METHOD OF MEASURING MOISTURE PERMEATION

Abstract: In a method of measuring moisture permeation through a surface protective film a specimen is prepared which comprises a substrate (1) which is impermeable to water and is transparent to infra-red rays; a water-absorbent layer (2) formed on the substrate, the water-absorbent layer being composed of a material which exhibits an infra-­ red absorption spectrum which varies depending upon the absorption of moisture; and a covering of the surface pro­ tective film (3) formed on the water-absorbent layer so that the layer is completely covered by the protective film and the substrate. The prepared specimen is then exposed to steam at a temperature of about 120-1000°C under a pres­ sure of about 2 to 20 atmospheres to accelerate the mois­ ture absorption. The steam-exposed specimen is then sub­ jected to infra-red spectrophotometry, whereby the mois­ ture permeation through the surface protective film (3) is determined. The method can be advantageously utilised for judging water resistance of semiconductor devices, each of which has a surface protective film formed thereon.

Description: MOISTURE PERMEATION MEASURING METHOD This invention relates to a method of measuring moisture permeation through a protective film. More particularly, it relates to an improved method of measuring moisture permeation through a surface protective film formed, for example, on an integrated circuit, by which method the moisture permeation can be measured within a short period of time and with a high reliability. In order to improve the reliability of an integrated circuit, it is necessary to prevent moisture absorption in active regions of the circuit. For this purpose, a waterproofing method has been widely adopted in which the surface of an integrated circuit is protected by a water-resistance protecting film. Such protecting film is required to have a good water resistance, that is, a low water permeability. A pressure cooker test method, which is usually abbreviated as "P.C.T. method", has heretofore been mainly used for testing the water permeability of various materials. In this P.C.T. method a specimen is maintained in steam at a temperature of 120 C and a pressure of 2 atmospheres to effect humidification. Therefore, this method can be regarded as one of the accelerated test methods. However, in using this method about 100 hours are necessary for attaining the desired degree of moisture absorption, and hence, a long time is required for completion of the test. Furthermore, since the degree of moisture absorption in the specimen is determined on the basis of examination of changes of the specimen by visual inspection through a microscope, it is difficult to evaluate the degree of moisture absorption precisely, and therefore, it is very difficult to recognize the difference in water permeability between specimens. It also has been proposed in the Fall Meeting of THE BLECTROCHEMICAL SOCIETY, INC., Extended Abstracts of BATTERY DIVISION, Abstract No. 268 (October, 1972), that moisture permeation through a protective film is measured by a testing method wherein a specimen is placed in a humidity furnace and maintained therein at a temperature of, for example, from 2000 to 5000C, and then the moisture-absorbed specimen is subjected to infra-red spectrophotometry. However, this proposed method also needs a substantially long period of time for attaining the desired degree of moisture absorption. It is an object of the present invention to provide an improved method for measuring moisture permeation through a surface protective film,by which method the moisture permeability of the surface protective film can be evaluated within a short period of time and with a high reliability. In accordance with the present invention, a method of measuring moisture permeation through a protective film, wherein a specimen to be tested is exposed to steam maintained at an elevated temperature and the moisture-absorbed specimen is subjected to infra-red spectrophotometry, whereby the moisture permeation through the surface protective film is determined; is characterised by the steps of (1) preparing a specimen comprising(9) a substrate whichois impermeable to water and transparent to infrared rays, (ii) a waterabsorbent layer formed on the substrate, the water-absorbent layer being composed of a material which exhibits an infra-red absorption spectrum varying depending upon the absorption of moisture, and (iii) a covering of the protective film, the moisture permeation through which is to be measured, formed on the water-absorbent layer so that the water-absorbent layer is completely covered with the protective film and with the substrate; and (2) exposing the prepared specimen to steam maintained at a temperature of from about 1200C to about 10000C and a pressure of from about 2 to about 20 atmospheres to accelerate the moisture absorption. An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig.1 is a sectional view illustrating the structure of a specimen to be tested according to the