MEMS Mechanical Sensors - Stephen Beeby Part 5 docx

Pressure Leadout Die attach Pressure sensor die Corrugated stainless steel diaphragm Oil-filled chamber Support chip Figure 4.7 Stainless steel isolation diaphragm... 4.4.3 Mechanical Is

liquid water forming at the interface [27] These polymers, however, offer poor lev-els of protection against alkaline solutions

Protective silicon oxide and silicon nitride films possess a much greater resis-tance to the diffusion of water molecules These films can be applied both at wafer level and on mounted chips using CVD processes They must be free from cracks and pinholes, and in the case of mounted chips, the films must be deposited on all the exposed surfaces, including wirebonds and contact pads The chemical resistance of these films is fundamentally important since they will only be deposited in thick-nesses of a few microns Even very low corrosion rates (27 angstroms/day) will remove a 1-micron-thick protective film after 1 year Silicon carbide thin-films have been found to offer the most promising levels of chemical resistance [28] A further consideration is the effects of thermal cycling, which can cause delamination of these films due to TEC mismatches

If the second order package is required to protect the device, the sealing processes developed by the IC industry and described in Section 4.3.3 can be used In the case of MEMS packaging, second order capping can be further complicated by the functionality of the device The most common example of this is in pressure sen-sors where a stainless steel diaphragm in the second order package is used to provide media isolation [29] Stainless steel offers excellent levels of chemical resistance and possesses good mechanical properties making it an ideal material for such a barrier diaphragm This diaphragm must not only protect the sensor but transmit the media pressure to it This is typically achieved by placing the sensing die in an oil-filled chamber behind the stainless steel diaphragm (see Figure 4.7) The pressure exerted

on the stainless steel diaphragm is transmitted through the hydraulic oil to the sensor diaphragm Both the stainless steel diaphragm and the oil used to fill the chamber will influence the behavior of the sensor The corrugated steel diaphragm shown in Figure 4.7 is an example of a mechanical design used to minimize its influence on the behavior of the sensor The thermal expansion of the oil will introduce another source of temperature cross sensitivity on the output of the sensor This approach also places limitations on the minimum attainable size, increases the costs of the device, and restricts the number of applications

Pressure

Leadout Die attach

Pressure sensor die

Corrugated stainless steel diaphragm

Oil-filled chamber

Support chip

Figure 4.7 Stainless steel isolation diaphragm.

4.4.2 Protecting the Environment from the Sensor

The MEMS package must also protect the environment from the presence and func-tion of the sensor Applicafunc-tion areas where this may be of particular concern include healthcare, food, beverage, and bioprocessing These typically require the microsen-sor to be isolated from the chemical or biological media by a mechanical interface or sensor housing made from a suitable material Types of interface materials include polymer membranes [30], ceramics, glass ceramics, and some metals The duration

of contact with the environment is a fundamental factor in choosing the material, and it must possess the following characteristics in the typical applications men-tioned earlier:

• Biocompatible;

• Nontoxic;

• Able to withstand sterilization

This is particularly important in biomedical applications, where the small size and performance characteristics of MEMS sensors make them highly attractive Examples of such devices include catheter blood pressure sensors and chemical monitoring systems (e.g., glucose) These have been successfully used in both in vivo and in vitro applications

Many of the applications discussed also impose space constraints upon the final packaged solution—catheter pressure sensors being the obvious example Tech-niques such as flip chip assembly and wafer level packaging can be employed to reduce the packages’ volumes In extreme cases where more than one sensing die or separate ICs are to be incorporated, chip stacking can be employed to further reduce package size [31, 32] Chip stacking introduces many potential difficulties including electrical interconnects, thermal issues, and packaging stresses [33] Electrical interconnects have been realized using through-wafer techniques discussed above, purpose-made intermediate chips with a suitable track layout, and also by forming metal tracks on the outside of the stack [34] A basic stacking approach can be used

to reduce packaging stresses, and this is discussed in the following section

Another interesting development that may suit some applications is the devel-opment of spherical semiconductors A 1-mm-diameter spherical semiconductor has three times the surface area of a 1-mm-square chip [35] Many sensing applica-tions have been suggested for this form of device including medical sensors and accelerometers

4.4.3 Mechanical Isolation of Sensor Chips

The mechanical isolation of the sensor chip is vital in avoiding unwanted cross sitivities and the transmission of external stresses through the packaging to the sen-sor Indeed, the package design itself must avoid introducing such undesirable effects and should provide relief from residual stresses trapped in the assembly dur-ing the packagdur-ing process Factors such as the long-term stability of packagdur-ing materials and methods are of fundamental importance A well-isolated sensor chip mounted on a carefully designed package will be less affected by changes in its envi-ronment, leading to improved long-term stability, resolution, and sensor accuracy

