Advanced Microsystems for Automotive Applications 2004

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Pressure Measure 1 - The event must power up the sensing element, con- vert its variation to a voltage, store that result using a sample and hold capac- itor and then shut down. Voltage Measure 3 - The voltage measurement is primarily to monitor the hattery. It is done using a voltage threshold comparator for the minimum bat- tery voltage. There must also be a ftiirly accurate voltage reference.

This event must power up the voltage reference and the comparator, store the result and then shut down. There will also be a requirement to perform this voltage measurement during both the highest current event usually the RF transmis- sion and during any event requiring the highest battery voltage usually an analog measurement.

Wheel module architecture. Data Processing 5 - The data collected from the above measurements must be analyzed according to a defined operational algorithm. The result will determine how the TPM will operate in the future and whether a transmission is to be made. The data processing will also format the data for the RF trans- mission.

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The power consumed by such digital logic will depend on a very small standby current term and a speed-related term that is linearly related to its clocking speed. Further, each data process can be separated out into individual events such as read data, calculate data, format data, send data, etc.

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Highly Integrated Tire Pressure Monitoring Solutions I 27 Wake Up 6 - The wheel sensor will normally be in a standby mode waiting for a need to measure and process data. This wake-up function is usually a very low power oscillator that outputs a pulse every 3 to 6 seconds. This event will be one of the long time standby current events that may exist for the complete life of the battery.

This is the highest current consumption module and creates the highest current pulse. However, while transmitting the operational voltage may be less than required for the meas- urement modules. Therefore the battery cut-off voltage will be different from that for the other modules. In all cases, a minimal number of RF transmissions are the most important feature of the operational algorithm. A confounding problem with the RF transmitter is how to handle the possibility of simulta- neous transmissions from multiple wheel sensors. This would result in data "collisions" causing the RF receiver to be unable to discern any data.

The usual method to solve this issue is to randomize the transmission times and also send multiple randomized data words within any given transmission. One purpose of the NVM is to store trimming values to make each sensor meet the required accuracy for pressure and temperature measure- ments.

One other purpose for the NVM is to store a unique serial code for each wheel sensor and to store the configuration bit of the device. The NVM may draw current for each programmed bit as well as some digital switching cur- rent during its access. This is usually done at frequencies around kHz using a transmitting coil mounted on the chas- sis or axle housing somewhere near each wheel and a small LF receiver coil within each wheel sensor.

The LF receiver can also be used to trigger a given wheel sensor to transmit within a given time frame.

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In this way the chassis receiver can match up the wheel sensor's serial ID with the location that was triggered and provide tire localization information. A further use of the LF receiver would be to receive test and other commands from a closely placed inductive transmitting coil.

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This can provide diagnostics and fast identification of wheel sensors in either the factory or field. Over-Temperature 10 - A concern with TPM is that extremely high tem- peratures may develop within the tire and wheel, which are beyond the nor- mal operational range for the wheel sensor components. The major source of such high temperatures is the braking system that can transfer heat through 28 I Safety the hub to the wheel rim.

Power Management 11 - The most critical operational issue with TPM is keeping total power consumption to a minimum. Therefore, power manage- ment requires special attention.


The drawback of this approach is that the MCU requires a number of instructions to manipulate the enables. If such power control sequences were controlled by logic the changes would be nearly instantaneous and consume much less power. Therefore as many tasks as possible should have their power sequenced using hardware logic in small state machines.

SV, 3. Although linearity errors can be improved with reduced pressure sensitivity, more circuit gain is required, and circuit noise and other errors can increase. Thus, there is a desire to have a high-pressure sensitivity and a low-pressure linearity error, and therefore, other approaches were investigated to reduce linearity errors. Transducer using an internal post diaphragm design a. Transducer using a rectangular diaphragm design b.

In addition to previously used shapes, such as circles and squares, "internal post" configurations were evaluated.

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  8. Internal post designs anchor the diaphragm both at its perimeter and at inter- nal locations. Simulations showed that some configurations of internal posts could produce more uniform diaphragm deflection, and significantly reduce linearity error at a given sensitivity level. Figures 3 a and 3 b show photo- graphs of an internal post design and a rectangular diaphragm design. The methodology used in the insertion of the sensor module into the CMOS process was based on the requirements that there is no impact, either ther- mally or topographically, on the CMOS fabrication.

    This is done by insertion of the micromachining steps before the temperature sensitive process steps of the CMOS process. In addi- tion, the precess integration was designed to reuse, as much as possible, the fabricatien steps of the CMOS process. The cross-sectien of the CMOS and sen- sor devices is shown in figure 4 illustrating the key features of the process inte- gration. Selected cross-sections of CMOS integrated capacitive pressure sensor process flow.

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    In addition to defining the active area of the CMOS devices, the field oxide is used as the area where the sensor is to be formed. The floating gate polysilicon is patterned and etched so that it also forms the bottom fixed plate of the capacitive pressure sensor.

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    The isolation of the fixed electrode to the substrate is provided by the CMOS field oxide isolation. The CMOS transistor gate oxide is then deposited followed by the gate polysilicon deposition. CMOS Integrated pressure sensor. At this stage, the CMOS fabrication is suspended and the sensor fabrication steps are inserted. The next step is the removal of the PECVD oxide, gate polysilicon and the gate oxide from the sen- sor area. This is done in such a way that the bottom polysilicon plate is not 32 I Safety affected by the etching process. After the gate oxide is removed, an isolation layer of low stress silicon rich nitride is deposited using an LPCVD process.

    The silicon rich nitride is chosen for its excellent etch resistance to HF based chemistries. The silicon nitride is used to isolate the top and bottom plates of the pressure sensor and also protect the CMOS area from subsequent micro- machining steps.

    The sacrificial layer of the sensor is next deposited using a phosphorus-doped glass. The thickness of this layer defines the spacing between the bottom fixed electrode and the moveable diaphragm. The spacer layer is also patterned for transducer designs, which utilize internal support posts for improved pressure linearity.

    The polysilicon is then doped, patterned and etched to define the diaphragm. The pressure sensitive diaphragm is then released by removing the sacrificial layer below the diaphragm polysilicon. Etch ports located at the periphery allow the HF based chemistry to completely remove the phosphorus-doped glass. The thickness of the PECVD oxide is chosen to ensure that the etch holes are plugged and the cavity between the fixed plate and the diaphragm is sealed at the deposition pressure of the PECVD oxide.

    This com- pletes the formation of the sensor module. The next step is the resumption of the CMOS fabrication process.


    This is done by first removing the sealing oxide, isolation nitride and protective oxide over the CMOS device area. This thermal treatment also serves to activate the dopants and provide the stress relief anneal in the sensor polysilicon film. The first interlayer dielectric layer is next deposited, patterned and etched to allow for the subsequent first metal layer to contact the various CMOS devices as well as provide the interconnect for the sensor to the circuit.

    The second interlayer dielectric layer is then deposited, patterned and etched to form vias for the second metal to contact the first metal with the routing required for the circuit blocks.

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