METHOD FOR HALIDE ETCHING IN THE PRESENCE OF WATER OF SEMI-CONDUCTOR SUBSTRATES

Abstract: A process for halide etching of a semi-conductor substrate in the presence of water. Etching is realized in a reaction vessel. The process steps comprise filling of the reaction vessel with a first gas to a first pressure and subsequently filling the reaction vessel with a second gas to a second pressure after which the substrate is left alone in the reaction vessel during several minutes to obtain the etching required.

Description: Method for halide etching in the presence of water of semi-conductor substrates This invention relates to the process of removing and vapori- i 5 zing small layers from semi-conductor substrate materials with halide gases in the presence of water. More particular it relates < to removing and vaporizing native oxides in contact holes etc., in order to prepare a wafer for a subsequent step, specifically to try to restore the interface to a virginal state so as to be able 10 to e.g. grow epitaxial silicon, grow selective W, selective metal suicides etc., or to realize very thin, very uniform and very reliable oxide layers to be used for gates, dielectrics etc. For all of this the interface has in any case to be free from organic or metal debris. In the art this is done in a wet etch. However, a 15 wet etch is of limited use if the feature size is such that the liquid surface tension prevents the reactant to reach the very bottom of e.g. the contact hole. Because of that a wet etch is incapable of completely removing the oxide built-up (typically 10 to 20 A thick) in deep contact holes. 20 Because of this in the prior art, e.g. US-A-4,749,4',0 it is proposed to use a gaseous etch to remove this so called native oxide in order to clean the interface. This gaseous etch does not have surface tension. In the method according to the above US specification halide gas does enter the reaction chamber at near 25 atmospheric pressure in the presence of water. The flow of the halide is controlled by a mass flow controller and with a vent continuously open to atmosphere or throttled to increase the pres¬ sure. The etch time is typically between and 30 seconds. The use of a mass flow controller in a dynamic process gives 30 rise to several problems. First of all the mass flow controller has to be constructed from a material resistant to the reactive gases used. Furthermore it has been found that it is extremely difficult to exactly control the rate of etching with a fluctu- ating flow of etchant gas and therefore at fluctuating partial 35 pressures. The nature of the application in the prior art requires a process on a single wafer to be finished within - 30 s. i.e. etching of hundreds to thousands of A at relatively high etch rate. It has been established that actual etching may suffer from a delay in time ranging from 1 second up to 5 minutes. This is a phenomenon not understood. Therefore in ~ 30 s. one can either have zero etching or many thousands of A removed either locally or all over the wafer. Of course this is unacceptable. This is particular true when dealing with patterned wafers. It should not be that when the removal of 30 A of a native oxide is required, intentially deposit¬ ed oxide layers are also for a substantially part removed because of an appreciable overetch. In US-A-4,229,247 etching of different metals is described being an Al/Cu alloy and chromium. For each of said metals a gas mixture is necessary: CCl^ and Ar for Al/Cu and CCl^/Al/I^O for chromium. Because in this US specification both metals are stacked in layers above each other, these layers have to be etched after each other: first Al/Cu and after that chromium must be etched. Because the gas mixtures neces¬ sary for etching, only differ in that for etching of chromium water is necessary as further admixture, switching from etching of Al/Cu (with CCl^/Ar) to chromium can be easily effected by adding water to the present CCl^/Ar gas mixture. It is not possible with this method to exactly monitor the etching process described above. The invention aims to obviate these drawbacks. According to the invention this is realized with a method for chemical etching with a halide in the presence of water of a silicon oxide substrate placed in an etching chamber, at sub at¬ mospheric pressure comprising the introducing of a first gas, until a first pressure is reached and subsequently introducing of a second reactive gas until a second higher pressure is reached and wherein only after introducing of the second gas etching is initia¬ ted. In contrast to the prior this is a static method. In contrast to the prior art the quantity of reactive gas and the mixing ratio with the other components present in the reaction chamber is not longer measured with a mass flow device, but is related to the pressure. This means that a relatively simple pres¬ sure measuring device can be used giving a considerable improved accuracy over the method according to the prior art. After the gases are introduced in the reaction chamber the substrate is left alone so that the reaction can take place. During the reaction the concentration of reactive gas will decrease such that the start and end of the reaction can be exactly determined. In contrast to the not very reliable mass flow controllers for halides, such as HF, where the output may vary between 20 to 50# over e.g. 100 runs, measuring the pressure can always be done exactly. It has to be remarked that the etching rate is dependent on the fourth power of the HF concentration. To further controll the reaction rate according to the inven¬ tion this rate is considerably decreased. The invention aims to realize an etching time in the order of minutes and not of seconds, such as with the prior art. This is realized in that the pressure within the reaction chamber is a subatmospheric pressure or prefe- rable lower than 50 Torr. Under these circumstances etching between 1 and 10 minutes becomes possible. In that