METHOD TO DETECT METAL IMPURITIES IN THE SEMICONDUCTOR PROCESS GASES

Abstract: A process for detection of metal impurities in a semiconductor process gas; wherein the leak of the sample gas into the clean room is miminised by sampling the gas near atmospheric pressure, redundant valves between the sampling device and the sampling line and the installation of a booth whose air is exhausted continuously outside the clean room. Preferably, polytetrafluoroethylene components in the sampling line and sampling device to minimise metallic contamination from the sampling device to the extent that no metal can be detected by ICPAES (Inductively Coupled Plasma Atomic Emission Spectrometry), ICPMS (Inductively Coupled Plasma Mass Spectromety), or GFAAS (Graphite Furnace Atomic Absorption Spectrometry).

Description: METHOD TO DETECT METAL IMPURITIES IN THE SEMICONDUCTOR PROCESS GASES BACKGROUND OF THE INVENTION : The invention relates to detection of metal impurities in the semiconductor process gases and more particularly immediately at the inlet/outlet of the semiconductor process machine. DESCRIPTION OF THE PRIOR ART : Trace metal contamination in the Ultra Large Scale Integration manufacturing process ULSI, causes poor electrical property in the devices. Electrical properties of the integrated circuit can be altered by the presence of metal impurities in the process gases used during the manufacturing process of the integrated circuits, even with impurity level as low as 10 ppb or even 1 ppb. Therefore, it is an aim of the present invention to control the metal impurity levels in the process gases below those levels. As the integration of the semiconductor devices are advanced, the requirement for the metal contamination control will become even stricter. Various methods are known to detect metal impurities in the process gases. A first method known as the bubbling method consists of bubbling the process gases through pure water or a dilute acid solution to trap metal impurities in it. The water or the acid solution is then analysed by ICPAES (Inductively Coupled Plasma Atomic Emission Spectrometry), ICPMS (Inductively Coupled Plasma Mass Spectrometry), and/or GFAAS (Graphite Furnace Atomic Absorption Spectrometry) after the appropriate solution preparation, knows by the man skilled in the art. A second method known as the hydrolisis method consists of absorbing the process gas to be analysed into pure water together with metal impurities in the gas and to analyse the resultant solution by ICPAES (Inductively Coupled Plasma Atomic Emission Spectrometry), ICPMS (Inductively Coupled Plasma Mass Spectrometry), and/or GFAAS (Graphite Furnace Atomic Absorption Spectrometry) after appropriate solution preparation, well known by the man skilled in the art. A third method known as the filtration method consists of passing the gas through a filter to trap metal impurities in the gas, then washing out the trapped metals into a dilute acid solution, and analysing the solution by ICPAES (Inductively Coupled Plasma Atomic Emission Spectrometry), ICPMS (Inductively Coupled Plasma Mass Spectrometry), and/or GFAAS (Graphite Furnace Atomic Absorption Spectrometry) after appropriate solution preparation. These various methods are disclosed for example in Proceedings of Technical Conference in Semicon Europe 1992, « Metals and dopants analysis in electronic specialty gases » by Martine Carre, which is incorporated herein as a reference. Semiconductor manufacturing, such as etching, is made by using pure or diluted gases. These gases are usually provided to the semiconductor manufacturer in cylinders which are installed usually in gas cabinets at a remote location from the reactor where semiconductors are manufactured and gas is provided therein through pipings. Most of the semiconductor process gases are either reactive or corrosive and metal contamination can be introduced not only from the source gas in the cylinder but also from various components in the gas delivery system. Therefore, it is desirable that the metal impurities can be monitored and controlled at the immediate inlet of the process machine or reactor (the point of use) rather than at the exit of the gas cylinder in order to know exactly which quantity of the metallic impurities is introduced in the process machine. Semiconductor process machines are generally placed in a clean room where workers are present. In a monitoring/controlling process at the point of use, as new devices are introduced in the gas delivery pipings, there is an increased risk of leaks and the safety of the workers in the clean room not including the damage to various process, equipment there, can be very serious. On the other hand, since the required control level of the metal impurities in the process gases are at least in the order of a few ppb or a few tens of ppb, it is desired to avoid metallic materials in the sampling device as much as possible which may in turn, increase the level of metallic impurities. The problem, if using non metallic sampling device, however, is that it is difficult to obtain sufficient pressure resistance and leak tightness with non-metallic materials. It is therefore an aim of the present invention to detect the metal impurities in a process gas at the point of use substantially at the inlet of the process machine without additional contamination