GB2384303A

GB2384303A - Detection of metals in semiconductor wafers

Info

Publication number: GB2384303A
Application number: GB0122932A
Authority: GB
Inventors: Gary Lewis Jones
Original assignee: Pure Wafer Ltd
Current assignee: Pure Wafer Ltd
Priority date: 2001-09-24
Filing date: 2001-09-24
Publication date: 2003-07-23
Also published as: TW544750B; GB0122932D0; WO2003027649A1

Abstract

A method of detecting for the presence one or more metals in a semiconductor wafer 10 comprises directing a laser beam 13 at the wafer 10 to cause localised vaporisation thereof and analysing the spectral characteristics of the electromagnetic radiation emitted from the resultant plasma plume 12 to detect for the presence of the metal(s). The method is performed on wafers 10 which are to be reclaimed by the steps of removing films of foreign matter from the surface of the wafer 10 by etching and then polishing opposite sides of the wafer in a slurry. Any metals present in the wafers are detected prior to reclamation, so that those wafers can be rejected, thereby avoiding contamination of the slurry and the resultant risk that the metal contaminants will be ground into the surface of wafers which are subsequently polished.

Description

Detection of Metals in Semi-Conductor Wafers This invention relates to a

method for the detection of metals such as Copper, Boron, Phosphor and Aluminium in semi-

conductor wafers..

Semi-conductor wafers comprise a substrate of semi 5 conductor material onto which metals such as copper are deposited to form conductive pads and tracks. A disadvantage of this is that the metal can diffuse into the semi-conductor substrate, thereby degrading the electrical characteristics of the semi-conductor and affecting its performance.

10 Accordingly, its desirable to be able to determine the degree of metal contamination in the substrate. US Patent Number 6,140,131 discloses a method for determining the degree of metal contamination in semiconductors by x-ray analysis.

A disadvantage of this method is that it is both time 15 consuming and expensive.

It is also known to determine the decree of metal contamination within a semi-conductor wafer using a mass spectrometer to analyse a portion of the wafer. However, a disadvantage of this method is that the wafer has to be 20 destroyed in order to remove a portion of the wafer for analysis. Furthermore the method is extremely time consuming.

Semi-conductor wafers are expensive to produce and often many wafers are wasted, either as a result of them being used for test purposes or as a result of a fault in the 25 manufacturing process. Accordingly, its desirable to be able to reclaim such wafers for re-use.

Our co-pending British Patent Application Number 0027626.1 describes a process for reclaiming wafers in which the surface depositions including the metal tracks and pads 30 are removed chemically. The doped semiconductor regions in the substrate are then removed by polishing using an abrasive slurry. This slurry is expensive and has to be re-used.

However, a disadvantage of this is that the slurry may contain metals from a contaminated wafer which metal contaminants are liable to be ground into the surface of wafers which are subsequently polished. Accordingly, it is desirable to be 5 able to determine the degree of metal contamination in a semi-

conductor wafer prior to the reclamation process, so that the contaminated wafers can be rejected and the contamination of slurry avoided.

It is also desirable for a company which reclaims 10 wafers on behalf of a manufacturer to be able to prove to the manufacturer that any metal contamination in the reclaimed wafer was present prior to the wafer being reclaimed.

Accordingly, its an object of the present invention to provide a method of detecting metals in a semi-conductor 15 wafer which is reliable and yet which can be performed quickly and easily without any of the abovementioned problems.

In accordance with this invention there is provided a method of detecting one or more metals in a semi-conductor wafer comprising directing a laser at the wafer to cause 20 localised vaporization thereof and analysing the spectral characteristics of the electromagnetic radiation emitted from the resultant plasma plume to detect for the presence of said metal(s). The laser causes rapid heating of a localized area of 25 the semi- conductor wafer causing vaporization of the wafer material, disassociation into its atomic species, and ionization which produces a plasma plume. The excited species relax the plasma cools and emit electromagnetic radiation at the characteristic wavelength. This radiation can be 30 spectrally analysed to determine the concentration of the metal(s) present in the wafer.

The method is relatively simply and can be performed quickly and easily to give a reliable indication of the presence of the or each metal under consideration.

35 The laser only causes localized heating of the wafer

and thus the method is non-destructive of the wafer.

Preferably the magnitude of the electromagnetic radiation at the characteristic wavelength of the or each metal is measured to determine the concentration of the metal 5 in the wafer.

Preferably the magnitude of the electromagnetic radiation at the characteristic wavelength of copper is measured to determine the concentration of copper in the wafer. 10 Preferably the laser is directed adjacent an edge of the wafer.

Preferably the laser is used to form a marking such as a bar code or a serial number on the wafer, the above-

mentioned method being carried out at least one point during 15 the laser marking process.

In one embodiment the laser is pulsed at one point on the wafer to cause a sequential localised vaporization at increasing depths, the spectral characteristics of the electromagnetic radiation emitted from each resultant plasma 20 plume being analysed in order to give an indication of the level of metal contamination at different depths of the wafer.

In an alternative embodiment the laser is continuously directed at one point on the wafer to cause localized vaporization at increasing depths, the spectral 25 characteristics of the electromagnetic radiation omitted from the plasma plume being analysed at least two points in time during the process.

Preferably the spectral characteristics are analyzed continuously to a depth of between 4-40-pm.

