Abstract:
A method for monitoring an ion implanter is disclosed. In one embodiment, the method comprises providing a wafer, forming a barrier layer on the surface of the wafer wherein the barrier layer has a substantial blocking effect on ion implantation, performing an ion implantation process to the wafer, performing a thermal treatment process, removing the barrier layer, and measuring a physical property of the wafer. The measured physical property of the wafer can be used to ascertain the status of the ion implanter. For instance, the measured physical property can be used to determine whether the ion implanter has problems when the energy or concentration of the implanted ions is changed.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   This application claims priority from R.O.C. Patent Application No. 092137173, filed Dec. 26, 2003, the entire disclosure of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a method for monitoring, and more particularly to a method for monitoring an ion implanter. 
   In the semiconductor industry, the technique of ion implantation is broadly utilized to produce electronic devices. In the process of ion implantation, the doped atoms or molecules, as the form of charged ions, are accelerated to directly hit and enter a target with a specific energy level. Therefore, the depth profile of implanted ions in the target can be accurately controlled by the energy of implanted ions, and the dosage of implanted ions can be accurately controlled by the implantation time and the current of the ion beam. The use of ion implantation not only can accurately control the depth profile and dosage of the implanted ions but also produce more well-distributed and purer dopants. 
   Nowadays, ions are implanted in a wafer by an ion implanter in the semiconductor doping process. Since the quality of a semiconductor device will be affected by the quantity and distribution of ions doped in the wafer, how to judge whether an ion implanter has problems or not in time when the energy or concentration of the implanted ions is changed is an important task for the semiconductor doping process. 
   Although there is a relative control device for the operation condition of the ion implanter, it is controlled by electronic signals and cannot reflect the real situation in the operation process. 
   Therefore, how to develop a method for monitoring an ion implanter, which can overcome the above shortcomings of the prior art and judge whether the ion implanter has problems or not, is an urgent problem needed to be solved now. 
   BRIEF SUMMARY OF THE INVENTION 
   Embodiments of the present invention provide a method for monitoring an ion implanter, which can overcome the problem that the prior art cannot monitor the real situation in the operation process, and quickly judge if there is any change of the energy of the implanted ions or not. 
   As known by one skilled in the ion implantation art, implanted ions will be distributed to a specific depth after the ions are implanted in a substrate. For example, the concentration of the ions will gradually increase to a specific value from the surface of the substrate and then gradually decrease after a specific depth. 
   If there is a barrier layer on the top of the substrate, it can substantially trap some implanted ions, and thus the state of the ions in the substrate will be different. 
   The different state of the ions is able to reflect the physical property change of the substrate, e.g., resistance, optical property, acoustic property, surface property of the substrate and the like. 
   If the energy of the implanted ions is changed, the physical property will deviate from the normal value. 
   The above features and advantages of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1(   a )–( d ) show cross-sectional views illustrating the method for monitoring an ion implanter according to an embodiment of the present invention; 
       FIGS. 2(   a ) and ( b ) show the distribution of the measuring points on the surface of the wafer according to an embodiment of the present invention; 
       FIG. 3(   a ) is a table showing the resistance variation upon different energy of ion implantation according to the method of an embodiment of the present invention; and 
       FIG. 3(   b ) is a table showing the resistance variation upon different dosage of ion implantation according to the method of an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1(   a )–( d ) show cross-sectional views illustrating the method for monitoring an ion implanter according to an embodiment of the present invention. As shown in  FIG. 1(   a ), first a wafer  10  is provided, and a barrier layer  11  is formed on the surface of the wafer  10 . The barrier layer  11  is composed of a material that has a substantial blocking effect on ion implantation. The thickness of the barrier layer is determined by the energy or concentration of the implanted ions. With the specific thickness, the property of the to-be-measured region of the wafer has a comparison meaning (such as on statistics or quality control) for measuring and judging whether it deviates from the normal value after the process of ion implantation. The barrier layer  11  can be an oxide layer formed by thermal oxidation, or a silicon nitride layer or a polysilicon layer formed by chemical vapor deposition. When the barrier layer  11  is an oxide layer, the thickness of the barrier layer  11  is about 1000–2000 Å, and the barrier layer  11  is substantially silicon dioxide. Then, an ion implantation process  12  is performed on the wafer  10  and the barrier layer  11  by an ion implanter (not shown), as shown in  FIG. 1(   b ). The majority of the ions are blocked by the barrier layer  11  and retained in the barrier layer  11 , and only a portion of ions can penetrate the barrier layer  11  and implant in the surface of the wafer  10  to form a conductive (e.g., electrically conductive) region  101  on the surface of the wafer  10 , as shown in  FIG. 1(   c ). Subsequently, a thermal treatment process, e.g., rapid thermal annealing (RTA), is performed on the wafer  10  and the barrier layer  11  to rearrange the silicon lattice of the wafer  10  to an acceptable degree (e.g., the rearrangement degree of silicon lattice derived from the operation with RTA parameters for production). 
