Section formation observing method

Details Section formation observing method Section formation observing method

1999-03-18

2001-12-18

Berman, Jack (Department: 2881)

Radiant energy

Irradiation of objects or material

Irradiation of semiconductor devices

C250S310000, C250S307000

Reexamination Certificate

active

06331712

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a method for forming a section of a sample, such as a semiconductor integrated circuit and ceramics substrate, having a plurality of conductive layers by using a focused ion beam to observe the section.
There is known, as a method to form and observe a section for observing a fine structure of a sample, a method of repeatedly irradiating a focused ion beam to a region having as a side a section observation position of a sample to form a recess (hole) in the sample as disclosed for example in JP-A-2-123749 so that a section of the sample appeared in a sidewall of the recess is observed by scanning and irradiating another charged particular beam.
FIGS. 1A and 1B
show, as an example of a sample including a plurality of conductive layers, an interconnect leading to a gate of a MOSFET. Explanation will be made on a method to observe a section obtained by cutting the interconnect.
FIG. 1A
is a plan view of a gate interconnect
51
portion. A source region
52
and a drain region
53
are formed on both sides of the gate interconnect
51
in a surface portion of a substrate
50
. The source interconnect
55
and the drain interconnect
56
are respectively connected to the source region
52
and the drain region
53
through contacts
54
. Explanation is made for a case that the gate interconnect
51
is observed, from a direction of an arrow C, in a section at a position (broken line A-B) between the source region
52
and the drain region
53
. A work frame
60
(hole) is defined as in
FIG. 1A
, which is a region where a focused ion beam is irradiated to form a recess. Subsequently, a focused ion beam is scanned within the work frame
60
to open a hole. As a result, a recess
61
is formed as shown in FIG.
1
B. The section of the gate interconnect
51
can be observed by observing in a direction of an arrow D shown in FIG.
1
B.
At this time, the gate interconnect
51
(on a D side) leading to the MOSFET gate is separated from the conductive layers, such as other interconnects and semiconductor substrate, by the section formation, and is thus rendered in an electrically floating state. Because the floating-state gate interconnect
51
leading to the gate is formed on a substrate
50
surface through a gate oxide film
57
, they form a capacitor. In this state, if charged particles, such as a focused ion beam or electron beam, are irradiated for observation, the charges are charged in the capacitor thus formed. A so-called charge-up phenomenon occurs wherein the potential of the floating gate interconnect
51
increases with respect to a potential of the substrate
50
when irradiating a focused ion beam, and decreases in the case of an electron beam. That is, a problem occurs because of the charge-up phenomenon when the section is observed by irradiating with a focused ion beam or electron beam in a D direction.
In particular, generally in the case of an ion beam the effected charge-up is prominent. That is, in the case of a focused ion beam, the secondary electrons are withdrawn in the section due to charge-up in the observation surface (interconnect section) to a positive potential. This makes it impossible to detect sufficient secondary electrons for obtaining an image. As a result, there is a problem in that the interconnect rendered in charge-up for the image is dark and observation is impossible.
As a countermeasure to this, there is a method disclosed for example in JP-A-7-45681 that the conductive layer in electrically floating is formed with a section separately from one for observation, an exposed new section is irradiated by a focused ion beam while blowing a metal compound gas thereby forming a metal film, and the conductive layer in floating is electrically connected to a sample substrate to avoid a charge-up. This method is effective as a means to avoid a charge-up phenomenon. However, there have been such problems that a metal compound gas blowing mechanism is required besides the focused ion beam irradiation system, and there is necessity of operations of second section formation and the succeeding metal film formation.
Meanwhile, in recent years line widths have become narrow and the gate oxide film formed between the substrate
50
and the gate interconnect
51
has become thin. In particular, the area of the floating conductive layer has been decreased by section formation due to refining in a forming region. Due to this, there has been a drastic decrease in capacitance of a capacitor formed in the process of section formation, and there has been increase in relative potential difference of the gate to the substrate due to floating phenomenon. As stated before, because the decrease in gate oxide film thickness results in a lower withstand voltage, the possibility of sample damage due to charge-up is increased. In particular, before forming a floating conductive layer in the process of section formation, the electric charges due to focused ion beam irradiating are discharged through the conductive layer. The section is observed by irradiating a scanning focused ion beam or scanning electron beam to the formed section and detecting secondary charged particles occurring from the section. However, after a recess
61
for section observation is formed and the conductive layer is rendered in a floating state, electric charges are accumulated in the floating conductive layer during formation of a portion lower than the conductive layer, causing a charge up phenomenon. There is a problem in that a high voltage is applied to the thin gate oxide film that is low in withstand voltage, resulting in damage to the sample as the case may be.
SUMMARY OF THE INVENTION
The present invention has an object to solve these problems.
That is, in order to avoid the problem due to the floating conductive film, a second conductivity layer region formed in a electrically floating state by the section forming process is electrically connected to another non-floating first conductivity layer region (substrate). At this time, realization is made by continuously irradiating a focused ion beam to one point without scanning, or otherwise irradiating it with scanning over an extremely narrower region than the recess formed for section observation.
That is, the present invention is a sample section formation observing method, in a sample including a second conductive layer overlying a first conductive layer through an insulating layer, characterized by comprising: a first process of irradiating a focused ion beam repeatedly scanning at a predetermined region of the sample and forming a recess to form a sidewall in which the second conductive layer and at lease the first conductive layer are exposed, a second process of irradiating the focused ion beam at a region other than the predetermined region from above the second conductive layer rendered in an electrically floating state by the first process to provide a hole reaching the first conductive layer thereby electrically connecting between the first conductive layer and the second conductive layer, and a third process of observing a desired exposed portion in the recess using a charged particle beam.
Although scan irradiation is best effective at a site where the second conductive layer in a floating state is overlapped with the first conductive region positioned thereunder through the insulating film, it is possible to realize the effect of the desired object if an electrical connection is realized between the two electrically independent conductive regions, by a resistance with a sufficient degree for discharging charges that typically are a cause of charge-up.
FIGS. 2A-2B
show section of a hole
62
formed by scanning/irradiating a focused ion beam. In the case where the recess formed as in
FIG. 2A
is shallow in depth (where the hole is opened shallower in depth than a broadest diameter), the substance sputter-etched is discharged from the hole by irradiating focused ion beam ions
71
. However, where the recess formed as in
FIG. 2B
is deep in depth (where the hole is ope

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