Chip Scale Packaging of a MEMS Accelerometer

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					                                 Chip Scale Packaging of a MEMS Accelerometer

                                     L.E. Felton, N. Hablutzel, W.A. Webster and K.P Harney
                                      Micromachined Product Division, Analog Devices, Inc
                                             21 Osborn Street, Cambridge, MA 02139

                                                                  trademarks, by assembly contractors. These include MLF®,
                                                                  QFN, MLP®.) Figure 1 shows a cross-section of a typical
    Accelerometers fabricated with Micro Electro Mechanical
                                                                  LFCSP package.
Systems (MEMS) technology are widely used in automotive
crash detection and vehicle dynamic control systems.
                                                                            Epoxy Mold Compound               Die
Recently, there has been increased interest in the use of
MEMS accelerometers in smart handheld devices. The most
demanding new requirement for this application is package
size. This paper details the fabrication a 2 axis accelerometer
in a 4 mm x 4 mm x 1.45 mm thick lead frame chip scale
package. The fabrication of the device depends on three
technologies: (1) An integrated MEMS technology that yields
                                                                             Lead Frame                        Die Attach
MEMS structure and all associated signal processing circuitry
on a single die; (2) The development of a wafer scale capping        Figure 1. Schematic cross section of LFCSP package
process that provides protection for the MEMS sensor and
renders it suitable for transfer molding; and (3) modifications       The main advantage of the QFN package for mobile
of the standard Lead Frame Chip Scale Package (LFCSP)             electronics devices is reduced size. The thickness of a typical
assembly process to accommodate the capped MEMS sensor.           LFCSP is less than 0.9 mm, compared to a Thin Small
The performance of the accelerometer is assessed and              Outline Package which has a typical thickness of 1.1 mm.
compared to the same device in a leadless chip carrier.           Compared to a leaded package with the same number of
                                                                  leads, the LFCSP package requires significantly less board
                                                                  area. For example an 8 lead SOIC has a foot print of 5 mm x
    MEMS accelerometers have been used as crash sensors in
                                                                  4.5 mm, while an 8 lead LFCSP package can be as small as 2
automotive airbag systems for more than 10 years. Over this
                                                                  mm x 3 mm.
time, crash sensors have been the single largest application of
                                                                      The LFCSP package is also attractive from a
the technology. Thus, the packages offered for MEMS
                                                                  manufacturing point of view. First, the manufacturing
accelerometers have reflected the requirements of automotive
                                                                  process is very similar to that used for leaded transfer molded
applications including very high quality, reliability, and
                                                                  packages. This minimizes the amount of capital investment
performance at the lowest possible cost. Several packages
                                                                  required for implementation. Second, the device density on a
meet these requirements and have found broad use for
                                                                  lead frame is much higher than for a leaded package, thereby
automotive MEMS applications. These include hermetic
                                                                  reducing the cost of assembly.
cavity packages such as the CERDIP, LCC and BGA and
transfer molded packages such as the PDIP, SOIC, and                  The advantages of LFCSP for consumer applications have
PLCC.                                                             led to the desire to assembly MEMS devices in this package.
                                                                  This paper describes the key elements of the development of a
    More recently, MEMS accelerometers have found
                                                                  2 axis low g-accelerometer assembled in a 4 x 4 1.45 mm
application in consumer products, including laptop
                                                                  LFCSP package.
computers, cell phones, and hand held computers. The
package requirements for these applications differ somewhat       Fabrication Technology
from those for automotive sensors. As for automotive, quality         The manufacturing process for a 2-axis MEMS
and reliability are still critically important. In many cases,    accelerometer assembled in a LFCSP package consists of
lower cost is required than for automotive applications, but      three major steps are described in this section. (1) fabrication
the performance requirements are often somewhat relaxed.          of an integrated MEMS accelerometer; (2) a wafer scale
Most importantly however, the size and weight of the package      capping process for the accelerometer; and (3) assembly of
become critical.                                                  the capped accelerometer in the LFCSP package
    The hand held market has driven packaging technology              The first element of the overall accelerometer system is
for many semiconductor devices to smaller, lighter                the iMEMS® process that Analog Devices has developed and
packages[1]. The most prevalent example of the trend to           applied to the fabrication of accelerometers and gyroscopes.
smaller and lighter packages is the development of the Lead       The defining feature of this process is that the MEMS sensor
Frame Chip Scale Package (LFCSP). (The LFCSP package              is fabricated on the same die as the signal process circuitry.
has been given several different names, some of which are         Core, et. al, have described the process [2,3]. A very mature
and well characterized BiCMOS fabrication circuit process is             advantage of the integrated approach is size. The two axis
used to provide the signal processing circuitry. The circuit             accelerometer in Figure 3 is just 2.3 mm x 2.3 mm. A two
process also includes laser trimmable thin-film resistors.               chip accelerometer of with the same functionality would be
This circuit process has been integrated with a surface                  much larger. A second advantage is ease of assembly, which
micromachined MEMS process. The MEMS process uses in-                    translates into assembly cost. The two-chip accelerometer
situ doped poly-silicon as the device material and silicon               requires two die attach operations and wire bonding between
dioxide as the sacrificial layer. Figure 2 is a cross section            die. Overall, the use of integrated MEMS facilitates the use
showing the circuit and MEMS portions of a wafer as well as              of a smaller, less expensive package.
the interconnection between the two.
                                                    Poly-Si MEMS Structure
                        Die Passivation
     Al Metallization

