Abstract:
Fluid injection devices comprise M sets of fluid injection units. Each fluid injection unit comprises N injectors separately connecting to a driver. A controller separately transmits a signal to the driver, thereby simultaneously driving a selected injector of each of the M sets of fluid injection units. A non-selected injector of each of the M sets of fluid injection units does not trigger bipolar junction transistors (BJTs).

Description:
BACKGROUND 
     The invention relates to fluid injection devices, and more particularly, to fluid injection devices preventing activation of a bipolar junction transistor (BJT) therein. 
     Typically, fluid injection devices are employed in inkjet printers, fuel injectors, biomedical chips and other devices. Among inkjet printers presently known and used, injection by thermally driven bubbles has been most successful due to reliability, simplicity and relatively low cost. 
       FIG. 1  is a cross section of a conventional monolithic fluid injector disclosed in U.S. Pat. No. 6,471,338, the entirety of which is hereby incorporated by reference. A conventional monolithic fluid injector  10  is fabricated by micro-electro-mechanical system (MEMS) and metal oxide semiconductor field effect transistor (MOSFET) processes. The conventional monolithic fluid injector comprises a base such as a silicon substrate  38  with a field oxide layer  50  thereon. A structural layer  42  is formed on the field oxide layer  50 . A fluid chamber  14  is formed between the silicon substrate  38 , the field oxide  50 , and the structural layer  42 . The fluid chamber  14  connects a fluid reservoir (not shown) via a channel  16 . A first heater  20  and a second heater  22  are formed on the structural layer  42 . A dielectric layer  45  is disposed overlying the structural layer  42  defining a nozzle  17 . The nozzle  17  adjacent to the first and the second heaters  20 ,  22  connects the fluid chamber  14 . The first and the second heaters  20  electrically connect a driver via a signal transmitting circuit  44 . The driver is a MOSFET comprising a drain  107 , a gate  105  with a gate dielectric layer  52  between the  105  and the base  38 , and a source  106 , wherein the drain  107  electrically connects the signal transmitting circuit  44 . A passivation  46  is disposed on the fluid injection device and the driver. 
     As the development of fabrication processes has progressed, fluid injection devices with high density nozzles and multiple activation methods thereof to increase printing quality and speed have been introduced. A driver integrated with conventional fluid injection devices comprises a MOSFET device. When multiple nozzles are activated simultaneously, parasitic bipolar junction transistors (BJT) can be triggered, causing abnormal injection. The abnormal injection not only reduces printing quality, but also overheats the heaters, reducing the lifetime of the fluid injection device. 
     Accordingly, fluid injection devices with high density nozzles and multiple activation methods which do not activate parasitic bipolar junction transistors (BJTs) are desirable. 
     SUMMARY 
     The invention provides fluid injector devices integrating MOSFET doping with low concentration dopant to reduce junction capacitance between a drain and a base, preventing activation of parasitic bipolar junction transistors (BJTs) and abnormal injection. 
     The invention further provides a fluid injection device, comprising M sets of fluid injection units, each fluid injection unit comprising N injectors, each injector separately connecting to a driver, and a controller separately transmitting a signal to the driver, thereby simultaneously driving a selected injector of each of the M sets of fluid injection units, wherein a non-selected injector of each of the M sets of fluid injection units does not trigger a bipolar junction transistor (BJT). 
     Note that the injector comprises a structural layer disposed on a substrate, a fluid chamber formed between the substrate and the structural layer, a channel connecting the fluid chamber, at least one fluid actuator disposed on the structural layer and opposing the fluid chamber, and a nozzle adjacent to the at least one fluid actuator passing through the structural layer connecting the fluid chamber. 
     The invention also provides a fluid injection device, comprising M sets of fluid injection units, each fluid injection unit comprising N injectors, each injector separately connecting a MOS transistor comprising a drain, a gate, a source, and a base, wherein the drain connects the injector via a signal transmitting circuit, and wherein the junction capacitance between the drain and the base is equal to or less than 1.139×10 −14 (F/μm 2 ), and a controller separately transmitting a signal to the driver, thereby simultaneously driving the injector of each of the M sets of fluid injection units, wherein the injector is driven by the driver without triggering a bipolar junction transistor (BJT). 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description in conjunction with the examples and references made to the accompanying drawings, wherein: 
         FIG. 1  is a cross section of a conventional monolithic fluid injector; 
         FIG. 2  is a block diagram of an embodiment of a fluid injection device according to an embodiment of the invention; 
         FIG. 3  is a cross section of a nozzle of a fluid injection device according to an embodiment of the invention; 
         FIG. 4  is a schematic view of an exemplary embodiment of the active matrix driving circuit; 
         FIG. 5  shows driving signals of the active matrix driving circuit to activate the fluid injection device; 
         FIG. 6  is an equivalent circuit of a fluid injection device according to an embodiment of the invention; 
         FIGS. 7A-7D  are voltage and current waveforms of P 1 -P 16  dependent on driving loads under CS on and off states; 
         FIG. 8  is a relationship of substrate capacitance dependent on driving loads with dosage concentration variations; 