water permeation test method of the present invention; Fig.2 is a sectional view illustrating one method of carrying out the step of humidifying the specimen in the water permeation test method, and Fig.3 is a diagram illustrating changes in the infrared absorption spectrum due to the moisture absorption of phospho-silicate glass. Referring to Fig.1, a specimen 12 to be tested comprises a substrate 1, a water-absorbent layer 2 formed on the substrate 1, and a covering 3 of a protective film, the moisture permeation through which is to be measured, formed on the water-absorbent layer 2 so that the water absorbent layer 2 is completely covered by the protective film covering 3 and by the substrate 1. The substrate 1 is made of a material which is impermeable to water and transparent to infra-red rays. For example, the substrate 1 may be made of a two-surface-polished silicon plate having a specific resistivity of at least 100 ohm-cm and a thickness of about 1 mm. The water absorbent layer 2 is made of a material which exhibits an infra-red absorption spectrum which varies depending upon the absorption of moisture and which includes, for example, phospho-silicate glass (PSG) and boro-silicate glass (BSG). The thickness of the water-absorbent layer 2 may be varied, usually in the range of from about 3,000 angstroms to 3 microns. For example,the waterabsorbent layer 2 may be composed of a PSG layer having a thickness of about one micron, which has been formed on the substrate 1 by a chemical vapor deposition (CVD) method. The protective film covering 3 to be tested may be composed of, for example, silicon nitride (SiON), silicon dioxide (SiO2), silicon nitride (SiN) and thermal resistant synthetic resins, such as polyimides and organopolysiloxanes. The thickness of the protective film covering 3 may be varied, preferably in the range of from about 50 angstroms to about twenty microns. One typical example of the protective film coverings is a SiON (silicon oxynitride) film having a thickness of about 500 angstroms, which has been formed by a plasma CUD method. Referring to FIG. 2,'an optional number of specimens 12, each having the above-mentioned laminate structure, are placed within a quartz capsule 11. A heat -resistant vessel 13 charged with an appropriate amount of water 14 is also placed within the quartz capsule 11. After the specimens 12 and the water-charged vessel 13 are placed in the quartz capsule Il, one end of the quarts capsule 11 is sealed in vacuo. In FIG. 2, this sealed portion is represented by reference numeral 15. The quartz capsule 11 sealed in vacuo is placed in a high temperature oven, such as a diffusion oven 16 > to heat the specimens 12. Heating is carried out so that the quartz capsule 11 is maintained at, for example, 3200C. The amount of water in the vessel 13 is adjusted so that, at this heating step, water in the vessel 13 is evaporated and the quartz capsule 11 is filled with steam maintained, for example, at 10 atmospheres. then this humidifying method is adopted, in case of, for example, the specimen illustrated in Fig. 1, steam permeates into the water-absorbent layer 2 through the protective film 3, and the water-absorbent layer 2 can be humidified to the desired extent in about 8 hours. In general, the temperature of the specimens and the pressure thereof may be selected in the ranges of from about 1200C to about 1,0000C and from about 2 to about 20 atmospheres. If the specimens are placed in doubled quartz capsules (not shown in FIG. 2), the specimens can be maintained at higher temperatures and higher pressures. The period of time for which the specimens are exposed to steam varies depending upon the temperature and pressure of the s-eam, and the thickness of the surface protective film to be tested. It is convenient, however, that the temperature and pressure of the steam be selected so that the waterabsorbent layer 2 reaches the desired moisture absorption in a time period of from about 5 minutes and to about 10 hours. For examining the degree of moisture absorption in the humidified specimens, each specimen 12 is taken out from the quartz capsule 11, and then, subjected to infrared spectrophotometry. If an appreciable amount of water is absorbed into the water-absorbent layer 2 through the protective film 3, the P=O bond in PSG disappears, and hence, the infrared absorpotion band at 1320 cm inherent to the P=O bond is not observed. It is well known that P=O bond in PSG is caused to disappear when PSG absorbs moisture, and absorption of infrared rays is changed owing to this disappearance of the P=O bond. In order to accomplish the above-mentioned test, it is indispensable that the substrate 1 should be water-impermeable and transparent to infrared rays. The above-mentioned silicon substrate is suitable for this test because it satisfies the above requirements very well. Fig. 3 is a diagram illustrating changes of the infrared absorption spectrum owing to moisture absorption of PSG. In Fig. 3, the uppermost curve shows the state where no moisture absorption has