                                    n+ runner                                                      alignment keys


                                                                                            cut capture          cavity
                Oxide                                                                         cavity
                                                Si Substrate                                        seal glass
Figure 2. Cross section showing interconnection between
MEMS and circuit in iMEMS®
    Figure 3 is a photograph of the XL202, a two axis, low g
accelerometer fabricated with the iMEMS® process. Note
that the MEMS sensor is in the middle of the die, surrounded
by the signal processing circuitry. The bond pads for the
device are distributed around the periphery of the die, as is                        (d)
typical for an analog circuit.

                                                                                                    sensor wafer


                                                                         Figure 4.     Cross sectional view of capping process for

                                                                             Analog Devices has developed a wafer scale capping
                                                                         process for iMEMS® [6]. Clearly, some kind of cap is
                                                                         required for a transfer molded package; without a cap, the
                                                                         MEMS structure would be impeded by the plastic mold
                                                                         compound. A secondary goal of the cap is to protect the
                                                                         delicate MEMS structure from damage during assembly. The
                                                                         key steps in the process are depicted schematically in Figure
                                                                         4. The process begins with a double side polished silicon
                                                                         wafer. In the first step, the alignment keys are etched into the
Figure 3. Photograph of XL202 die, which is fabricated in                top side of the wafer and sensor and cut capture cavities are
the iMEMS® process                                                       etched into the bottom side. Next, a seal glass is screen
                                                                         printed onto the cap wafer. After screen printing, the cap
   In the inertial MEMS industry, there are two broad classes            wafer is bonded to sensor wafer. In the fourth step, the cap
of MEMS processes, integrated and two-chip. The two chip                 wafer is separated into individual caps. At this point the
approaches consist of a MEMS only die and an ASIC which                  capped wafer is ready for standard assembly in a wide variety
provides the signal processing [4,5].        One significant
of packages. Figure 5 is an SEM image of a iMEMS®                 Figure 7shows the major steps in the assembly process of
accelerometer capped with this process.                       the 1.5 mm thick MEMS accelerometer. One of the major
                                                              advantages of capping MEMS devices prior to assembly is
                                                              that it reduces the complexity of the dicing operation. Once
                                                              the MEMS sensor is protected with a cap, the die can be
                                                              singulated using a standard dicing process. Similarly, the cap
                                                              protects the MEMS sensor during die attach, so no
                                                              modifications are required for that process either. The third
                                                              step, wire bonding requires minor process modifications to
                                                              accommodate the presence of the cap. After the ball is
                                                              formed on the die bond pad, the wire bond capillary moves
                                                              toward the cap in order to form a low loop bond to the lead
                                                              frame. If the capillary were to follow the same path used on
                                                              standard die, it would collide with the cap. This required
                                                              reprogramming the wire bonder to avoid collision with the
                                                              cap. The height of the capped sensor was too thick for
                                                              molding in a standard 0.85 mm nominal depth mold tool.
                                                              Instead, a custom mold tool, with a nominal thickness of 1.45
                                                              mm and a maximum thickness of 1.45 mm was fabricated.
