Package Design of Pressure Sensors for High Volume ... - IEEE Xplore

Package Design of Pressure Sensors for High Volume Consumer Applications Mark Shaw, Federico Ziglioli, Chantal Combi *, Lorenzo Baldo * STMicroelectrionics, Corporate Packaging and Automation (CPA), Agrate Italy, * MEMS Business Unit, Castelletto, Italy, [email protected] Abstract In this abstract we outline the critical design aspects for the design of high volume pressure sensor for MEMS applications. Pressure sensor designs by their nature require the active device to be in contact with the pressure to be measured, this requirement has until now restricted the possibility of applying large volume semiconductor package manufacturing techniques to pressure sensors and hence their large scale deployment in consumer applications. The current designs of pressure sensors rely on either a separate protective membrane, a Gel-Liquid to transmit the pressure to the active device or a Gel applied on top of the sensor. The active part of the sensor is normally made up of two separate chips , the active membrane which flexes in response to external pressure the movement of which is translated into electrical signals by resistive or capacitive elements and a support chip with trapped between them a reference pressure volume. In this paper the design of a novel LGA based MEMS Pressure sensor and barometer/altimeter design are demonstrated. A single chip sensor solution is outlined with no need for the trapped reference volume between the support chip and the sensor membrane. In this single chip sensor solution the reference pressure volume is trapped within the chip itself improving both the chip dimensions and overall reliability by eliminating the joint between the two chips. The Packaging for a this single chip sensor is based on the now well established LGA sensor platform, the problem of interfacing the pressure sensor to external pressure is solved by using an exposed silicon in the over moulded package which allows the pressure sensor access to the outside pressure, reducing the overall package size for the absolute pressure sensor to 3x3x1mm. MEMS sensor devices in general depend on their mechanical performance to translate mechanical movement into an electrical signal, the interface to the outside world, the package, is a mechanical structure that has a large influence on the device performance and must be thought of a part of the device not just as protective housing. In the case of the pressure sensor the flexible silicon membrane is influenced by the thermal stresses in the package. The design aspects that have influence on the performance of the MEMS devices are outlined, the Package and sensor devices were extensively modeled using FEA thermo-mechanical simulation with the results showing a good correlation to the final device performance. Problems encountered during the development related to device manufacturability are outlined and the solutions detailed. The final performance and reliability data is also presented demonstrating the robustness of the design solution.

978-1-4244-2231-9/08/$25.00 ©2008 IEEE

Introduction The original MEMS (micro electro-mechanical systems ) applications such as collision sensors for automotive airbag accelerometers and pressure sensors for automotive applications such as engine manifolds, these involved bulky packages using ceramic or pre moulded plastic packages. MEMS Packaging is currently evolving from these original formats into newer more low cost styles more suitable to consumer applications. New applications for example accelerometers such as free fall protection for hard disk drives on portable computers and cell phones, gaming interfaces, pedometers etc or pressure sensors for barometers and altimeters, require lower cost and smaller packages in order to open up these markets [1,2]. The MEMS pressure sensor packages currently available range from oil filled steel housing for high end industrial applications to ceramic or pre moulded plastic lead frame open packages with a gel fill to protect the pressure sensor and its wirebond connections. To achieve low cost solutions the semiconductor packaging industry has for many years used full over moulded technology, obviously we can not over mould completely over the pressure sensor as it has to have access to the outside pressure to function. Some recent packages have been developed using a pressure sensor which has a gel covering the sensor which is then over moulded leaving the gel exposed [3,4]. Any gel over the membrane however will compromise the performance of the pressure sensor. The most recent solutions using a technique initially developed for large chip fingerprint scanners involves over moulding using either an insert with a rubber tip or a film to protect the sensitive membrane during moulding. These techniques have been used to realise Lead frame based packages [5,6] mainly for automotive applications such as Tyre pressure Management Systems. For consumer applications lower cost packages are required such as QFN or LGA packages already used for other MEMS applications such as accelerometers [7] . A low cost packaged pressure sensor can then find applications in diverse fields. Hard disk drives where it can be used to sense atmospheric pressure in order to maintain the head at a constant height, this then will maximise the amount of information that can be stored independent of the instruments altitude. Together with a custom ASIC they canbe used as altimeters or barometers for cell phone or watches and can perform altitude measurement for navigation systems where in certain countries have multiple stacked carriageways. Vensen Pressure chip VENSENSTM process begins with a standard silicon wafer. A proprietary combination of wet and dry silicon