While this applies equally to capacitive, piezoresistive, and resonant devices, the per-formance advantages offered by resonant sensing can only be achieved with capable package design One of the major undesirable influences is the effect of temperature changes on the packaged sensor assembly Uneven thermal expansion coefficients of the different materials making up the packaged assembly often induce stresses across the sensor chip Similar packaging stresses can also be induced by the application of mechanical forces onto the second order packaging, changes in humidity, the pres-ence of vibrations, or be in-built in the assembly during the packaging process The following techniques for providing mechanical isolation of a sensor chip have been applied to a simple pressure sensor The pressure sensor in this case con-sists of an etched diaphragm with some form of strain-sensing mechanism fabricated

on the top surface, as shown in Figure 4.8 This example assumes direct contact of the pressurized media with the sensor chip, and therefore, other packaging require-ments, such as oil filling, are not considered in this case Pressure sensors are dis-cussed in more detail in Chapter 6

The simplest and lowest cost form of sensor package is to bond the sensor chip directly to the second order packaging, in this a case a simple TO header as shown schematically in Figure 4.9 Coupling to the sensor diaphragm is facilitated by a pressure port formed in the header Such an arrangement is based upon microelec-tronic device packaging and effectively has no first order packaging stage As a result, mechanical stresses are transmitted directly to the sensor chip and the trans-ducer housing is likely to be thermally incompatible with silicon due to TEC mis-matches The overall accuracy of the sensor will therefore be poor Thermal stresses can be compensated for to some degree by the sensing electronics, but associated drift cannot be compensated for

The above packaging solution is impractical in the vast majority of applications Improved mechanical isolation can be achieved by the following range of tech-niques, the suitability of which will depend upon each application and its particular packaging requirements:

• Use of a first order packaging stage (i.e., placing an intermediate, or support chip, between the sensor chip and housing);

• Mechanical decoupling on the sensor or support chip;

• Displacing the sensor away from the second order packaging;

• Die attach using of soft ductile bond materials;

Pressure-sensing diaphragm

Pressure

Figure 4.8 Typical pressure sensing die.

4.4.3.1 Basic First Order Packaging Stage

A basic yet typical first order packaging arrangements is shown in Figure 4.10 The support chip used in Figure 4.10 can be fabricated from either thermally matched lead borosilicate glass, such as Pyrex 7740 or Schott Borofloat 33, or silicon itself The glass constraint is typically anodically bonded to the silicon chip, providing and extremely strong molecular bond This bond can be performed at wafer level, ena-bling all devices to be simultaneously mounted If the glass constraints are not exactly matched to the silicon, some thermally induced stresses will occur because

of the TEC mismatch This drawback is exaggerated by the anodic bond, which is carried out at temperatures of around 400°C As the bonded assembly cools, resid-ual stresses will be inevitably introduced across the sensor chip Thermal matching between the sensors and constraint will naturally be improved if the constraint is made from silicon [36, 37]

Another factor that should be considered in certain applications is that the presence of the support chip can alter the sensitivity of the sensor to the measurand

In the case of high-pressure sensors, for example, the pressure will not only be

Pressure port

Leadout Die attach

Wirebond Pressure sensor die

Header cap

TO header

Figure 4.9 Basic packaging scheme.

Pressure port

Leadout Die attach

Pressure sensor die

TO header

Glass or silicon support chip

Figure 4.10 Basic first order pressure sensor packaging.

applied to the diaphragm itself but also to the exposed surfaces of the intermediate [38] The resulting stresses induced in the intermediate will be transmitted in part to the sensing elements and will therefore contribute in some manner to the sensor output The magnitude of the effect will depend upon the particular design and the application Differential pressure sensing is another example application where this effect can be important, especially when attempting to detect small differential pressures imposed on high line pressures Comprehensive modeling of the assemble sensor diaphragm and the first order packaging can be used in the design stage to predict this effect