been caused at all. An absorption band inherent to the P=O bond is clearly observed -l at 1320 cm , and the absorption band inherent to the Si-O bond appears at 1080 cm 1 This state is indicated by rating symbol "A". The second curve shows the state where slight moisture absorption has taken place. It is seen that the absorption band at 1320 cm , inherent to the P=O bond, is slightly reduced as compared with the uppermost curve. This state is indicated by rating symbol "B". The third curve shows the state where a considerable quantity of moisture has.been absorbed. In this state, the absorption band inherent to the P=O bond is observed only slightly at -l 1320 cm 1 This state is indicated by rating symbol C". The lowermost curve shows the state of complete moisture absorption, wherein the absorption band inherent to the P=O bond is not observed at all. This state is indicated by rating symbol RDn, The invention will be further illustrated by the following example. Example A phospho-silicate glass layer (i.e., a waterabsorbent layer) having a thickness of about one micron was formed on a two-surface-polished silicon plate having a thickness of about one mmwby a CUD procedure. Then, a silicon oxynitride (SiON) protective film having a thickness of about 500 microns was formed on the water-absorbent phospho-silicate glass layer by plasma CUD procedure, to prepare specimens having a laminate structure similar to that illustrated in FIG. 1. The plasma CUD was conducted at a growth temperature of 350"C, under a total pressure of one torr., with a high frequency electric power of 10 W, by using a mixed gas of SiH4 (silane), NH3 (ammonia) and N20 (nitrous oxide), diluted with Ar (argon). The composition of the SiON protective film formed was varied by changing the value of "x", which indicates the ratio of NO2/(NH3 + N2O), and the influences of the value "x" on the water permeability were examined. The obtained results are shown in Table I, below, using rating symbols given in Fig. 3. Table I Run Heating and Humidifying Conditions Value "X" (%) Tt. Temrablrei( C.) Pressure (atms.) 0 10 50 90 100 1* 320 1 A A A B B 2 120 2 A A A B B 3 320 2 A A A C C 4 320 10 A A C D D 5 320 20 A C D D D * Cariparative Example The results shown in Table I clearly demonstrate that the water permeability of the SiON protective film greatly varies depending upon the composition of the protective film. This dependency is especially conspicuous under high-temperature and high-pressure conditions (3200C., 10 and 20 atmospheres). In contrast, under conditions adopted in the conventional P.C.T. method, the difference of the water permeability is not mainfested so prominently as mentioned above. The results in Run No. 2 are similar to those in Run No. 1 (Comparative Example). It is to be noted, however, that the procedure employed in Run No. 2 is advantageous over that employed in Run No. 1 because the low temperature of 1200C has no adverse effect on semiconductor devices. In Table I, in the case of x = O, Si N (silicon 34 nitride) grows, and in case of x = 1.0, SiO2 (silicon dioxide) grows. Accordingly, the results shown in Table I teach that as the composition of the SiON protective film is brought close to Si 3N4 , the water permeability is reduced, whereas as the composition of the 5iON protective film is brought close to S02 , the water permeability is increased. As pointed out hereinbefore, according to the water permeation test method of the present invention, the water permeability of the protecting film can be evaluated definitely in a short tSime and with a high reliability, and the test procedures are very simple. The moisture permeation measuring method of the present invention can be advantageously utilized for judging water resistance of a plurality of semiconductor devices, each of which has (i) a substrate which is impermeable to water and transparent to infrared rays, such as a silicon substrate, (ii) a water-absorbent layer formed on the substrate, said water-absorbent layer being composed of a material which exhibits an infrared absorption spectrum varying depending upon the absorption of moisture, such as phospho-silicate glass, and (iii) a protective film made of, for example, a polyimide or silicon oxynitride (SiON), said protective film being formed on the water-absorbent layer so that the water-absorbent layer is completely covered with the protective film and with the substrate. The judgment of water resistance of a plurality of such semiconductor devices is effected by the steps of: (1) first, one or more samples to be tested are taken from the semiconductor devices; (2) second, the sample or samples are exposed to steam maintained at a temperature of from about 1200C to about 10000C and a pressure of from about 2 to about 20 atmospheres to accelerate the moisture absorption, and then, (3) the steam-exposed sample or samples are subjected to infrared spectrophotometry, whereby the moisture permeation through the respective protective film or films is determined according to the infrared absorption spectrum.