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2008 Electronic Components and Technology Conference

etching steps enables the formation of a sacrificial layer on top of which a monocrystal silicon layer is grown. The thickness of the sacrificial layer is less than 3 µm and the thickness of the structural layer can reach 20 µm. The end result is very similar to what it’s possible to get with bulk micromachining and wafer to wafer bonding where an etched silicon wafer is bonded to another silicon or glass wafer to create the reference pressure cavity (6,8) but with the big advantage to have thinner, smaller and mechanically more robust chips (Fig 1). Moreover, the sealing of the cavity doesn’t require any wafer-to-wafer bonding and thus the reliability of the sealing joint is higher. Membrane~10µm with air cavity~1µm

Membrane~50µm with air cavity~100µm

Monolithic Silicon sensor with hermetic cavity

Glass/Silicon

Hole in moulding compound created by protrusion in mould cavity

~1mm

~300µm Intrinsic Stopper

to reduce cost the devices are packed closely in an array format. A protrusion in the top mould cavity makes a hole in the moulding compound so that the pressure sensor membrane is exposed. The protrusion can either be made from a rubber compound to protect the sensor or a plastic film is inserted between a metal protrusion in the cavity and the sensor membrane [5]. The array of devices particularly for the stand alone device means that the type of over moulding so far used for pressure sensors becomes difficult due to the large number of protrusions required reach the die surface, and still allowing the moulding compound to cover the wirebonds (Fig 3). A typical substrate strip for high volume LGA 3mmx3mm device would have 4 blocks of 15x15 arrays which would require 900 protrusions per strip which makes this type of mould cavity relatively expensive and difficult to maintain

Silicon membrane bonding with glass/Silicon wafer to create cavity

Fig 3 Moulded package schematic using protrusion

Fig 1 Vensen Pressure sensor (left) and standard pressure sensor design (right) Thanks to good electrical properties of the monocrystal silicon, stable resistors can be integrated in the structural layer through implantation or diffusion process. Then these resistors are connected with an aluminum metal layer to realize the four branches of a Wheatstone bridge. The bridge is sensitive to pressure changes thanks to the excellent piezoresitive properties of the monocrystal silicon layer. The metal layer is then covered with a standard dielectric, like silicon-oxy-nitride, to provide the required protection against the external corrosive agents. The thickness of the membrane can be altered to suit the pressure to be measured, moreover there is a built in stop which protects the membrane for over pressure damage. The membrane can also be made thinner than ‘standard’ membranes increasing the device sensitivity

The alternative is to transform the pressure sensor into a structure where the exposed area is level with the top of the moulding compound eliminating the need for protrusions in the moulding cavity. A dam is added above the Pressure sensor, the height of which is sufficient to allow the wirebond loop to connect the sensor to the substrate. This Dam prevents ingress of the moulding compound to the sensitive diaphragm area. This eliminates the need for protrusions in the top mould cavity (Fig4) Dam/Cap layer

Membrane

Substrate Fig 4 Moulded package schematic with Film assisted moulded cap

Fig 2 SEM of membrane and air cavity and Photograph of pressure sensor showing membrane Packaging A pressure sensor for consumer applications can be packaged as a stand alone unit or with an ASIC in either case

The dam can be produced by wafer bonding, another silicon wafer using standard MEMS techniques used in the production of other devices such as accelerometers. Wafer bonding for Accelerometer devices is now a standard process using for example glass frit technology. The cap wafer for the accelerometer is first etched to produce the cavity for the accelerometer and the holes for the wirebond pads. The wafer is then bonded to the sensor wafer using a screen printed glass paste, after the wafers are bonded together the wafer sandwich is lapped from the top surface to expose the wirebond opening area which is etched to a greater depth than

