Method of and apparatus for conducting analysis of buried oxides

A beam of light is projected through an electrolyte to a SOI substrate to scan the surface of the SOI substrate. When the light passes through a pinhole in a buried oxide layer, the light excites a semiconductor layer beneath the buried oxides. An ammeter measures electric charges derived by this light excitation to indicate the presence of pinholes in the buried oxide.

BACKGOUND OF THE INVENTION 
1. Field of the Invention 
The present Invention relates to a method of and an apparatus for 
conducting analysis of insulator films, for example, buried oxides. 
2. Description of the Related Art 
Insulator films in semiconductor or SOI substrates are required to be 
sufficiently condensed. There tend to occur pinholes through insulator 
films due to film forming conditions. Such pinholes in insulator films 
cause insulator breakdown. Various proposals have been made to examine 
insulator films to find pinholes. 
According to JP-A 7-86757, a frame is placed in liquid-tight manner on an 
insulator film that extends over a conductive layer on a substrate. 
Contained in the frame is test liquid including electrolyte and 
pH-indicator. A power supply with sufficient voltage is hooked up to an 
anode plate, immersed in the test liquid and a cathode probe pin connected 
to the conductor layer below the insulator film. Current flows through a 
pinhole in the insulator film. At the point of conduction above the 
pinhole, a colored spot will occur in the test liquid. 
This method appears to be effective in determining the presence or absence 
of pinholes. However, it is difficult to evaluate the size of each pinhole 
and the number of pinholes owing to diffusion of colored spots. If, for 
example, two small pinholes lie in close proximity to each other, two 
colored spots spread into one spot. 
An article entitled "ELECTROCHEMICAL ANALYSIS OF SIMOX BURIED OXIDES" by L. 
P. Allen et al., "Electrochemical Society Proceedings Volume 96-3" pages 
18 to 27 (1996), explains an experimental set-up for electrolytic analysis 
of SIMOX or SOI wafer. A power supply with sufficient voltage is hooked up 
to an Al or Cu cathode and a Cu anode plate. Cleanroom Texwipes.TM. (Trade 
Mark), cut to fit an exclusion area of the wafer edge, are first highly 
soaked in a saturated copper sulfate solution. The Texwipes.TM. are placed 
on the top surface of the SIMOX substrate. The substrate, with surface 
tension attached Texwipes.TM., is placed face down upon the Cu anode. A 
drop of water is placed upon the backside of the wafer. The cathode weight 
is placed over the water drop to complete the circuit. Cu atoms are 
deposited on the Texwipes.TM. at the point of leakage current. The number 
of Cu plates is counted, divided by the area of the wafer, and a Cu plate 
density is obtained. 
This method is found to be unsatisfactory because it takes a long time for 
Cu atoms to be deposited and Cu atoms are deposited on the buried oxides, 
failing to establish itself as a tool for quality control of substrate 
manufacturing. 
An object of the present invention is to provide a method of and an 
apparatus for conducting analysis of insulator films, such as buried 
oxides, so that quick quantitative measurement of pinholes can be made. 
SUMMARY OF THE INVENTION 
According to one aspect of the present invention, there is provided a 
method of conducting analysis of a semiconductor substrate including a 
semiconductor layer under an insulator film, the method comprising the 
steps of: 
immersing the insulator film of the semiconductor substrate to electrolyte; 
projecting a spotlight through said electrolyte to the insulator film; 
scanning the insulator film with said spotlight; and measuring electric 
charges that have been excited in the semiconductor layer and passed 
through the insulator film into said electrolyte. 
According to another aspect of the present invention, there is provided an 
A method of conducting analysis of a semiconductor substrate including a 
semiconductor layer under an insulator film, the method comprising the 
steps of: 
immersing the insulator film of the semiconductor substrate in an 
electrolyte; 
projecting a spotlight through said electrolyte to the insulator film; 
scanning the insulator film with said spotlight; and 
measuring electric charges that have been excited in the semiconductor 
layer and passed through the insulator film into said electrolyte. 
According to still another aspect of the present invention, there is 
provided a method of conducting analysis of a semiconductor substrate 
including a semiconductor layer under an insulator film, the method 
comprising the steps of: 
immersing the insulator film of the semiconductor substrate in an 
electrolyte; 
projecting a spotlight through said electrolyte to the insulator film; 
scanning the insulator film with said spotlight; and 
measuring electric charges that have been excited in the semiconductor 
layer and passed through the insulator film into said electrolyte.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIGS. 1 and 2, a SOI substrate 1 is illustrated. The top 
surface of the substrate 1 is exposed to electrolyte 2 contained in a cell 
that is defined by a frame 4 in cooperation with the top surface of the 
substrate 1. The frame 4 is placed on the top surface of the substrate 1 
in liquid-tight manner via seal 12. A power supply 5 of sufficient dc 
voltage is hooked up to an anode electrode plate 6 that is immersed into 
the electrolyte 2 and a cathode electrode 7. The cathode electrode 7 is 
kept in contact with an ohmic electrode on the edge of the top surface of 
the SOI substrate 1 outside of the frame 4. A window 3 is placed on the 
frame 4 to keep the electrolyte 2 within the cell. A light irradiation 
unit 8 is provided above the window 3. The light irradiation unit 8 
projects a spotlight on the top surface of the SOI substrate 1 through the 
window 3 and moves the spotlight. An ammeter 10 is provided to measure 
electric current upon completion of the circuit. 