         FIG. 9  shows the relationship of depletion capacitance of drain junction C JD  and the number of driving loads under a dosage concentration of 10 20  atoms/cm 3 ; 
         FIG. 10  shows the relationship of depletion capacitance of drain junction C JD  and the number of driving loads under increasing 20% dosage concentration of 10 20  atoms/cm 3 ; and 
         FIG. 11  shows the relationship of depletion capacitance of drain junction C JD  and the number of driving loads under reducing 20% dosage concentration of 10 20  atoms/cm 3 . 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
       FIG. 2  is a block diagram of an embodiment of a fluid injection device according to an embodiment of the invention. Note that the invention provides a monolithic fluid injection device with 300 nozzles for implementing different features of various embodiments. These are, of course, merely examples and are not intended to be limiting. It should be appreciated by those skilled in the art that other injection devices, such as high density piezoelectric injector, can also use the transistor disclosed hereinafter. 
     The fluid injection device  100  comprises M sets such as 16 sets of injection units P 1 -P 16 . Each set of injection units P 1 -P 16  comprises N number of such as 19 nozzles A 1 -A 19 . Each nozzle A 1 -A 19  connects to a driver (not shown). A controller  150  transmits a control signal to each driver separately, thereby one nozzle A 1 -A 19  in each set of injection units P 1 -P 16  can be triggered simultaneously. The un-selected nozzles A 1 -A 19  are not triggered by parasitic bipolar junction transistor (BJT) of the corresponding driver. 
       FIG. 3  is a cross section of an exemplary embodiment of nozzle A 1  of the fluid injection device  100 . The nozzle A 1  is fabricated using standard micro-electro-mechanical system (MEMS) and metal-oxide-semiconductor (MOS) transistor processes. A base such as a silicon substrate  338 , with field oxide  350  thereon is provided. A structural layer  342  is disposed on the silicon substrate  338  and the field oxide  350 . A fluid chamber  314  is formed in the field oxide  350  between the substrate  338  and the structural layer  342  for receiving fluid. The fluid chamber  314  connects a fluid container (not shown) through a fluid channel  316 . A dielectric layer  345  is disposed overlying the structural layer  342  defining a nozzle  317 . The nozzle  317  is formed between heaters  320 , and  322 , communicating with the fluid chamber  314 . A first heater  320  and a second heater  322  are disposed on the structural layer  342 . The first heater  320  and second heater  322  can be electrically coupled to a driver. The driver can be a metal-oxide-semiconductor field effect transistor (MOSFET) comprising a drain  307 , a gate  305  with a gate dielectric layer  352  between the  305  and the base  338 , a source  306 , for example. The drain  307  can electrically connect to a signal transmitting circuit  344 . The junction capacitance between the drain and the substrate can be reduced by reducing the doping concentration of the source  306  and drain  307 , thereby preventing an unselected nozzle from being triggered by the parasitic bipolar junction transistor (BJT). Thus, optimized printing results can be achieved. For example, the n-type doping concentration of the source  306  and the drain  307  is preferably in a range of 10 20 -10 21  atoms/cm 3  with corresponding junction capacitance between the drain and the substrate of less than or equal to 1.139×10 −14  F/μm 2 . A passivation layer  346  covers the fluid injection device  100  and driver. 
       FIG. 4  is a schematic view of an exemplary embodiment of the active matrix driving circuit. According to some embodiments of the invention, the fluid injection device  100  can be divided into 16 groups (P 1 -P 16 ), for example. Each group can be divided into 19 addresses (A 1 -A 19 ). In order to reduce the total number of the I/O pads on the tape automatic bond (TAB) board, the addresses A 1 -A 19  can be further grouped into three pads (AG 1 , AG 2 , AG 3 ).  FIG. 5  shows driving signals of the active matrix driving circuit which activate the fluid injection device. 
     Referring to  FIG. 4 , when a specific nozzle is selected, a selected address (A 1 -A 19 ) and group (P 1 -P 16 ) are switched on. If a fluid injection device is selected, controller  150  applies bias on pad CS to turn on switches  203 ,  204  and  205 . Next, pads AG 1 , AG 2 , AG 3  can be sequentially biased to turn on switches of the addresses (A 1 -A 19 ). For example, a selected nozzle A 19 , i.e., pad A 19  of group AG 3  is triggered by turning on the MOSFET  215 . A current P 1  can pass through the MOSFET  215  to heaters neighboring the nozzle A 19 , thereby activating the nozzle A 19 . 
     For example, color and black inkjet heads of a printer commonly use electrical pads AG 1 , AG 2 , AG 3 , A 1 -A 8  and P 1 -P 24  to reduce costs. Whether the color or black inkjet head is triggered depends on which CS of the color or black inkjet head is switched on. Therefore, both the color and black inkjet heads can apply a driving voltage of 12V. Each MOSFET  215 , such as an NMOS, corresponding to each nozzle can be simplified as an equivalent circuit as shown in  FIG. 6 . When CS is switched off and the relationship of driving voltage change dependent on the driving time is 
                 ⅆ   V       ⅆ   t       =       12   ⁢           ⁢   V       2   ⁢   us             
for P 1 -P 16 , the total capacitance of the substrate can be expressed as 300 C db  in parallel. The resistance of the substrate can be R b . A parasitic NPN bipolar junction transistor (BJT) is triggered when substrate current I d2  is great enough that the result of R b ×I d2  is greater than the forward bias of the NPNBJT. Furthermore, if charges accumulated at the junction of the substrate and the MOSFET  215  are not conducted to ground, the trigger time of NPNBJT can be prolonged causing burnout of the fluid injection device.
 
