JPS5941303B2 - Method for measuring deep impurity levels in semiconductors - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a method for measuring deep impurity levels contained in a semiconductor, and is intended to more accurately and easily obtain information on deep impurity levels.

To measure deep impurity levels, there is a method of examining the time change in capacitance based on the depletion layer in a pn junction or Schottky barrier.Recently, various measurement methods based on this principle have been proposed, and semiconductor evaluation technology has improved. is being run as.

In these measurement methods, the time constant of the temporal change in capacitance is directly determined by the thermal radiation rate (emission) of deep impurity levels.
It is based on the assumption that it corresponds to the reciprocal of the capacitance change ΔC (rate), and therefore, in reality, the signal obtained by directly converting the time change ΔC of the capacitance into the time change of the voltage is measured and analyzed. This is how the measurements are taken.

As a specific example of this type of conventional measurement method, the case of a pfn junction will be described.

The electric capacitance Co due to the depletion layer of the pfn junction when applying the reverse bias VR is C(5■qksεoA″NIO
/2(VD+VR)...Given by (1).

Here, q is the amount of charge, ksεO is the dielectric constant of the semiconductor forming the p+n junction, A is the cross-sectional area of the junction, Nlo is the net ionized impurity concentration in the depletion layer, and qVD is the diffusion potential. Now, if we apply an injection pulse of minority carriers or majority carriers to the junction in this state and then calculate the change in capacitance ΔC() in seconds, we get Δat:Co((1+ΔNi(tl//Nlo) − 1
) ......Given by (2).

Here, ΔNi() is the change in Nlo after . ΔNi( ) is ΔNi( ) when there is an impurity level.
)■ΣΔNi(O)exp(--)・・・・・・(3
) becomes lτ・.

Here, ΔNi(O) is the direct value of ΔNi(O) with The time constant is given by

If ΔNI(t)〈NlO in equation (2), then
ΔC(t) is approximately given by using equations (2) and (3).

In this way, equation (5) shows that the time constant of the capacitance change over time is directly the reciprocal of the heat radiation rate of the deep impurity level τl. Conventionally, therefore, first, the time change ΔC(t) of the capacitance is directly extracted as a signal converted into a voltage change, and this is stored in a computer, for example, or only the values of tl and T2 are selected at a certain time and the analog signal is converted into a voltage change. τi or the heat radiation ratios Ep, en were determined based on equation (5) using a method such as a method such as processing according to the method described above. However, as mentioned earlier, the conventional measurement method has been perfected (5)
The equation is an approximate equation when the condition ΔNI(t)<<NlO holds true.

On the other hand, in general, it is often unclear whether a semiconductor p+n junction satisfies this condition. Therefore, the conventional methods described above do not necessarily provide accurate measurement and analysis results, and often result in misunderstandings.

The present invention has been made in view of the above, and unlike the conventional
We aim to provide a measurement method that can accurately and easily measure the deep impurity quasi-9 position based on accurate Pn junction and Schottky barrier theory without directly analyzing and processing the (t) signal. It is something.

Hereinafter, the measurement method of the present invention will be explained as follows, as in the case of the conventional example.
An embodiment applied to an fn junction will be described with reference to the accompanying drawings.

In the present invention, measurement and analysis are performed based on equation (2), which is a theoretical equation that does not include mathematical assumptions as in conventional methods.

Therefore, first, ΔC(t) on the left side of equation (2) is extracted by applying an optical pulse or voltage pulse 2 from the p-n junction sample 1 to which a reverse bias VR has been applied. It is sufficient to use a capacitance meter 3 such as a transient capacitance meter that can take out the ΔC(t) or C(t). However,
ΔC(t) extracted in this way is also difficult to analyze if the formula (2) is used as it is because it is a non-exponential function.

Therefore, one feature of the present invention is that analog processing is performed using the analog function generator 4 to obtain a function f(ΔC(t)) suitable for processing, and in the case of this embodiment, ( Transform the equation 2) into the following equation (6).

In this equation (6), instead of ΔC(t), (ΔC(t)/C
It is shown that if the time change of the signal O)2+2ΔC(t)/CO is determined at a certain temperature, the time constant will be rl, which is the reciprocal of the heat radiation rate based on the deep impurity level. And also, the output signal f(ΔC(t)) of this analog function generator 4=(ΔC(TVCO)゛+2ΔC
(t)/CO can be easily analyzed using an existing analysis device 5 such as a minicomputer. As described above, according to the present invention, since the above equation (2) is used as the basis and this is analog-processed into a signal convenient for analysis, data analysis is much more accurate and easier than conventional methods. effective. *Although the above examples relate to pn junctions, in general, evaluation methods for the capacitance of the depletion layer in pn junctions and shottock junctions are based on accurate theory for each of the various cases. The equations corresponding to equation (6) differ depending on the method of joining. For example, when there is a distribution in the impurity concentration of the n layer in a p+n junction, the electric capacitance CO due to the depletion layer of the p+n junction when applying a reverse bias VR is (1
) becomes the formula. In this case, a function that replaces equation (6) would be ΔNI(t)/NlO.

In order to obtain the numerical value of equation (8) from the signal of ΔC(t),
It can be created by a combination of an analog function generator whose input signal is cubed and an analog function generator whose input signal is squared. As described above, since the analog function generator 4 itself is well-known, it is easy to obtain a function f(ΔC(t)) that is easy to process and corresponds to reality in accordance with the idea of the present invention.

[Brief explanation of the drawing]

The drawing is a schematic structural diagram of an example of an apparatus for implementing the method of the present invention, and in the drawing, 1 is a sample to be measured, 3 is a capacitance meter, 4 is an analog function generator, and 5 is an analysis device.

Claims (1)

[Claims] 1. A method for measuring information on deep impurity levels in a semiconductor from time-based changes in the capacitance of depletion layers of p-n junctions and Schottky barrier junctions of a semiconductor, in which the time-varying signal of the capacitance is generated using an analog function. A method for measuring deep impurity levels in semiconductors, which is characterized by performing analog processing in a device and inputting it into an analysis device as a function signal suitable for analysis.