circumstance it is of course preferable to work batch-like. In contrast to the prior art the method according to the invention can be used with batch-wise construction of substrates. At high pressure and short reaction times it is hardly possible to obtain uniform etching over the several substrates in a reaction vessel. Almost certainly problems with diffusion of the reaction gas which must be equal at all points of all substrates, will occur. In the method according to the invention first of all time is gained to more uniformly mingle the reactive gases. Furthermore by having a lower pressure the diffusion constants will increase as they are inversely proportional to the pressure. Because of this only now it has be found possible to etch batch-like. Preferably etching is done at room temperature and if the wafer temperature increases the etch rate does decrease. This rate of varying is quite considerable at room temperature. This means that an increase or decrease of 5 * C has a considerable effect on the etching rate. However, means to exactly control the temperature of the wafer are complicated. According to the invention wherein the etch time is increased relative to the prior art the process is controlled in that the temperature is measured and referring to these values the etch time or the HF/H 2 0 ratio is determined. E.g. this can be done with a processor in which the known exact temperature and HF/H2O ratio dependent etch rate are stored. In this way the effect of changing ambient temperatures can be taken into account to changing the etch time or partial pressure of the halide and water rather than trying to force the substrate to be a certain fixed temperature. Most of the reactions proposed in the reaction chamber will give a change in pressure. If this variation in pressure is measured with the pressure sensors being provided, problems described above with regard to inhibition time can be obviated because a clear indica¬ tion is found of starting, course and completion of the reaction. The invention will be further elucidated referring to the following example: Example: The method according to the subject invention was realized in a reaction chamber equipped with three inlets: one for anhydrous HF, one for water vapor and one for a dry N 2 purge. The inlet lines were combined at a distance of about 0 cm above the reactor. Subsequently the reactor chamber was loaded with a batch of wafers, comprising 1-100 silsicon semiconductor wafers, being kept at ambient temperature. After this the reaction chamber loaded with wafers was evacuated and filled with the reactant gases. Monitoring the pressure during a 20 - 50 A/min etch process, it potentially makes possible to accurately etch away e.g. no more than 30 A by pumping off the reactant gases after a certain pressu¬ re change. The reactor and associated equipment according to the invention was made from materials chemically resistant to the reactive HF-H2O mixture A proper choice is of great importance as the entire purpose of the etch is defeated if the removal of the native oxide would lead to an excessive generation of particles from either the reactor walls, the wafer support structure, the gas feed lines, etc. After evacuating of the reaction chamber the valve to the pump is closed. After this first of all the inlet of water vapor is opened and water vapor is injected into the reactor up to a certain pressure. The water vapor inlet is closed and the wafers are left to reside in the water vapour ambient till the next step. After this the inlet of anhydrous HF vapor is opened and anhydrous HF is supplied in the reactor. The amount of anhydrous HF supplied into the reactor is measured as to lead to the required HF/H 2 0 ratio. By measuring the partial pressures the quantity of HF intro¬ duced can be measured exactly. The wafers are subsequently kept isolated for a time being the sum of the inhibition time plus the actual etch step time, the latter being typically between 2 and 5 minutes. The temperature of the wafers is fed in a microprocessor in which a map is stored comprising the dependencies of the etching process on the temperature, pressure, HF/H 2 o ratio etc. With this microprocessor the actual etching time or HF/H2O ratio is determin¬ ed required to give the desired etch. After said time interval has elapsed the pump valve is opened and the process gases are pumped away. By monitoring the pressure drop the start of the reaction can be determined and the course of the reaction can be followed. Although the invention is described referring to a preferred embodiment it has to be understood that the invention is not limit¬ ed to this application. E.g. any other gas then HF might be used.

Claims: C L A I M S 1. Method for chemical etching with a halide in the presence of water of a silicon oxide substrate placed in an etching chamber, at sub atmospheric pressure comprising the introducing of a first gas, until a first pressure is reached and subsequently introducing of a second reactive gas until a second higher pressure is reached and wherein only after introducing of the second gas etching is initiated. 2. Method according to claim 1, wherein the method is conducted at a pressure lower than 0 Torr. 3- Method according to claim 2, wherein the halide comprises HF gas. 4. Method according to claim 1, wherein the method is conducted at near room temperature. 5- Method according to claim 1, wherein the substrate is exposed to both gases between 1 and 10 minutes. 6. Method according to claim 5» wherein the time of exposure to both gases is determined depending from a temperatuee measured during the reaction. 7- Method according to claim 1 wherein in the etching chamber the pressure is measured during the etching time.

Inventors: PIEKAAR HANS WILLEM, WESTENDORP JOHANNES FRANCISCUS, PIEKAAR, HANS, WILLEM, WESTENDORP, JOHANNES, FRANCISCUS, MARIA
Applicants: ASM INT, ASM INTERNATIONAL N.V.
Application year: 1990
Main classification: H01L21
IPC codes: H01L21