30 Preferably the method is performed at more than one point on the wafer, so as to avoid the risk of the method being performed at a localized point where no metal exists.

Preferably the method is formed along a line extending across the wafer, with the plume being sequentially or 35 continuously analysed along the said line.

In order to perform the methods at different points on the wafer, the laser may be displaced or deflected.

Alternatively, the wafer may be moved or rotated relative to the laser.

5 Preferably the laser is directed at the front or rear surface of the wafer in a direction which preferably extends substantially perpendicular to the plane of the wafer.

Alternatively, the laser may be directed at a side edge of the wafer in a direction which co-extends with the plane of 10 the wafer.

An embodiment of this invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a plan view of an apparatus for carrying 15 out a method in accordance with this invention for detecting metal in a semi-conductor wafer; Figure 2 is a diagram depicting one method of movement of the laser of the apparatus of Figure 1 relative to the wafer; and 20 Figure 3 is a diagram depicting an alternative method of movement of the laser of the apparatus of Figure 1 relative to the wafer.

Referring to Figure 1 of the drawings, there is shown a semi-conductor wafer 10. In order to detect the level of 25 copper or other metals in the wafer 10, a laser beam 11 is fired at a point adjacent the side edge of the front surface of the wafer 10, in a direction which extends perpendicular to the plane of wafer 1O. The laser beam 11 causes rapid heating of a localised area of the semi-conductor wafer 10, causing 30 the vaporization of the wafer material, disassociation into elemental species and ionization which produces a plasma plume 12. As the plasma plume 12 cools the excited elemental species relax and emit light at their characteristic wavelength. 35 In the case of copper, as little as 5parts per

billion can affect the electrical characteristics of the wafer and thus the system has to be capable of detecting 2-15 photons radiating at the characteristic wavelength of copper.

In order to analyse the plume 12, the light is 5 conducted by an optical fibre and/or lenses 13 to a prism 14 or a reflection grating, which breaks the light down into its spectral components. A diffraction grating 15 is used to select the required emission wavelength(s) of the metal(s) to be detected. The light passing through the diffraction 10 grating 15 is then detected by a photomultiplier tube 16. The output of the photomultiplier tube 16 is fed to a computer (not shown) to provide an indication of the level of copper and/or other metals present in the wafer 10.

The laser 11 may be continuously pulsed at one point on 15 the wafer 10 and each time the pulse is produced, the depth of penetration into the wafer increases, thereby enabling the level of metal contamination at different depths to be determined. Referring to Figure 2 of the drawings, it is often 20 desirable to be able to mark the wafer with a serial number or bar code around its edge. In order to achieve this, the wafer can be marked with a laser and during the marking process, the plume produced by the laser can be analyzed at one or more points 20, 21 to detect the level of metal contamination in 25 the wafer.

In order to ensure that the method is not carried out on an isolated point of wafer 10 comprising little or no metal contamination, the laser may be scanned across an area, as shown in Figure 3 of the drawings.

30 It will be appreciated that the present invention provides a reliable and non-destructive method of detecting the level of metal contamination in semi-conductor wafers, which can be performed as a part of the process for marking serial numbers etc. on the wafer.

Claims

1. A method of detecting one or more metals in a semiconductor wafer comprising directing a laser at the wafer to cause localised vaporization thereof and analysing the 5 spectral characteristics of the electromagnetic radiation emitted from the resultant plasma plume to detect for the presence of said metal(s).

2. A method as claimed in claim 1, in which the magnitude of the electromagnetic radiation at the characteristic 10 wavelength of the or each metal is measured to determine the concentration of the metal in the wafer.

3. A method as claimed in claim 2, in which the magnitude of the electromagnetic radiation at the characteristic wavelength of copper is measured to determine the concentration 15 of copper in the wafer.

4. A method as claimed in any preceding claim, in which the laser is directed adjacent an edge of the wafer

5. A method as claimed in any preceding claim, which is performed whilst the laser is forming a visible marking on the 20 wafer.

6. A method as claimed in any preceding claim, in which the laser is pulsed at one point on the wafer, the spectral characteristics of the electromagnetic radiation emitted from each resultant plasma plume being analysed in order to give an 25 indication of the level of metal contamination at different depths of penetration into the wafer.

7. A method as claimed in any preceding claim, in which the laser is continuously directed at one point on the wafer

to cause localized vaporization at increasing depths, the spectral characteristics of the electromagnetic radiation emitted from the plasma plume being analysed at at least two points in time during the process 5

8. A method as claimed in claim 7, in which the spectral characteristics are analysed continuously to a depth of 4-40,um in the wafer.

9. A method as claimed in any preceding claim, which is performed at more than one point on the wafer.

10 10. A method as claimed in claim 9, which is performed along a line extending across the wafer, with the plume being sequentially or continuously analysed along the said line.

11. A method as claimed in claims 9 or 10, in which the laser is displaced or deflected to move between the points.

15

12. A method as claimed in claims 9 or 10, in which the wafer is moved or rotated relative to the laser to move between the points.

13. A method as claimed in any preceding claim, in which the laser is directed at the front or rear surface of the wafer 20 in a direction which preferably extends substantially perpendicular to the plane of the wafer.

14. A method as claimed in any of claims 1 to 12, in which the laser is directed at a side edge of the wafer, in a direction which co-extends with the plane of the wafer.