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the internal cavity. To expose the silicon membrane of the pressure sensor it is just necessary increase the depth of the etching inside the cap wafer. The hole in the top of the cap wafer will then be produced after the lapping process arrives at the etched level within the cap. LGA package and substrate design The LGA (Land grid Array) package consists of a laminate substrate with a BT resin Core, so far MEMS sensor devices have required only a top and bottom metal tracks with corresponding solder mask but as more complicated integrated designs develop these can be easily accommodated with extra layers. The exposed soldering pads (lands) and wirebond pads are suitably plated. One of the advantages of the LGA substrate design is its flexibility, in the ability to route the Cu tracks to permit different designs of sensor or ASIC to have the same pin out the same designs to have different pin outs with no change to the assembly process including even the wirebonding layout, only a different substrate design (7). This gives a large amount of flexibility which is not available in more traditional (e.g QFN) designs. The devices produced so far have been with a stand alone sensor or with the ASIC by the side to the Pressure sensor. Further work is however underway to produce a stacked Pressure sensor with the sensor on top of the ASIC. The sensor die wafer is laminated with a combined die attach and dicing tape applied to the wafer prior to dicing. The tape has characteristics such that for small die the adhesion is sufficient in the uncured state that the small diced chip remains in place during dicing and the die can then be picked with the die bonding tape adhered to it from the dicing tape. The dicing blade therefore cuts through the bonding tape but not the dicing tape. Alignment structures for the dicing must be placed within the wirebond opening as there is no alignment features in the cap layer. In selecting the die bond tape the bonding epoxy should flow sufficiently to take up the non planarity in the substrate and be sufficiently rigid after curing so that ultrasonic energy is not dispersed during wirebonding at elevated temperature.

new sealing surface. The exposed die will protrude slightly above the moulding compound due to the fact that it is pressed slightly in to the tape. The clamp pressure, the pressure at which the two moulds come together, the transfer pressure, the pressure at which the moulding compound is forced into the cavity and the pressure of the mobile insert in the top cavity all have an influence on the quality of the final moulded product. Vacuum

insert

Top Mould Chase

Tape

device Bottom Mould Chase Fig 6 Schematic of film assisted mould In addition to the standard problems of cavity filling and voiding etc problems present themselves with bleed, resin flowing on top of the exposed die or wrinkles , a furrow around the edge of the die, (fig 7,8). Though essentially aesthetic problems in the case of bleed as long as it does not reach the hole in the Cap wafer and for wrinkle as long as the depth of the wrinkle does not expose the wirebonds they can be avoided be correct setting of the moulding parameters.

Fig 7 Moulded devices with moulding compound over the exposed die, bleed (left) and wrinkles around the exposed die

Fig 5 Pressure sensor and pressure sensor +ASIC wirebonded on substrate array

Fig 8 Section of pressure sensor showing wrinkle and slight protrusion of exposed die above moulding compound

Film assisted moulding The basic process of film assisted moulding consists of using a compliant tape between the die and the top moulding cavity. Vacuum is used to hold the tape into the top part of the mould cavity and the die to be exposed then presses into the tape which forms a seal between the die and the top mould to prevent ingress of the moulding compound. Once the moulding process is complete the tape is wound on to create a

Bleed is produced when there is no seal along the edge of the exposed die as it is pressed into the moulding film. It is a combination of the film thickness and hardness, if the clamping pressure and insert pressure is too low or the transfer pressure too high. Wrinkle is produced when the moulding film buckles. If the vacuum pulling the tape onto the top cavity is low or the insert pressure too high, wrinkles

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will form in the tape which will then reproduce themselves in the final moulded product. Also if the transfer pressure is low the opening for the wirebond pads will not be filled. Once the ideal conditions are found for a particular device they will not be the same for another device with a different amount of die area in contact with the film or a different packing density. This can be seen with the difference between the stand alone pressure sensor and the sensor + ASIC. The pressure sensor die is the same but for the stand alone device a matrix of 15x15 devices is used , in the case of the sensor +ASIC there is a matrix of 9x9 a difference in surface area of 40%.

metal traces which will enable parallel strip testing of the devices before the final singulation producing the holed LGA 3mm x 3mm stand alone sensor or 5mm x 5mm sensor with ASIC, HLGA331or HLGA551 package Fig10.