       FIGS. 7A-7D  are voltage and current waveforms of P 1 -P 16  dependent on driving loads under CS on and off states. Referring to  FIGS. 7A and 7B , when CS is turn on triggering less than nine P-lines, curves I and II exhibit perfect voltage and current waveforms of P 1 -P 9  without overshoot current I os . Optimized injection quality can be achieved when current waveforms without overshoot current I os  are provided. If driving more than 9 P-lines simultaneously, overshoot current I os  may cause more power consumption. Hot carrier effect may trigger parasitic NPNBJT, reducing lifetime of the injection device. 
     Referring to  FIGS. 7C and 7D , when CS is at the off state, curves I′ and II′ voltage and current waveforms of switching on P 1 -P 16  and P 1 -P 9  respectively. Different overshoot currents I os  caused by different loading may turn on parasitic NPNBJT. 
     For example, when driving loads less than 9, i.e., less than 9 P-lines are triggered simultaneously, the driving current waveforms can be square. A drain junction capacitance C JD  of each NMOS  215  can be 1.139×10 −14 (F/μm 2 ).  FIG. 9  shows the relationship of depletion capacitance of drain junction C JD  and the number of driving loads under a dosage concentration of 10 20  atoms/cm 3 . When reducing the dosage concentration of 10 20  atoms/cm 3  by 20%, the driving current waveforms can be square when driving loads more than 10, i.e., when more than 10 P-lines are triggered simultaneously. A depletion capacitance of drain junction C JD  of each NMOS  215  can be 1.059×10 −14 (F/μm 2 ) as shown in  FIG. 10 . When increasing the dosage concentration of 10 20  atoms/cm 3  by 20%, the driving current waveforms can be square when driving loads less than 8, i.e., when less than 10 P-lines are triggered simultaneously. A depletion capacitance of drain junction C JD  of each NMOS  215  can be 0.991×10 14 (F/μm 2 ) as shown in  FIG. 11 . 
       FIG. 8  shows the relationship of substrate capacitance dependent on driving loads with varied dosage concentration. In order to achieve a high printing rate, more P-lines being triggered simultaneously is required. Preferably, 16 P-lines can be triggered simultaneously. When 16 P-lines can be triggered simultaneously, C db  of  FIG. 6  can be expressed as: 
     
       
         
           
             
               
                 C 
                 db 
               
               = 
               
                 
                   C 
                   JD 
                 
                 × 
                 
                   A 
                   D 
                 
               
             
             ; 
           
         
       
       
         
           
             
               
                 C 
                 JD 
               
               = 
               
                 
                   C 
                   
                     j 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     0 
                   
                 
                 
                   
                     1 
                     + 
                     
                       
                         V 
                         DB 
                       
                       
                         ϕ 
                         0 
                       
                     
                   
                 
               
             
             ; 
             and 
           
         
       
       
         
           
             
               
                 C 
                 
                   j 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   0 
                 
               
               = 
               
                 
                   
                     
                       qK 
                       s 
                     
                     ⁢ 
                     
                       ɛ 
                       0 
                     
                     ⁢ 
                     
                       N 
                       D 
                     
                   
                   
                     2 
                     ⁢ 
                     
                       ϕ 
                       0 
                     
                   
                 
               
             
             ; 
           
         
       
     
     where C JD  is the depletion capacitance of the drain junction, A D  is the area of the drain junction, ø 0  is built-in voltage, q is 1.602×10 −19 C, ε 0  is 8.854×10−12 F/m, K s  is relative permittivity of silicon, N D  is dosage concentration. 
     According to some embodiments of the invention, in order to drive P 1 -P 16  simultaneously under predetermined injection parameters, i.e., with constant driving voltage and heating time, C JD  of a MOSFET less than or equal to 1.139×10−14(F/μm 2 ) is required. That is, the concentration of n-type drain doping can be reduced to 10 20 -10 21  atoms/cm 3  to ensure driving P 1 -P 16  simultaneously without generating overshoot current. Alternatively, C db  can also be reduced by shrinking the drain/source area. 
     While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. On 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.