from the metal impurities sampling device and with sufficient safety. SUMMARY OF THE INVENTION : According to a first aspect of the invention, the gas to be analysed is sampled at pressure near the atmospheric pressure so as to allow the use of non-metallic materials such as polytetrafluoroethylene material or any other appropriate polymer material in the sampling device with no metallic contamination from the sampling device and with sufficient safety. According to another aspect of the invention, redundant valves in the inlet and outlet of the sampling device are provided to minimise the potential gas leakage; According to another aspect of the invention, a plastic booth is provided to cover the sampling device whose air is extracted so as not to contaminate the clean room air in case of gas leakage from the sampling device. BRIEF DESCRIPTION OF THE DRAWINGS : Fig. 1 is an example of a sampling device according to the invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION On fig. 1, the various numerals have the following meaning : 1 1 : connection to points to sample the gas near the process machine, 12: orifice or needle valve (if there is no flow controller in the process machine side), 13, 14, 15, 16, 18, 19, 20, 21 : valves 17: bubbler or impinger, 22: connection to the abatement system, 23 : a plastic booth, 24: exhaust out of the clean room. A bubbler, an impinger or a filter is alternatively represented by 17 on fig. 1 according to the selected sampling method. The sampling device of fig. 1 is connected to the semiconductor process machine at point 11 by an appropriate fitting. The gas flow rate can be controlled by the process machine if it has a controlling device, otherwise an orifice (12) should be installed to control it. The flow rate is determined by considering the typical operational flow rate which is usually between 100 and 1000 seem, i.e. between 13 and 21. Before actually sampling the gas, the gas is first circulated (and thereafter discarded) through 11-12-13-16-20-21-22 during at least 5 minutes, and preferably about 60 minutes, to minimise the effect on the sampling results of the contamination due to the connection of the sampling device. During this first step of the process, valves 14 and 18 are closed and valve 16 is open. In a second step of the process, the gas is sampled by closing valve 16 and opening valves 14 and 18. The total sampling volume must be determined by taking into account the basic physical and chemical property of the gas. The maximum sampling volume must be determined by solubility of gases into water and the volume of the water in the impinger or bubbler. If the solubility of the metallic compounds which are expected to exist in the sample gas, is small in the acqueous diluted acid solution in the impinger or bubbler, the sample gas volume must also be determined by the expected contaminant concentration in the sample gas and the volume of the water or diluted acid solution in the impinger or bubbler. The sampling method must be selected among bubbling method, hydrolysis method, and filtration method according to the physical/chemical property of the gas. Bubbling method is applied to gases which shows small solubility into water or diluted acid solution although the metallic contaminant in those gases exist as chemical form which can dissolve into water or diluted acid solution. Hydrolysis method is applied to gases which are able to dissolve into water. When the sampling is completed, valves 15, 19, 14 and 18 are closed and valve 16 is opened. In order to remove the sample gas out of the gas line, said line is purged with filtered N2 gas having a metal impurities content less than about 10 ppt (by weight) and preferably less than 1 ppt(by weight). Then the valve 16 is closed and valves 14, 18, 15, 19 are opened to purge the remaining sample gas from the sampling device with the same filtered nitrogen gas. It is preferred to close 15, 19, 14 18 and open 16 in this order. Thereafter, the sampling device is disconnected at point 25 between valves 14 and 15 and that 26 between valves 18 and 19. By doing this post-purging procedure, the contamination of the clean room by the sample gas is minimised. Valves 14, 16 and 18 can be replaced with a 6-port rotary valve for the same function. Valve switching procedure must be done quickly so as not to minimise pressure build up in the sampling device and to avoid this pressure to become significantly higher than the atmospheric pressure. This invention can be used for any gas except gases which react with polymers such as polytetrafluoroethylene which is used in the sampling device of the invention. For example, F2 and C1F3 which react with polytetrafluoroethylene should not be sampled using the invention. DETAILED EMBODIMENT OF THE INVENTION : Example 1 : Metals in N2 gas N2 gas is sampled near the process machine which utilises this N2 gas. The sampling flow rate is 250 seem which is controlled by a mass flow controller built in the machine. The exhaust gas is abated with an abatement system installed with the machine. The sampling is done by bubbling, and the resultant solution is analysed by ICPMS (Inductively Coupled Plasma Mass Spectrometry) and GFAAS (Graphite Furnace Atomic Absorption Spectrometry). The results are shown in Table 1. The sampling was done using the sampling kit