4.4.3.2 Mechanical Decoupling

Mechanical decoupling in the form of stress-relieving flexible regions may be incor-porated on either the sensor or intermediate chip The flexible regions take the form

of micromachined corrugations that absorb stresses rather than transmit them to the sensing element within the assembly This corrugated decoupling zone may be fabricated on the sensor chip itself, as shown in Figure 4.8 [39, 40] The pressure-sensing diaphragm is located at the sensor of the chip and is supported by an inner rim The sensor chip is fixed to its surroundings at an outer rim and the decoupling corrugations lie in between the two rims The placement of the corrugations on the sensor chip could remove the need for any first order packaging (as depicted in Figure 4.11), but this does increase the overall size of the chip and reduces the number of devices that can be realized on each wafer Also, the fabrication processes

of the corrugations and the sensing mechanism employed on the sensor chip must be compatible Another disadvantage is the difficulty in forming conductive paths over the corrugations to the outer rim, which would be the preferred location for the bond pads This could be overcome by placing the bond pads on the inner rim or by providing planar paths, or bridges, over the corrugations [41]

Alternatively, the use of silicon intermediate support chips offers the opportu-nity of micromachining the stress-relieving regions on the constraint chip rather than the sensor chip itself Finite element analysis employed to investigate various decoupling designs identified the structured washer style support chip, shown in Figure 4.12, as the most promising solution [41, 42] The mechanical decoupling is provided by V-grooves etched into both sides of the constraint wafer, forming a thin corrugated region between the sensor chip and its mounting When packaging stresses are present, the corrugations absorb the deflection rather than transmitting

Pressure port

Leadout Inner rim

Outer rim and die attach

Pressure-sensing diaphragm Decoupling V-grooves

Figure 4.11 Decoupling zones on sensor chip.

them to the sensor chip itself In this manner, a 99% reduction in packaging stresses transmitted to the sensor chip is possible The support chips can be fabricated and bonded to the sensor die at wafer level, therefore enabling the simultaneous process-ing of all the devices on a wafer

This approach should improve the performance of the sensor and reduce the complexity and cost of the second order packaging The disadvantages are the reduced strength of the assembly—because less area is available to bond the inter-mediate to the transducer housing—and the increased cost of the first order packag-ing due to the processpackag-ing of the silicon intermediate Also, as discussed previously, the presence of the support chips may influence the output of the sensor in certain applications and this may be further exaggerated by the corrugations

The economic benefits of placing mechanical decoupling on the sensor chip or the silicon intermediate will depend upon the relative processing costs required by the sensor chip and the intermediate If the sensor has a complex design requiring many processing steps, then it would be more economical to maximize the device density upon the wafer and incorporate the mechanical stress relief on the interme-diate chip If, on the other hand, the processing of the sensor is straightforward and not affected by incorporating the corrugations alongside the sensor structure, that approach could be favorable

4.4.3.3 Displacing the Sensor from the Second Order Packaging

Other stress-relieving first order packaging designs involve removing the sensor as far away from the transducer housing as is practical This can be achieved with both vertical and horizontal separation Vertical separation can take the form of tall glass

or silicon supports chips similar to the design shown in Figure 4.10 The packag-ing of the Druck resonant pressure sensor [43], described in Chapter 6, is an example of vertical separation The package design is shown in Figure 4.13 The pressure-sensing diaphragm and resonator is mounted on a silicon support chip, which is in turn attached to a glass tube The glass tube serves both to move the

Pressure port

Leadout Die attach

Pressure sensor die

TO header

Corrugated silicon support chip

Figure 4.12 Corrugated silicon intermediate.

sensor away from the transducer housing and, by sealing the end in a vacuum, trap a vacuum around the resonating element This approach, however, is time consuming and expensive to assemble; wafer level vacuum encapsulation is greatly preferred Horizontal, or lateral, separation of the sensor chip away from transducer hous-ing or supporthous-ing substrate is achieved by fixhous-ing the chip only at an insensitive part

of the die (i.e., away from the location of the sensing elements) [44] The sensing ele-ment is therefore separated from the substrate by a small gap, as shown in Figure 4.14, and packaging stresses will only be transmitted directly to insensitive regions of the sensor chip This approach will not be suitable for many applications, but where it is applicable, experimental work has shown packaging stresses reduced

by a factor of 10 This approach certainly offers a very simple isolating technique, but it may involve increasing the size of the sensor chip in order to include an insensi-tive region of sufficient area to enable robust mounting

A similar approach has been employed in the packaging of a silicon high-pressure sensor designed for use in refrigeration and fluid power applications The pressure-sensing membrane and associated piezoresistive elements are located

at the end of a silicon needle [45] This needle is housed within a metallic collar, and

Resonator chip Wire bond

Temperature sensor

Drive electronics Transducer housing

Adhesive

Carrier pcb

Glass tube

Support chip

Figure 4.13 Packaging of the Druck resonant pressure sensor (After: [43].)

