# Integration and System Aspects

## Introduction

In this section we describe concepts for pixel modules. We also present
an overview of the structures that support and cool the modules. These
concepts and examples are described only for applications in a
high-energy, hadron-collider environment. A pixel module consists of
integrated circuits (Chapter 3) bump-bonded to sensors (Chapter 2) and
the integration of power and signal connections that are presented in
this Chapter. Pixel detectors require a high level of systems
integration among the electrical and mechanical elements and we present
here some examples of this integrated approach. . The pixel modules must
be supported by a stable mechanical structure but one in which the
radiation length is minimized to prevent unwanted interactions of
particles. The heat generated by the integrated circuits, by power and
signal connections to these circuits and by the leakage current of the
sensors under a bias voltage must be controlled. The local cooling
needed to provide temperature control is most often integrated into the
mechanical structure. Finally, we briefly mention issues associated with
radio-activation of materials in pixel detector systems at high
luminosity hadron colliders.

## Modules

Modules are the "building blocks" of a pixel system. The module is an
identical(or nearly identical) unit replicated many times to create the
active part of the pixel system. A hybrid pixel module is one in which
the sensor and the read-out electronics are produced separately and
joined by bump bonds. Hybrid pixel detectors have been built for the
WA-97 experiment[^1] and the DELPHI experiment at CERN.[^2] Only hybrid
pixel modules are planned for use in the next generation of pixel
detectors for the Large Hadron Collider at CERN[^3]^,^[^4]^,^[^5] and
the Tevatron collider at Fermilab.[^6] Only hybrid pixel modules can
currently meet the high-speed requirements for pixels detectors at these
high luminosity hadron colliders. The basic concept of a hybrid pixel
module is illustrated in Figure 1. A photograph of a pixel module is
shown in Figure 2.

Figure 1. Conceptual view of a hybrid pixel module showing the sensor,
read-out integrated circuits and flex-hybrid connections.

There are a variety of possible bump-bonding techniques that can, in
principle, be used to make the connection between the electronics and
the sensor. The most common bump-bonding methods are to use
electroplated solder bumps or indium bumps deposited by evaporation.
Bump-bonding methods are described below.

![](media/image1.jpeg){width="4.60625in" height="2.064583333333333in"}

Figure 2. An of pixel module that will be used in the ATLAS experiment
at the CERN Large Hadron Collider. Sixteen integrated circuits are
bump-bonded to a silicon sensor. This is a placeholder.

The signals from the integrated circuits must be routed within a module
and transmitted from a module to other electronics outside the active
region of the pixel detector. There are two general methods for routing
within a module. In the first of these, the \"bare\" module(sensor
bonded to electronics only) is connected to a flex-circuit hybrid(or
possibly a ceramic hybrid substrate) that routes the power and signals
to and from the integrated circuits. In the second general method,
multi-chip-module(MCM) technology is used to deposit metal and
insulating layers directly on the sensor to route power and signals. In
either case, power must be delivered to the module. This is typically
done by chain of a variety of cable types. Fast signals from or to the
module are transmitted on cables alone to external electronics or on
cables for a short distance and then converted to optical signals that
then travel on optical fibers to remote electronics for additional
processing. We describe in this Chapter some of the types of hybrids
used and the MCM approach. We also give some examples of power and
signal distribution.

An example of the steps necessary to assemble modules is given in Figure
3.

![](media/image2.png){width="4.839583333333334in"
height="3.0590277777777777in"}

Figure 3. An example of the process flow necessary to make a pixel
module for the ATLAS detector.

### Bump bonding

Flip-chip bonding or bump bonding is extensively used in the electronics
industry for the attachment of integrated circuit die to printed circuit
boards or other substrates. [^7] There are two basic steps to the bump
bonding process. The first of these is the deposition of bumps or metal
onto the sensor and integrated circuit substrates. The second is
flip-chip placement of the individual integrated circuits onto the
sensor. In this section we first present an overview of the bump
deposition process and then of the flip chip process. We give some
examples of bump bonding as used in particle physics detectors. The bump
bonding processes used for pixel detectors in particle physics typically
have much smaller diameter bumps than used in the electronics industry.
The typical bump diameter for hybrid pixel detectors at the LHC or for
the BTeV experiment is about 20 microns whereas the typical minimum size
in the electronics industry is 5-10 times larger. However, the bump
bonding process used for infrared sensor arrays does use indium bump
bonding techniques of comparable or even greater density than particle
physics detectors. [^8]

#### Bump deposition

We present here a very brief overview of the two methods most widely
used so far in pixel detectors for particle physics experiments. The two
techniques mostly widely used so far are solder bump deposition by
electroplating and evaporative deposition of indium. Other techniques -
gold bump deposition or conducting polymer bumps - have not yet been
used for particle physics pixel detectors.