                                                              The mold process had to be optimized to meet wire sweep
                                                              specifications and to eliminate internal voids. No other
Figure 5. SEM image of capped iMEMS® accelerometer            modifications were required to the molding process. After
                                                              molding, a matte Sn lead finish is applied via electroplating.
                                                              Finally, the packages are singulated with a standard sawing
    Once the wafer has been capped, the accelerometers are
                                                              process. Figure 8 is a photograph of the finished part.
ready for assembly via standard transfer molding processes.
Figure 6 is a schematic diagram of the cross section MEMS
accelerometer assembled in a 1.45 mm nominal thickness
LFCSP package. Table 1 summarizes the thickness of each of
the elements in the system. The total thickness of the
structure is 1275 um, which leaves 175 um of headroom
between top of the sensor cap and the top of the package.
Note that the headroom required between the top of the
capped accelerometer and the top of the package is required
to accommodate a laser brand on the top of the package.

          Epoxy Mold Compound                 Silicon Cap

                                                              Figure 7. Major steps in LFCSP assembly process

                                                                 One of the key reliability characteristic of plastic packages
                                                              is the JEDEC Moisture Sensitvity Level (MSL). This
                                                              package was characterized to JEDEC MSL Level 3 and a with
          Lead Frame     Die Attach    Sensor Die             a 260 C relflow.
Figure 6. Schematic cross section of          capped iMEMS®   Performance
accelerometer in LFCSP package.                                   There are a number of important performance
                                                              specifications for accelerometer, including sensitivity, cross
        Feature              Thickness (um)                   axis sensitivity, noise, frequency response, and 0-g offset.
        Leadframe                 150                         The drift of these parameters is with temperature is also a key
        Die Attach                 25                         performance specification. Of these parameters, 0-g offset
        Capped Sensor            1100                         and the drift of 0-g offset with temperature are most directly
                                                              dependent on package design. Figure 9 is a plot of stationary
        Total                    1275
                                                              (0-g) accelerometer output as a function of temperature for
                                                              the same accelerometer packaged in the LFCSP package
Table 1. Summary of cross-sectional thicknesses of iMEMS®     described above (Figure 9.a) and in a LCC (Figure 9.b). The
accelerometer in LFCSP package.                               accelerometer in the LCC is not capped. In both cases, the
                                                              data were obtained from components soldered to printed wire
boards. The board assemblies were placed in a temperature          and the LFCSP explain the difference. First, the CTE of the
chamber and heated from room temperature to 125 C at 10            LCC is 7.0 x 10-6 /K, which is much closer to that of the Si
C/min, cooled from 125 C to -40 C at 5 C a minute and then         sensor (2.8 x 10-6 /K) than the copper (17 x 10-6 /K) and epoxy
returned to room temperature at 10 C/min.                          mold compound (12-16 x 10-6 /K) that comprise the LFCSP
                                                                   package. Second, the only point of contact between the
                                                                   package and the die in the LCC is the die attach, while the die
                                                                   in the LFCSP is completely encapsulated. Clearly, the
                                                                   amount of package induced strain from the LFCSP package is
                                                                   much larger than that of the LCC. Thus, the magnitude of
                                                                   the 0-g offset distribution and the 0-g drift with temperature is
                                                                   much larger. While the LFCSP package does result in
                                                                   performance that is not as good as the same part assembled in
                                                                   an LCC, the overall performance of the device in the LFCSP
                                                                   package is excellent





                                                                       0-g Output, V


Figure 8. Photograph of 2 axis accelerometer assembled in                                    2.4

4mm x 4mm x 1.45 mm LFCSP package.