Insert Wrinkles Bleed LGA Film Clamp pressure pressure Size 3x3 A,B High No High No Low medium 5x5 A,B High No High yes Low medium 5x5 A low low yes 5x5 A No Medium low No 5x5 A low High Yes low 5x5 B No High yes Low medium Table 1 Moulding parameters effect on bleed and wrinkle

Fig 10 Singulated HLGA551 and HLGA331 packages Simulations and Test Results Finite element analysis of the package design was performed to optimize the package design focusing primarily on the package stress-warpage. The warpage is important both for the whole strip as if it bends to much handling problems in manufacturing can occur and stress on the individual components, from simulation the unit warpage of the package is equal to -2.5µm.

From the table above it can be seen that the tapes and parameters selected for the 3x3 devices did not function well with the reduced die contact area of the 5x5 device and the insert pressure had to be reduced. This effect has also to be taken to account when the strip to be moulded is not fully populated.

Fig 9 Moulded strip array Laser marking is performed on the strip and care needed for the 3x3 device due to the small amount of space available. It is also important that the die attach placement tolerance is at around 100um to maintain sufficient space for marking. Singulation is performed using traditional sawing techniques; tests have shown that no contamination enters the hole in the sensor cap that changes the device performance from the singulation process. A partial saw cut can be used though the

Fig 11 Simulation showing package warpage and warpage across the membrane A correlation of the measured pressure against the warpage of the membrane with temperature is clearly seen (Fig 12,13), translating the pressure measurement against the movement of the membrane a 25mBar change is equivalent to 6nm of displacement in line with the FEA simulation. In the

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HLGA551 device this can easily be corrected for using the co-packaged ASIC. Warpage of Membrane vs Temperature 0.008

TYPE

0.006 W a rp a g e (µ m )

Reliability testing of this package structure over high temperature, Humidity and temperature cycling (table 2) has shown the package to be robust with little drift in the voltage offset or the sensitivity of the device under test during the various environmental tests (Fig 15,16) DESCRIPTION

TIME

HTS

High Temp Storage @ 150ºC Temp & Humidity Storage @ THS 85ºC/85%RH TC Thermal Cycles -40ºC/85ºC PPOT Pressure Pot 121ºC/2Atm LTS Low Temp Storage @ -20ºC Table 2 Environmental tests performed

0.004 0.002 0 -0.002 -60

-40

-20

0

20

40

60

80

100

-0.004 -0.006

500hrs 500hrs 1000cyc 96hrs 500hrs

-0.008 Sensitivity

Temperature (°C)

5.0%

Upper limit

HTS THS TC PRPOT LTS

4.0%

Fig12 FEA simulation showing warpage on the pressure sensor membrane with temperature

3.0%

% F u ll s c a le

2.0% 1.0% 0.0% 0

1

2

3

4

5

6

7

-1.0% -2.0% -3.0% Lower limit

-4.0% -5.0%

Sample No

Fig 15 Graph of the Variation of offset during environmental tests Sensitivity 5.0% Upper limit

Fig13 Graph of pressure measurement against temperature

HTS THS TC PRPOT LTS

4.0% 3.0%

The performance of the stand alone sensor in the HLGA331device against pressure is linear as can be seen in graph 13

% F u ll s c a le

2.0%

Vout (mV)

140.0

1.0% 0.0% 0

120.0

-2.0%

100.0

-3.0%

80.0

100

0.7

0.9

Pr essu re (Bar)

Fig 14 Graph of pressure vs voltage

1.1

6

7

Fig 16 Graph of the Variation of sensitivity during environmental tests

0.0 0.5

5

Sample No

85

0.3

4

Lower limit

55

20.0

3

-5.0%

25

40.0

2

-4.0%

T ( C)

60.0

1

-1.0%

1.3

The HLGA551 with ASIC with its digital output can be calibrated to perform both as an altimeter and a barometer. As an altimeter it is shown in Graph 17 with the altitude variation inside a lift. The floors are shown showing the resolution of the device down to easily below the height between floors of

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approx 3m. In the barometer configuration ( Graph 18) the device is shown tracking the atmospheric pressure against the pressure monitored at the local Milan airport .

The resist is then exposed, the surplus removed and then hardened in a post bake process. The use of the resist also has the benefit that as it will not be handled as a separate wafer the connecting walls along the side of the wirebond pad opening can be removed. The samples in fig 19 were made using the standard pressure sensor structure where the orientation of the dies is turned through 90° to facilitate the formation of the hole in the cap wafer layer for the wirebond opening. This is not necessary in a structure designed for manufacture with the SU-8 cap as the resist layer does not need to be managed separately, the devices were therefore designed with a a larger opening space for the wirebond pads. A space is also left between the individual caps to prevent contamination of the dicing saw blade.

