Advanced Methods for the Analysis of Semiconductor Manufacturing Process Data - Advanced Techniques in Knowledge Discovery and Data Mining - page 32

packaging make up more than 50% of the production cost. Wafer processing
takes places in a so-called wafer fab or manufacturing line and is often further
divided into front-end-of-line (FEOL) and back-end-of-line (BEOL) process-
ing. Simply speaking, the FEOL processing provides the active devices within
the silicon, BEOL processing produces the connections between the devices,
and the back-end processing provides the connections to the outside world
as well as protective packaging. To simplify the fab logistics, wafers typically
run in lots of 25, 7 although some tools demand batches (see Fig. 2.3) of up
to six lots to be used effectively, while other tools can't process a complete
lot, which will then be split into smaller batches or even single wafers.
All wafer processing, whether FEOL or BEOL, has the same general struc-
ture of producing so-called layers, one after another. The whole wafer is sub-
jected to some processing, like producing a thin film of oxide or metal. Then
a mask is transferred to the wafer, most commonly by optical lithography, to
selectively protect parts of the wafer from the following process steps. Then
the wafer is subjected to further processing, like etching or implantation of
ionized dopants. Manual and automatic inspections are inserted at various
stages (Fig. 2.4). Then the mask is removed and the next layer is processed.
Layers vary widely in the number, complexity, and cost required to make
them. This leads to a distinction between critical and uncritical layers. Mod-
ern technologies make use of 20 to 30 layers, and this number continues to
go up. The number of layers that make up the actual devices stays relatively
constant. However, as the minimum feature size

continues to shrink, the
exponentially growing number of devices requires much more interconnect
between them. For this reason, the number of interconnect or metal layers in
the BEOL, another commonly cited characteristic of a technology, increases
quite rapidly. In fact, the interconnect of the devices (the BEOL) is now more
costly to produce then the devices themselves (the FEOL).

F

2.2.3 Data from the Fab - Inline Data

Historically, each lot was accompanied by a stack of paper, called the process
record. Each sheet detailed one process step and the operator would set up the
tool accordingly, run the process, sign off, note remarks and the result of any
measurements taken, look up the next operation, and hand the lot over to the
next operator. This is really where the term semiconductor manufacturing
stems from. The process record has been replaced 8 by a database and the
lots are moved to the next operation by automatic transport systems (Fig.
2.5) coupled to that database. The so-called process flow is defined by the
layer sequence at the top level. This has to be broken down into individual
process steps, often called moves. Each of the process steps is made up of a

7 The lot size is somtimes reduced to 12 wafers for 300-mm wafers, as a lot of 25
wafers is too heavy to be handled manually.
8 Some fabs still use printouts to accompany the lots.