    There are several key features common the both plots.                                    2.3
                                                                                                   -50   -25   0   25         50          75   100   125   150
First, there is a distribution of 0-g output at room                                                                    Temperature, C
temperature. This distribution is the result of thermal stresses
resulting from CTE mismatches in packaging materials. To                                           (b)
understand the origin, consider the less complicated case of                                 2.7

the LCC and the effects of die attach. The die attach is a
thermoset polymer material that is cured at 150 C. Prior to
cure, the die attach is viscous and cannot produce a strain in                               2.6

the die. After curing, the die attach becomes rigid, and upon
cooling, the CTE difference between the die, the die attach
                                                                            0-g Output, V

and the package substrate cause a thermal strain in the die.                                 2.5

This strain causes a displacement of the MEMS structures,
which causes an offset in the device output. The distribution
of 0-g offset at room temperature reflects the normal                                        2.4

variations in manufacturing processes. The same mechanism
is operative in the plastic, but the situation is more complex                              2.35

because of the greater number of materials and the fact that                                 2.3

the entire devices in encapsulated in plastic.                                                     -50   -25   0   25         50
                                                                                                                         Temperature, C
                                                                                                                                          75   100   125   150

    The second feature that is common to both plots is that the
0-g offset varies with temperature. The explanation of this           Figure 9. 0-g bias as a function of T for (a) capped
effect is a simple extension of that for the distribution in 0-g   accelerometer assembled in LFCSP package and (b) uncapped
offset. As the packaged device is heated and cooled, the           accelerometer assembled in LCC.
thermal strain changes, producing a change in the output.          Summary
                                                                       The application of inertial sensors in hand-held
    Observations of the differences in between the data for        information appliances has created a need for accelerometers
LCC and LFCSP reveal the effects of packaging on this key          assembled in very small packages. This paper has presented
performance parameter. First, the distribution of 0-g offset is    the processes used in fabricating a 2-axis accelerometer in a 4
larger for the LFCSP part than the identical device packaged       mm x 4mm x 1.45 mm LFCSP package. Achieving this small
in the LCC. Second, the magnitude of the drift of the 0-g bias     size requires three major processing steps: (1) fabrication of
with temperature is larger for the LFCSP than the LCC. A           the MEMS accelerometer with integrated electronics using
comparison of the package materials and designs for the LCC        Analog Devices iMEMS® process; (2) wafer scale capping of
the accelerometer to protect the MEMS structure from
damage during assembly; and (3) assembly using standard
LFCSP processes. The 0-g bias stability over temperature
was measured and compared to that of the same accelerometer
assembled in a LCC.         The comparison shows some
degradation in performance, which is attributed to the
increase thermal stresses expected in the transfer molded
package.      The performance of the device in the LFCSP
package exceeds the requirements for most handheld
   The authors thank David Hollocher and Gail Edwards for
support in collecting and interpreting the offset behavior over
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Description: CSP (Chip Scale Package) package, chip-scale packaging means. CSP packages the latest generation of memory chip packaging technology, its technical performance has a new upgrade. CSP CSP package installed seal allows the chip area and packaging area ratio exceeds 1:1.14, very close to 1:1 has been the ideal situation, the absolute size is also only 32 square millimeters, about the ordinary BGA 1 / 3, only very TSOP memory chips in the area of ??1 / 6. Compared with the BGA package, under the CSP and the same space can be three times more storage capacity.