Claims: CLAIMS 1. A method of measuring moisture permeation through a protective film, wherein a specimen to be tested is exposed to steam maintained at an elevated temperature and the moisture-absorbed specimen is subjected to infra-red spectrophotometry, whereby the moisture permeation through the surface protective film is determined, characterised by the steps of (1) preparing a specimen comprising (i) a substrate (1) which is impermeable to water and transparent to infra-red rays, (ii) a water-absorbent layer (2) formed on the substrate, the water-absorbent layer being composed of a material which exhibits an infra-red absorption spectrum varying depending upon the absorption of moisture, and (iii) a covering of the protective film (3), the moisture permeation through which is to be measured, formed on the water-absorbent layer so that the water-absorbent layer is completely covered with the protective film and with the substrate; and (2) exposing the prepared specimen to steam maintained atria temperature of from about 1200C to about 1000 C and a pressure of from about 2 to about 20 atmospheres to accelerate the moisture absorption. 2. A method according to Claim 1, characterised in that the water-absorbent layer (2) is composed of phospho-silicate glass. 3. A method according to Claim 1 or Claim 2, characterised in that the water-absorbent layer (2) has a thickness of from about 3,000 angstroms to about 3 microns. 4. A method according to any preceding claim, characterised in that the substrate (1) is composed of silicon, 5. A method according to any preceding claim, characterised in that the protective film (3) is composed of at least one substance selected from the group comprising silicon oxynitride, silicon nitride, silicon dioxide, a polyimide and an organo-polysiloxane. 6. A method according to any preceding claim, characterised in that the protective film (3) has a thickness of from about 50 angstroms to about 20 microns. 7. A method of judging water resistance of a plurality of semiconductor devices, each of which has (i) a substrate which is impermeable to water and is transparent to infra-red rays, (ii) a water-absorbent layer formed on the substrate, the water-absorbent layer being composed of a material which exhibits an infra-red absorption spectrum which varies depending upon the absorption of moisture, and (iii) a protective film formed on the water-absorbent layer so that the waterabsorbent layer is completely covered with the protective film and with the substrate, which method is characterised by the steps of: : taking one or more samples from the semiconductor devices; exposing the sample or samples to steam maintained at a temperature of from about 1200C to about 100000 and a pressure of from about 2 to about 20 atmospheres to accelerate the moisture absorption; and subjecting the steam-exposed sample or samples to infra-red spectrophotometry, whereby the moisture permeation through the respective protective film or films is determined according to the infra-red absorption spectrum0

Inventors: MAEDA, MAMORU, TAKAGI, MIKIO, TAKASAKI, KANETAKE
Applicants: FUJITSU LIMITED
Application year: 1980
Main classification: G01N15
IPC codes: G01N15, G01N21