Gap under sensing element Bond

Substrate or housing

Sensing element Insensitive part of die

Figure 4.14 Lateral isolation of the sensing element.

the sensing elements protrude beyond the end of the collar and are in direct contact with the pressurized media This low-cost packaging approach provides a good degree of mechanical isolation, but the drawbacks include the increased size of the sensor chip and the fact that it is in direct contact with potentially corrosive media

4.4.3.4 Use of Soft Adhesives

The die mount material and the method of attachment will also have an influence on the mechanical isolation of the sensor and the level of in-built stresses trapped within the assembly The various methods of die attach used in the IC industry and discussed in Section 4.3.2 are equally applicable to MEMS packaging Typical parameters of these processes are shown in Table 4.3 The TECs of the bond materials, along with common packaging materials, are given in Table 4.4 The TEC of silicon varies with temperature and is listed against different temperatures in Table 4.5 The TEC of these materials is of fundamental importance to the MEMS designer since the stresses arising from TEC mismatches account for the majority of packaging-induced error

The use of soft, ductile bond materials in the mounting of the die can provide a high degree of isolation from undesirable mechanical stresses These soft adhesives absorb the stress in a manner similar to the mechanical decoupling structures described previously In addition, the lower temperature die attach processes associ-ated with typical soft adhesives are advantageous since the magnitude of thermally induced stresses trapped in the final assembly will be reduced The drawbacks of soft adhesive typically relate to their bond strength, which is very weak compared to harder epoxies and especially solder and eutectic bonds Soft adhesives are not suited to applications that place the sensor die under shear of tensile stress Where harder, stronger bonds have to be used, trapped thermal stresses and the resulting temperature cross-sensitivity can be minimized by keeping the adhesive film as thin

as possible

Soft adhesives, such as RTV silicone, must be applied in a controlled thickness

to achieve maximum benefit Experimental analysis showed that the thermal behav-ior of the sensor shown in Figure 4.14 was improved by increasing the adhesive thickness up to 50µm, but no further improvement was observed beyond this [46] Glass spheres can be used in the assembly of the sensor to control this thickness, as shown in Figure 4.15

Table 4.3 Typical Die Mounting Process and Material Parameters

Attachment Method Adhesion Material Process Temperature

(°C)

Thermal Conductivity (W/m °C)

Young’s Modulus (109

N/m2

)

A commercial low absolute pressure sensor has also been successfully packaged using soft adhesives for use in space applications and in particular a mission to Mars [50] The application requires the sensor to survive shocks of up to 100,000g, oper-ate in temperatures as low as –80°C with fluctuations of 50°C and resolve 0.05 mbar over a 14-mbar range with an overall accuracy of 0.5 mbar Given the size and

Table 4.4 Thermal Expansion Coefficients of Common Packaging Materials

/°C)

1

Below glass transition temperature.

2

Above glass transition temperature.

Source: [48].

Table 4.5 Thermal Expansion Coefficient of Silicon Versus Temperature

Temperature (°C) TEC (10–6

/°C)

Source: [49].

Package Glass sphere spacers

Soft adhesive

Silicon intermediate

Pressure sensing die

Figure 4.15 Soft adhesive die mount with glass spacers.

weight restrictions, the sensor was packaged alongside the electronics using an MCM, as shown in Figure 4.16 The MCM incorporated epoxy-mounted ICs with thick-film tracks, surface mount capacitors, and thick-film resistors The sensor and ICs were flush mounted to enable shorter wirebonds

The mounting of the sensor die to the ceramic package is shown in Figure 4.17 The pressure sensor is bonded with a 25-µm-thick layer of soft adhesive (Silicone RTV 566) to a silicon support chip Silicone RTV 566 was used because it has a glass transition temperature of –115°C, and therefore it maintains its ductile prop-erties at the specified operating temperatures The support chip is then bonded to the ceramic substrate using a much thicker layer of silicone (250µm), which pro-vides isolation from packaging and impact stresses This layer of silicon could not be thicker than 250µm because it would put the wirebonds under excessive strain The support chip serves to isolate the sensor from the effects of the TEC mismatch between the silicon and the RTV silicone

4.4.3.5 Summary of Techniques for Mechanically Isolating the Sensor Chip

Table 4.6 presents a summary of techniques for mechanically isolating the sensor chip

Pressure sensor ICs Discrete components

Multilayer ceramic Low-temperature epoxy

Titanium electronics housing

Figure 4.16 MCM packaging of Martian pressure sensor and electronics (After: [50].)

MCM

250- m RTV silicone µ

Silicon support chip Pressure sensor die 25- m RTV siliconeµ

Figure 4.17 Soft adhesive mounting of pressure sensor (After: [50].)