illustrated on fig. 1. A 125 ml impinger filled with 100 ml of 1% HN03 in DI water was set at the point of 17. Valves 14, 15, 19, 18 were closed and 13, 16, 20 and 21 were opened, and the purge of the sample gas N2 were done for 5 minutes. Valves 15 and 19 were opened, and then valve 16 was opened, immediately 14 and 18 were closed. The N2 gas was started to bubble through the DI water in the impinger 17. After the bubbling for 30 minutes, valves 15 and 19 were closed, 14 and 18 were closed, and then immediately valve 16 was opened. In case of N2, there is no risk of leakage of small gas volume remained in the lines between valve 14 and 15, 18 and 19. Therefore no post-purge was applied. At the points of 25 and 26, the impinger was removed from the sampling kit. The solution in the impinger was analyzed by ICPMS (Inductively Coupled Plasma Mass Spectrometry) and GFAAS (Graphite Furnace Atomic Absorption Spectrometry). In all of the following Tables, units for measured values are "wt.ppb". Table 1 : Metals detected in N2 gas coming into the process machine. The results are all less than the detection limits except Ni, Pb and Zn. From these results, it can be seen that the N2 gas used in the semiconductor process machine has an excellent purity and that the nitrogen gas delivery system and the sampling device are all very clean. Example 2: Metals in HCI Gas HCI gas is sampled near the semiconductor process machine after the sampling of N2 in example 1. The sample gas flow rate is 250 seem which is controlled by a mass flow controller built in the process machine. The exhaust gas is abated with an abatement system installed with the process machine. Sampling is done by hydrolysis, and the resultant solution is analysed by ICPMS (Inductively Coupled Plasma Mass Spectrometry) and GFAAS (Graphite Furnace Atomic Absorption Spectrometry). The results are summarised in Table 2. Table 2: Metals detected in the HCI gas coming into the process machine Al, K, Ni and Pb are detected from HCI gas used in this particular process machine. Since the cleanliness of the sampling device is proved by the previous N2 analysis, all the detected elements are carried to the process machine by the Hcl gas. Example 3: SiH4 Filtration SiH4 gas is sampled near the process machine which utilises SiH4 gas. The sampling flow rate is between 500 - 3000 seem which is controlled by a mass flow controller built in the machine. The exhaust gas is abated with an abatement system installed with the machine. Sampling is done by filtration, and the trapped metals are leached into a dilute solution. The resultant solution is analysed by ICPMS (Inductively Coupled Plasma Atomic Emission Spectrometry) and GFAAS (Graphite Furnace Atomic Absorption Spectrometry). The results are shown in Table 3. The result indicates that the sampled SiH4 gas is very clean. The above examples show that the present method is very sensitive to detect metals in gases.

Claims: CLAIMS 1. A process for detecting metal impurities in a gas or gas mixture used as a process gas in a machine having an inlet and an outlet for said gas or gas mixture comprising the steps of : (i) removing said gas or gas mixture, through a non-metallic piping, from a point close to at least one of the inlet and outlet of said machine; (ii) directing said gas or gas mixture through said non-metallic piping to a sampling device for further analysis and detection of metal impurities, said sampling device being enclosed in a gaseous atmosphere which is isolated from said machine and continuously circulated; and (iii) continuously partially removing an excess of said gaseous atmosphere enclosing said sampling device, said removed atmosphere being exhausted in isolation from said machine. 2. The process of claim 1, wherein said machine is a semiconductor manufacturing process machine. 3. The process of claim 2, wherein said machine is located in a clean room. 4. The process of claim 3, wherein the gaseous atmosphere removed in step (iii) is removed outside said clean room. 5. The process according to one of claims 1 to 4 wherein said sampling device is surrounded by a booth which encloses said gaseous atmosphere. 6. The process according to one of claims 1 to 5, wherein said non-metallic piping comprises polytetrafluoroethylene. 7. The process according to one of claims 1 to 6, wherein said sampling device comprises polytetrafluoroethylene. 8. The process according to one of claims 1 to 7, wherein said gas or gas mixture directed to said sampling device is at about atmospheric pressure. 9. The process according to one of claims 1 to 8 further comprising the steps of : first circulating said gas or gas mixture into said piping an sampling device; and then discarding said first gas or gas mixture for at least five minutes before actually analysing said gas or gas mixture. 10. The process according to one of claims 1 to 9, further comprising the step of purging said sampling device with filtered nitrogen having a content of metal impurities of less than about 10 ppt.

Inventors: SUZUKI ITSUKO, TARUTANI KOHEI, SUZUKI, ITSUKO, TARUTANI, KOHEI
Applicants: AIR LIQUIDE, SUZUKI ITSUKO, TARUTANI KOHEI, L'AIR LIQUIDE, SOCIETE ANONYME POUR L'ETUDE ET L'EXPLOITATION DES PROCEDES GEORGES CLAUDE, SUZUKI, ITSUKO, TARUTANI, KOHEI
Application year: 1998
Main classification: G01N33
IPC codes: G01N1, G01N33