FIG. 12 , the chip  30  on a ceramic base  91  is encapsulated with epoxy resin Epo to provide electrical insulation for the bonding wires and exposed silicon regions of the chip  30 . Next, the encapsulated chip  30  is immersed in a buffer solution  92  by 1 minute in order to measure and analyze the sensors of the chip  30  through the sensitive window  90  with SnO 2  membrane later. In practice, the HP4145B semiconductor parameter analyzer  93  was used to obtain the sensing values of the sensors through the integrated readout circuit via the membrane  90  and refer to the voltage of the reference electrode EGREF to generate corresponding output Vout. 
     FIG. 13  is curves of the sensing current-reference voltage (I DS -V EGREF ) under the different pH buffer solutions for the EGFET pH sensor of FIG.  12 . In  FIG. 13 , the lateral axis is the reference voltage V EGREF  of reference electrode and the vertical axis is the sensing current I DS  of pH sensor. When the EGFET and an Ag/AgCl reference electrode are immersed in the pH 2, 4, 6, 8, 10 buffer solutions  92  by 1 minute at room temperature and then performs the measurement operating conditions, a family of curves can be generated by applying the drain-source voltage V DS =0.2V for the reference voltage V EGREF . As shown in  FIG. 13 , the resulting pH values of the solution  92  and the reference voltage V EGREF  are negative proportional to the current I DS . That is to say that the pH value is smaller as the current I DS  measured is higher. Therefore, the sensing purpose is achieved by means of the transfer characteristics of the sensing current I DS  output with significant changes at various pH buffer solutions. 
     FIG. 14  is a curve of the output voltage-pH (Vout-pH) of  FIG. 13  according to the present invention. As shown in  FIG. 14 , the output voltages Vout and the pH values have a positive linear relation. As aforementioned, the sensitive configuration of  FIG. 12  can generate a linear relationship between the potential of SnO 2  membrane Metal 2  and the output voltage. In this case, the SnO 2 /Al extended gate E GREF  of pH sensor M 1  has a linear pH sensitivity of approximately 58 mV/pH after analysis. 
     FIG. 15  is curves of the sensing current-voltage (I DS -V DS ) under the different pH buffer solutions for the EGFET pH sensor of FIG.  12 . As shown in  FIG. 15 , when such a pH sensor is placed in different pH buffer solutions, the change between the drain-source current I DS  and voltage V DS  under the different pH buffer solutions is analyzed and found that drain-source current varies with the turn-on voltage of pH buffer solution. In this case, for applying the reference electrode voltage V EGREF =2.0 V in operation, the drain-source voltage V DS  is varied from 0 to 3 V, and so the device is operated in saturation region. It is clearly shown in the figure that the current I DS  changes as a function of square root. 
     FIG. 16  is a curve of the output voltages Vout of the pH sensor of  FIG. 12  at the different time t. After the pH sensor is connected to its readout circuit through the switches as shown in  FIG. 12 , the measurement for the pH values is performed respectively in the pH 2, 4, 6, 8, 10 buffer solutions and the outputs are shown in FIG.  16 . In the figure, the vertical axis is output voltage Vout and the lateral axis is time t. It is clearly shown that the output changes have a linear pH sensitivity of approximately 58 mV/pH under different measurement time and pH buffer solutions. 
     FIG. 17  is a curve of the output voltages of the temperature sensor of  FIG. 12  at the different time. After the temperature sensor, here a p-n diode with standard CMOS IC process, is connected to its readout circuit through the switches as shown in  FIG. 12 , the measurement for the temperature values is performed respectively at 24° C., 30° C., 36° C., 40° C., 45° C., 50° C. and the outputs are shown in FIG.  17 . In the figure, the vertical axis is output voltage Vout, the lateral axis is time t and the measurement time is about 1 minute. It is clearly shown that the outputs are very stable and present linear. 
     FIG. 18  is a curve of the output voltage-temperature (Vout-T) of  FIG. 17  according to the invention. As shown in  FIG. 18 , after the sensitivity analysis, the output curve clearly presents a highly linear sensitivity of approximately −1.83 mV/° C. 
     FIG. 19  is a curve of the output voltages of the photo-sensor of  FIG. 12  at the different time. After the photo-sensor, here a photodiode with standard CMOS IC process, is connected to its readout circuit through the switches as shown in  FIG. 12 , the measurement for the photo-intensity values is performed respectively at 0 lux, 1000 lux, 2000 lux, 3000 lux, 4000 lux, 5000 lux, and the outputs are shown in FIG.  19 . In the figure, the vertical axis is output voltage Vout, the lateral axis is time t and the measurement time is about 1 minute. It is clearly shown that the outputs are very stable and present linear. 
     FIG. 20  is a curve of the output voltage to photo-intensity (Vout-PIL) of  FIG. 19  according to the invention. As shown in  FIG. 20 , when a 234 Ω feedback resistor Rf is applied to the sensitivity analysis for the output voltage to photo-intensity relation in  FIG. 19 , the output curve presents a highly linear sensitivity of approximately −92.66 μV/C. 
   As cited, all sensors (including the pH, temperature, and photo-intensity sensors) and corresponding readout circuits are fabricated in a single chip using the standard CMOS IC process, and the entire area used by the chip is only about 1.8 mm×1.8 mm including wiring points. Therefore, the inventive chip is suitable for mass-production due to the features of low cost and small physical body. The integrated chip can additionally provide the compensation for the temperature and optical effects and output desired precise pH sensing value. 
   The invention can also be applied to other fields. For example, the cited readout circuit can convert biochemical signal and physical signal respectively to an electrical signal to be output to measure the pH value of environment water for pollution monitors desired by some industries and for pH reaction controls desired by bio-industries. 
   While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.