##### Solder bump process

The basic process steps for solder bump deposition by electroplating are
given in numerous references.Error: Reference source not found However,
the details of the electroplating process can vary significantly from
vendor to vendor and we only give a brief sketch of the process here.
Solder bumps are created by a sequence of plating and photolithography
steps to appropriately pattern thin metals for adhesion to the
underlying aluminum pads on the integrated circuit or sensor followed by
plating of other metals and finally solder. A micrograph of solder bumps
is shown in .

Figure 4. A scanning electron micrograph of typical solder bumps.
Courtesy IZM.

So-called under bump metallization(UBM) is deposited on the integrated
circuit wafers by sputtering and electroplating and selective etching.
The UBM consists of metals for adhesion to the aluminum, to act as a
diffusion barrier and to provide a wettable surface for the solder. A
typical UBM is Ti/W with copper(or Ni/Au) a few microns thick to adhere
to the solder. Openings in the passivation on the integrated circuit
wafer are about one-half the bump diameter. Eutectic solder(37% Pb and
63% Sn) has most often been used for high energy physics detectors.
After the solder is electroplated, the wafer is etched to remove the UBM
except under the solder. The final step is to reflow the solder to form
spheres as shown in Figure 5.

![](media/image3.png){width="5.4319444444444445in"
height="3.4972222222222222in"}

Figure 5. A scanning electron micrograph of a single solder bump showing
the metal composition described in the text. Courtesty VTT Electronics.

##### Indium bump process

Although it is possible to also deposit indium bumps by electroplating,
indium bumps used in particle physics pixel detectors have been
typically deposited by evaporation. In this process metals for adhesion
to the aluminum and to prevent indium diffusion(barrier metals) and
finally indium are evaporated in vacuum. The pattern on the integrated
circuit or sensor wafer(or individual die) is determined by a
photoresist mask on the wafer that has been itself patterned to open
small holes at the locations of the aluminum pads. The evaporated metals
pass through the hole and are built up on the pads. A micrograph of a
typical indium bump is shown in. Indium bumps are created on both the
integrated circuit and sensor wafers.

Figure 6. A scanning electron micrograph of a typical indium bump.
Courtesy Alenia Marconi Systems

#### Flip chip assembly

Flip-chip assembly is the placement of integrated circuit die onto the
sensor substrate to make a pixel module. The die are placed precisely by
a flip chip bonder, either manually or automatically. For solder bumps,
a flux may used to \"tack\" the die temporarily in place during a solder
reflow cycle and to enhance the soldering reliability or a fluxless
process has also been used. In the case of indium bumps, pressure is
used to mechanically bond the indium bumps on the sensor and integrated
circuit parts.

Inspection before and after flip-chip assembly is important to obtain
the highest yield for functional pixel modules. Inspection of the bumps
begins at the wafer level. It is possible(but extremely tedious) to
perform a visual inspection. Automated inspection at the wafer level is
now possible using equipment from a number of manufacturers. Combined
automated 2D(camera and pattern recognition) and 3D(laser
interferometry) is now possible to find missing bumps, merged bumps,
deformed bumps and other defects as well as to measure bump heights.[^9]
X-ray inspection has been routinely used to inspect assemblies after
flip-chip assembly and provides a direct means to correlate bump defects
with missing or misbehaving pixels seen in the electronics readout.

The defect rates(ie. non-functional pixels) that can be obtained after
flip-chip assembly and further handling for assembly of modules and
handling are typically in the range of 10^-4^ to 10^-3^. The bump defect
rate at the wafer level is often about ten times better but additional
defects are introduced during the flip-chip process.

### Module connections

Signal and power connections must be made within a pixel module. Control
and trigger signals are distributed to the integrated circuits within a
module. Power must be provided to these ICs. Signals from the ICs are
transmitted to other electronics outside the experiment. These functions
may be provided by a flex circuit, a rigid printed circuit board or
ceramic hybrid or by direct routing of signals using the MCM-D technique
on the sensor substrate. We describe here only the flex circuit and
MCM-D approaches. A low radiation length of the module package is
essential in almost all collider applications in particle physics, which
is only realistically met by either the flex circuit or MCM-D approach.
The flex circuit approach is the one currently preferred by the LHC and
Tevatron collider experiments under construction. The MCM-D approach is
still in the R&D stage.

#### Flex circuit approach

Flexible circuits for connections to the integrated circuits on a pixel
module are made by conventional commercial means.[^10] However, flex
circuits for pixel modules typically require line widths and spaces
together with an overall size that are aggressive for current, low-cost
commercial processes. In addition, there is also an emphasis placed on
minimizing material in the flex circuit that is absent in commercial
applications. Finally, in many cases the flex circuits must be function
reliably in an intense radiation field - up to about 50 MRad in some
cases.

An example of a flex circuit used in prototype modules for the BTeV
detector is shown in Figure 7 and Figure 8. [^11] The flex circuit is
mounted(glued) on the top of the silicon sensor. The signal and power
connections from the flex circuit to the front-end integrated circuits
are made by wire bonds.

![](media/image4.png){width="6.499305555555556in"
height="1.4215277777777777in"}

Figure 7. The cross-sectional view of the concept for a BTeV pixel
module.