The window 3 prevents electrolyte 2 from flowing out of the frame 4. Supply 
of electrolyte 2 to the inside of the frame 4 is effected through a supply 
pipe by a pump, not shown. 
The light irradiation unit 8 may include a white light lamp, a light 
irradiation head composed of a condensing lens and a relay lens, and a 
mirror that reflects the condensed light from the light irradiation head 
to project a spotlight on the SOI substrate 1 for scanning movement. The 
spotlight projected on the SOI substrate 1 can move in one direction. The 
SOI substrate 1 is placed on a carriage and can travel in a direction 
orthogonal to the scanning direction of the spotlight. Thus, the whole 
area of the top surface of the substrate can be exposed to the spotlight 
owing to the scanning movement of the spotlight and travel of the 
carriage. 
The white light, which is considered to be a good tool for obtaining light 
excited current at high efficiency, requires an expensive light 
irradiation head including a light condensing arrangement. Thus, in the 
preferred embodiment, a He--Ne (helium neon) laser oscillator, which is 
available at low cost on the market, is used as the light irradiation unit 
8. 
The manner of conducting analysis of buried oxides of SOI substrate 1 is 
explained. First of all, a SOI substrate 1 with buried oxides 9 is placed 
on a carriage, not shown. Frame 4 is placed on the top surface of the SOI 
substrate 1 and its window 3 is closed. Electrolyte 2, for example, 1% 
diluted hydrofluoric acid solution or diluted acetic acid solution, is 
supplied to the inside of the frame 4 by a pump, not shown, So that the 
electrolyte covers the top surface of the SOI substrate 1. 
Anode electrode plate 6, which is connected to positive side of dc power 
supply 5 and immersed in the electrolyte 2, and cathode electrode 7, which 
is connected to negative side of the power supply 5, are biased with dc 
voltage. That is, voltage is applied between the electrolyte 2 and the 
cathode electrode 7. Spotlight projected through the window 3 and the 
electrolyte 2 by light irradiation unit 8 to the SOI substrate 1 moves in 
the scanning direction. If the spotlight moves to a pinhole 17 in the 
buried oxides 9, a reading of electric current at ammeter 10 increases. An 
oscilloscope observes this increase in the form of a pulse-like wave. 
FIG. 3 is a view observing buried oxides of SIMOX (Separation by Implanted 
Oxygen) substrate with pinholes. 
The first embodiment is further described. The SOI substrate 1 shown in 
FIGS. 1 and 2 is a p-type substrate with carrier life time in the 
neighborhood of 30 Ms. As the electrolyte 2, a 1% HF solution (49% 
HF:H.sub.2 O) or a 1% acetic acid solution is used. Voltage of 5 V is 
applied across the electrolyte 2 and the cathode electrode 6 by the power 
source 5. The anode electrode 6 and the electrolyte 2 are positively 
biased, while the cathode electrode 7 is negatively biased. As the light 
irradiation unit 8, a laser oscillator is used which generates a laser 
beam with a wavelength of 670 nm and a spot diameter of 1 mm. The reason 
why the diameter of the spot is 1 mm is to provide resolution of 1 
pinhole/mm. 
Speed at which the spotlight moves is set so that two pinholes spaced a 
distance in the neighborhood of 1 mm can be detected separately. If the 
spotlight moving speed is high as compared to the lifetime of carrier, the 
excited carriers due to one pinhole occur before the excited carriers due 
to the other pinhole disappear. This overlap increases the possibility 
that the two pinholes are detected as one big pinhole. 
In the present embodiment, therefore, the setting is made that the 
spotlight moving speed is 25 mm/s taking it into account that carrier 
lifetime of the substrate is in the neighborhood of 30 .mu.s. With this 
speed, it took approximately 15 minutes to scan the whole surface area of 
the substrate. If the distance between the adjacent two pinholes is 1 mm, 
a limit spotlight moving speed is expressed by 1 mm/30 .mu.s=33.3 m/s. 
Thus, the spotlight moving speed is allowed to increase to 30 m/s, 
shortening time required for the spotlight to scan the whole surface area 
of the substrate. 