Fig 17 Example of pressure sensor as an altimeter

Fig 20 Over moulded device with SU-8 cap with section

Fig 18 Example of pressure sensor as a Barometer Further work The design outlined above shows a LGA package with good performance suitable for high volume production it has however to be noted that one of the main benefits of the Vensen process, that it does not require wafer bonding is partially lost with the requirement for a silicon cap to be bonded to the pressure sensor to form the ‘dam’ during the moulding. This wafer to wafer bonding is not however as critical as in a standard pressure sensor chip in that it does not need to have the same hermetic reliability requirements. Spin coated resist has been used to create structures for solder bumping and MEMS structures (9,10). An alternative process to create a Dam for the over moulding process can also be preformed using a spin coated photo resist material such as SU-8 on top of the pressure sensor to obtain the cap.

Fig 19 Wafer with SU-8 structure and detail

Devices assembled have shown good adhesion to the moulding compound with no evidence of delamination fig 20. Tests have also shown that there is no problem with residual resist material on the central area. The samples produced had the same cap height as the standard process but this technique also has the advantage that the overall package height can be reduced. In the silicon cap package the height is fixed by the thickness to which the cap wafer can be practically lapped without having problems due to the handling of such a holed wafer. The thickness of the SU-8 cap is limited only by the requirement to protect the wirebonds by the moulding compound An additional size reduction for the Pressure sensor with ASIC can be obtained by stacking the pressure sensor chip on top of the ASIC, trials have shown that the force exerted on the ASIC during the film assisted moulded process does not cause any damage to the chip below. The alternative is to integrate the control circuitry around the pressure sensor membrane. Conclusions In this paper we have outlined a new Holed LGA pressure sensor package based on high volume techniques already used for MEMS accelerometers with the addition of film assisted moulding to expose the pressure sensor to the atmosphere. This package opens up new possibilities for low cost, high volume applications in consumer devices. The HLGA package has the advantages of a flexible format that can easily be adapted to chip and package size and could be suitable for other sensor devices where the sensor surface need to remain exposed to the outside atmosphere. Acknowledgments The authors would like to thank Anne-Marie Grech and Roseanne Duca for the FEA modeling and Marco Tumiati, Giovanni Giuliani, Battista Vitali, Gaetano Caputo and Fulvio

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Viviani for their assistance with the process development and construction of the devices mentioned in this article. References 1. Vigna B., “Physical Sensors Drive MEMS Consumerization Wave” The 13th International Micromachine / Nanotech Symposium July 26th, 2007 2. Vigna B.,“Future of MEMS: An industry point of view”, Proc. of 7th International Conference on Thermal, Mechanical and Multiphysics Simulation and Experiments in Micro-Electronics and Micro-Systems, 2006. EuroSime 2006. Page(s):1 - 8 3. Frezza G., “Manufacturing method of electronic device package”, European patent No EP1211722B1 4. Van der Wiel A., “ low cost system in a package for tyre pressure monitoring systems”, Sensors 2003 http://www.melexis.com/Asset.aspx?nID=3888 5. Boschman F., “Film assisted molding technologies and applications for high volume MEMS and Sensor packages” Proceedings of the 16th European microelectronics and packaging conference EMPC2007 p208,209 6. Krondorfer R., “Finite element simulation of package stress in transfer molded MEMS pressure sensors”, Microelectronics Reliability, v 44, n 12, Dec. 2004, p 1995-2002 7. Shaw M. , “High Volume MEMS Packaging” Proceedings of the 16th European microelectronics and packaging conference EMPC2007 p182-187 8. Marek J., “Silicon Microsystems for Automotive Applications” Proceedings of the 27th European SolidState Device Research Conference, ESSDERC '97, p 10115 9. Rao V “A thick photoresist process for advanced wafer level packaging applications using JSR THB-151N negative tone UV Photoresist”, J. Micromech. Microeng. 16 (2006) 1841–1846 10. Conradie E “SU-8 thick photoresist processing as a functional material for MEMS applications ”, J. Micromech. Microeng. 12 (2002) 368–374

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