   After the process of ion implantation, the property of the wafer can be directly observed by a thermal wave method, which can be performed on Therma-Probe XPR Series provided by Therma-Wave Inc. in U.S.A. (1250 Reliance Way, Fremont, Calif. 94539, U.S.A.; Tel: 510-668-2200, Fax: 510-656-3863). 
   Afterward, as shown in  FIG. 1(   d ), the barrier layer  11  is removed from the surface of the wafer  10 . If the barrier layer  11  is an oxide layer, it can be removed by wet etching with an etching solution containing hydrofluoric acid. If the barrier layer  11  is a silicon nitride layer, it can be removed by wet etching with a solution containing phosphoric acid. Finally, the physical property of the conductive region  101  of the wafer  10  can be measured to judge whether the implantation energy of the ion implanter is greatly changed or abnormal by the change of the measured physical property. 
   In some embodiments, the measured physical property can be resistance (Rs), surface property of wafer, optical property of wafer and the like. The methods and instruments for measuring these physical properties are described as follows. 
   (1) Measurement of resistance: The resistance of the conductive region  101  on the surface of the wafer  10  can be measured by a sheet resistivity meter via a four-point probe method. The measuring method is to select plural measuring points  13 , such as 49 points shown in  FIG. 2(   a ) or 81 points shown in  FIG. 2(   b ), and then measure the resistance of each measuring point  13  and calculate the average thereof. 
   (2) Measurement of surface properties of wafer: Since ion implantation has influences on the surface roughness and stress of the wafer  10 , the ion implanter can be monitored by measuring the property change of the surface roughness and stress of the wafer  10  after ion implantation. The surface roughness can be measured by scanning probe techniques such as Scanning Tunneling Microscopy (STM), Atomic Force Microscopy (AFM) and Scanning Near-Field Optical Microscopy (SNOM), which have been relatively new microscopy techniques. The feature thereof is that an extremely tiny probe or a micro-sensor capable of measuring some specific physical properties is used to scan the wafer at a very short distance from the wafer and simultaneously obtain various information from the surface of the wafer, including, for instance, surface structure, topology, electrical property, magnetic property, optical property and surface potential. Additionally, the surface stress can be measured by an analytical electron microscope (AEM) performing convergent beam electron diffraction (CBED) to carry out symmetry examination for the wafer surface and derive relative information. 
   When the barrier layer is a polysilicon layer, since the resistance of the polysilicon layer has been changed by ion implantation, the polysilicon layer need not be removed and the resistance and the optical property or other physical properties of the polysilicon layer can be measured. 
   If the barrier layer is a polysilicon layer, the property of the wafer can be observed with a thermal wave method directly after ion implantation. The thermal wave method can be performed on Therma-Probe XPR Series provided by Therma-Wave Inc. of U.S.A. (1250 Reliance Way, Fremont, Calif. 94539, U.S.A.; Tel: 510-668-2200, Fax: 510-656-3863). 