In this example, the flex circuit is used only for power and signal
routing to external electronics via a connector. Other experiments(eg.
ATLAS -- see Figure 2) utilize the flex circuit also to mount another
integrated circuit on the flex circuit itself to provide another level
of interface to external electronics mounted far from the detector.

![](media/image5.png){width="6.499305555555556in"
height="1.8194444444444444in"}

Figure 8. A cross-section through the BTeV pixel module(not to scale)
emphasizing the flex circuit for interconnections.

We summarize in Table 1 some of the parameters of flex hybrid prototypes
made for collider detectors.

  ----------------- ----------------- ----------------- -----------------
  Detector          No. of metal      Line width/space  Comments
                    layers                              

  ALICE                                                 

  ATLAS                                                 

  BTeV                                                  

  CMS                                                   
  ----------------- ----------------- ----------------- -----------------

Table 1. Summary of some of the critical properties of prototype
flex-hybrids for future collider detectors.

#### MCM-D approach

A possible alternative to the flex-hybrid approach to make intra-module
connections is to use MCM-D technology to create these interconnections
directly on the sensor.[^12] This concept is illustrated in Figure 9.

![](media/image6.png){width="3.9652777777777777in"
height="2.5708333333333333in"}

Figure 9. The MCM-D concept compared with the flex hybrid approach for
module intra-connections.

Layers of dielectric and metal are deposited and patterned on the
sensor. The metal layers are connected by vias. The bump bonding process
as described above is then used to connect the front-end integrated
circuits to the sensor elements as shown in Figure 10.

![](media/image7.png){width="5.059027777777778in"
height="1.2756944444444445in"}

Figure 10. An MCM-D via structure with a bump bond.

A prototype module using this approach is shown in Figure 11.

![](media/image8.png){width="4.213888888888889in"
height="2.8270833333333334in"}

Figure 11. A prototype MCM-D module(to be improved.). The MCC is module
controller chip.

The MCM-D approach is currently in the R&D stage and is not planned for
use in the next generation of hadron-collider pixel detectors.

## Mechanics and Cooling

The pixel modules(and the associated power and signal cables or fibers)
must be supported by a stable mechanical structure. The modules must
also be cooled, since the integrated circuit electronics and, to a
lesser extent, the sensors under bias generate heat. Power connections
to the modules also generate heat. The rough order of magnitude of the
heat load in collider hybrid pixel detectors approaches 1W/cm^2^. Under
significant radiation, silicon sensors must be kept below temperatures
about 0^o^C. These two facts, considerable heat load and the requirement
to keep silicon sensors cold, lead to significant cooling systems with
capacities of 10\'s of kW at temperatures well below 0^o^C. The
mechanical and cooling systems must be tightly integrated to meet these
requirements and to minimize material.

In the sections below we describe some examples of elements of the
mechanical and cooling systems for collider pixel detectors. The module
supports are those mechanical/cooling elements in direct contact with
the pixel modules. Global supports hold the module supports. The cooling
systems for collider detectors are substantial and there have been some
novel developments in this area in recent years. We also briefly cover
integration of the electrical and other signal connections into the
mechanical structure.

### Design Considerations

### Module supports

These are the mechanical supports with integrated cooling in direct
contact with the pixel modules. The desirable requirements of these
supports are a low radiation length, good thermal performance, a
coefficient of thermal expansion(CTE) near zero(to reduce distortions
upon cooldown from room temperature) and high stiffness. Practical
considerations are low cost and high reliability. In the sections below,
we give some examples that differ in their implementations to meet these
goals.

In we summarized some of the properties of important materials that
might be used in module supports.

Table 2. Summary of some of the properties of vitreous carbon,
carbon-carbon, beryllium and aluminum that are components of module
supports described in the text.

## Systems Aspects

[^1]: "Experience with a 30cm^2^ Silicon Pixel Plane in the CERN
    Experiment WA97", F. Antinori et al,
    Nucl.Instrum.Meth.A360:91-97,1995.

[^2]: "The DELPHI Pixels", K. Becks et al, Nucl. Instr. and Meths.
    A386:11-17,1997.

[^3]: "ATLAS Pixel Detector Technical Design Report", CERN/LHCC/98-13,
    1998.

[^4]: CMS Technical Design Report

[^5]: ALICE Technical Design Report

[^6]: BTev Proposal

[^7]: See for example \"Flip Chip Technologies\", J. H. Lau ed, McGraw
    Hill, 1996.

[^8]: Reference to IR bump bonding\...

[^9]: See as an example "Solder Bump Inspection", R. Asgari and V.
    Romega-Thompson, Advanced Packaging, September 2002, pps. 31-34.

[^10]: Insert reference to example flex circuit book.

[^11]: "Development of a high density pixel multichip module at
    Fermilab", G. Cardoso *et al.,* IEEE Transactions on Advanced
    Packaging vol 25, no 1, February 2002 and references therin.

[^12]: MCM-D references.