Variations of reading of current at the ammeter 10 are observed via 
oscillograph or display. FIG. 3 shows locations of pinholes at white 
portions where the measured current is great. 
Referring to FIG. 4, the second embodiment is described. This second 
embodiment is substantially the same as the first embodiment except the 
provision of a conductive sheet 13 that is transparent and elastic and the 
provision of an electrolyte circulatory unit 21. 
The conductive sheet 13 is stretched on the bottom end of a frame 4a in 
direct contact with an insulator film 9a of a substrate 1a. The 
electrolyte circulatory unit 21 is provided to develop pressure in body of 
electrolyte 2 within the frame 4a to press the conductive sheet 13 against 
the insulator film 9a. 
The conductive sheet 13 is made of ethylene vinyl acetate containing 
graphite. The electrolyte circulatory unit 21 includes a compact bellows 
pump and a reservoir tank that stores electrolyte from the frame 4a. 
Activating the pump fills the inside of the frame 4a with electrolyte, 
urging the conductive sheet 13 into firm and close engagement with concave 
and convex surface of the insulator film 9a expelling air from the 
interface. This minimizes loss of light beam. 
Discharge of electrolyte from the Inside of the frame 4a is initiated by 
activating the pump with a drain side valve opened and a supply side valve 
closed. Thus, recovery of electrolyte is simple. Another advantage is that 
what is required is to operate the valves and the pump to contact and 
separate the conductive sheet 13 with and from the insulator film 9a. This 
fits requirements for automation. The second embodiment is different from 
the first embodiment in that a cathode electrode 7 contacts with an ohmic 
electrode formed on the backside of the substrate 1a. 
Referring to FIGS. 5 and 6, a third embodiment is described. The third 
embodiment is substantially the same as the first embodiment except the 
provision of a partition 11 and the arrangement of an electrode plate 7. 
The partition 11 divides the inside of a frame 4 into two sections, namely 
a left-hand section and a right-hand section, viewing in FIGS. 5 and 6. An 
electrode plate 6 is immersed into electrolyte 2 within the right-hand 
section inside the frame 4, and another electrode plate 7 is immersed Into 
electrolyte 12 within the left-hand section inside the frame 4. A power 
source 5 of sufficient voltage is hooked up to the electrode plates 6 and 
7. The polarity of the power source 5 switches in response to which one of 
left-hand and right-hand sections the beam of light passes through. When 
the beam of light passes through the left-hand section inside the frame 4, 
the electrode plates 6 and 7 are connected to positive and negative sides 
of the power source, respectively. When the beam of light passes through 
the right-hand section, the electrode plates 6 and 7 are connected to 
negative and positive sides of the power source, respectively. 
According to the third embodiment, no electrode is attached to a substrate 
1. Thus, contamination of substrate by metals is suppressed. If desired, a 
set of a light irradiation unit and a measurement circuit may be arranged 
for each of the left-hand and right-hand sections inside the frame 4. In 
this case, analysis of the substrate 1 below the left-hand section can be 
conducted simultaneously with analysis of the substrate below the 
right-hand section, thus shortening time required for analysis of the 
whole of the substrate. 
In this third embodiment, the same electrolyte as used in the first 
embodiment fills in both the left-hand and right-hand sections inside the 
frame 4 and the SOI substrate of the p-type is used. The light irradiation 
unit 8 can project a laser beam with a wavelength of 670 nm and a spot 
diameter of 1 mm. When the laser beam passes through the left-hand section 
inside the frame 4, the electrode plate 6 is connected to the positive 
side of the power source 5 and the electrode plate 7 is connected to the 
negative side of the power source 5. Bias voltage of 5 V is applied across 
the electrode plates 6 and 7. When the laser beam passes through the 
right-hand section inside the frame 4, the electrode plate 6 is connected 
to the negative side of the power source 5 and the electrode plate 7 is 
connected to the positive side of the power source 5. An ammeter 10 
measures current in the circuit. 
Analysis of SIMOX substrate with buried oxides conducted according to the 
third embodiment has given substantially the same measurement results as 
those given according to the first embodiment. 
In the case where no partition is used, the density of contamination of 
substrate by heavy metals is in the neighborhood of 10.sup.11 
atoms/cm.sup.2. The use of the partition has proven to be effective in 
reducing the density of contamination by heavy metals down to a level not 
exceeding 10.sup.10 atoms/cm.sup.2. 
From the preceding description, it is now understood that insulator film 
and semiconductor below the insulator film are involved in interference 
with electrolyte and beam of light. Thus, the insulator film and the 
semiconductor layer are free from damage and contamination. 
Ammeter measures electric charges that have occurred due to excitation of 
semiconductor layer by a beam of light penetrated through a pinhole of 
insulator film. The density of pinholes can be evaluated quantitatively 
without any difficulty and without any delay.