     FIGS. 3(   a ) and ( b ) show tables of resistance of the sample derived from the monitoring method according to an embodiment of the present invention, and it is performed on an ion implanter under a normal state. The resistance is measured with 49 measuring points as shown in  FIG. 2(   a ). The condition of ion implantation is to change the energy of implanted ions, as shown in the first column of the table in  FIG. 3(   a ). For example, when the condition of ion implantation is 31P/60 KeV/1.0E 14, it means that the implanted atom is phosphor with an atomic weight of 31, the energy of implantation is 60 KeV, and the dosage of implantation is 1.0E14. The second and fourth columns show the resistance and deviation derived from the wafer without a barrier layer after ion implantation. The third and fifth columns show the resistance and deviation derived from the wafer covered with a 1000 Å oxide layer. Comparing the second and third columns, when the energy of implantation is changed, the resistance variation of the wafer covered with a 1000 Å oxide layer is far greater than that of the wafer without a barrier layer. Therefore, according to the method provided by the present embodiment, whether the ion implanter is normal or not can be judged on the basis of whether the resistance of the sample deviates from the normal value when the energy of implantation is changed somewhat. 
   The normal values of other physical properties, e.g., property derived from the thermal wave method, optical property, surface property and so on, can be set in the same way. 
   Referring to  FIG. 3(   b ), the condition of ion implantation is to change the dosage of the implanted ions. The method for measuring resistance and the condition expression are identical to those of  FIG. 3(   a ). The meaning of each column is also identical to that of  FIG. 3(   a ). Comparing the second and third columns, when the dosage of implantation is changed, the resistance variation of the wafer covered with a 1000 Å oxide layer is far greater than that of the wafer without a barrier layer. Therefore, according to the method of the present embodiment, whether the ion implanter is normal or not can be judged on the basis of whether the resistance of the sample deviates from the normal value when the dosage of implantation is changed somewhat. 
   The method for monitoring an ion implanter provided by the present embodiment is to set constant parameters for the ion implanter first when it is under a normal state, and produce several samples according to the flowchart illustrated in  FIG. 1  to obtain a normal value (by the method of statistics or quality control). After the normal value is obtained, it can be used as a control standard. During regular operation, the measured value is derived from the sample produced according to the flowchart illustrated in  FIG. 1 . When the measured value deviates from the above-mentioned normal value to an unacceptable degree for quality control, the ion implanter will be judged as abnormal and can be further checked afterward. 
   When the barrier layer is a polysilicon layer and is not removed, several samples are also produced to obtain a normal value (by the method of statistics or quality control). Of course, during regular operations, the barrier layer is not removed in the method for monitoring an ion implanter. 
   When the barrier layer is a polysilicon layer, the thermal treatment process in  FIG. 1  need not be performed. 
   The thickness of the barrier layer of the present invention is not limited to the above-mentioned thickness; it depends on the ions and the energy of ion implantation, so it can be dynamically adjusted according to the energy of ion implantation needed in various semiconductor processes. Moreover, the ion to be implanted in the ion implantation process  12  can be boron, phosphorous, arsenic, antimony, or other elements of group 3 or 5. 
   In addition, the types of instruments used in each process of the present invention are not limited. In some embodiments, the ion implanter can be NH-20SR provided by Ion Equipment Co., Ltd. in Tokyo, Japan. The annealing process can be performed on AG8100, an RTA instrument provided by ALLWin21 Corp. Additionally, the deposition of the barrier layer can be performed on TEL IW-6D, a deposition instrument provided by OLM Enterprises. 
   In conclusion, the method for monitoring an ion implanter according to an embodiment of the present invention employs a barrier layer having a substantial blocking effect on ion implantation, and thus makes the wafer more sensitive to the penetration rate of implanted ions. Therefore, when the energy of ion implantation is changed, it can greatly increase the resistance variation, which is employed to judge whether the energy of ion implantation is normal or not. 
   While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.