Method for detecting removal of organic material from a semiconductor device in a manufacturing process

The present invention relates to a method for detecting removal of organic material from a semiconductor device in a manufacturing process inserting at least one semiconductor device with deposited organic material into a fluid for removing the organic material from the semiconductor device.

In the technical field of manufacturing of semiconductor devices, the semiconductor devices usually are formed like disks or wafers for producing e.g. semiconductor chips. In the process of manufacturing of e.g. a semiconductor wafer, it is common and necessary to apply various process steps to the wafer. Especially lithographic processes are applied frequently in the process. Before applying a lithographic process, the wafer usually is deposited with at least one material layer such as organic photoresist material. Structures over the front side of the wafer can be performed. The structured photoresist layer then usually serves as a mask for further etching processes.

After a final etching process, it usually becomes necessary to remove the remaining portions of the photoresist layer before a next process or process step is applied. For removing organic material such as photoresist material from a semiconductor wafer it is common to use fluid acids with ingredients such as sulphuric acid and hydrogen peroxide. To this end, the fluid acid is enclosed in a reactor or a fluid bath container in which at least one semiconductor wafer with a deposited layer of photoresist material is to be inserted. When removing the organic material carbon components of the photoresist material are oxidized and formed to reaction products such as carbon dioxide, the hydrogen components are formed to water. The fluid acid (so-called piranha bath) usually is heated up to e.g. 130 °C.

In such a process one often has to deal with the problem that photoresist residues are left on the wafer after process end. Also, some implanted kinds of resist are difficult to strip. Good tuning of the process is necessary. Under microscope it is very difficult to detect residuals of photoresist. Wafers having photoresist residues left on the wafer surface usually do not fulfil the strong requirements of modern semiconductor manufacturing. Photoresist residues can have negative influence with respect to the following process or process steps.

In view of the prior art, it is an object of the present invention to provide a method for removing an organic material from a semiconductor device of the above mentioned type which is suitable for detecting incomplete removal of organic material, i.e. photoresist deposited on the processed semiconductor device.

The object is solved by a method for removing an organic material from a semiconductor device according to claim 1.

The method is applicable to various semiconductor devices with deposited organic material such as wafers for manufacturing of semiconductor chips. For example, the semiconductor device comprises a deposited layer of photoresist material. Moreover, various types of fluids can be used which are suitable for removing the organic material from the semiconductor device. Especially, a fluid is used having ingredients for removing photoresist material. Preferably, sulphuric acid and at least one of hydrogen peroxide or ozone are inserted as fluid ingredients for removing photoresist material.

According to the invention, a clear identification even of small process problems which can lead to resist residues on a semiconductor device is made possible. The semiconductor device with deposited organic material is inserted into the fluid for removing the organic material from the semiconductor device. Then, measurement data are produced when measuring a concentration of at least one reaction product formed by the reaction of the organic material and the fluid. The measurement data are then processed to get a data curve.

In case of resist residues a concentrated dark piranha/resist mixture is dropping back into the fluid of the bath during wafer removal from the bath after process end. This results in a clear turning point of the curve, a local maximum point of the curve and a local minimum point of the curve each being significantly different from signal noise, since the concentration of the measured reaction product changes quite rapidly. So, according to the invention, with querying for at least one of such a turning point, a local maximum point or a local minimum point of the curve after process end an incomplete removal of organic material, i.e. photoresist deposited on the processed semiconductor device can be detected. To this end, only easy mathematical calculations are necessary. This information then can serve as a basis for deciding on further processing of the semiconductor device dependent on the query result.

Preferably, the concentration of the at least one reaction product is measured with an optical sensor system. To this end, the system measures the transparency of the process fluid. The optical sensor system emits optical radiation and receives emitted optical radiation. Thereby, the optical radiation is emitted towards the fluid. With the receiving of the optical radiation which is transmitted through the fluid the transparency of the process fluid can be detected. The process fluid is not clear if it contains unoxidized portions of the removed organic material. The method can be fully automated. The signal produced by the optical sensor system is independent on normal pollution effects since the signals influenced by such effects are usually not in the course of the data curve. Therefore, this effects can easily be filtered.

The insertion of at least one of the ingredients such as hydrogen peroxide can be controlled due to the sensor signal. The controlling of the insertion can be performed in a manner that the consumption of the hydrogen peroxide in relation to the process time is optimized.

If the received optical radiation intensity is filtered with respect to minima signal peaks an influence of bubbles induced by the process can be overcome. The bubble-induced minima peaks of the sensor signal resulting from scattered light are then filtered.

Further advantageous features, aspects and details of the invention are evident from the dependent claims.

The invention will be better understood by reference to the following description of embodiments of the invention, taken in conjunction with the accompanying drawings, wherein

figure 1

shows an embodiment of a processing arrangement;

figure 2

shows a more detailed view of an optical sensor system;

figure 3

shows a cross-sectional view of a wafer;

figures 4 and 5

show diagrams of a measured sensor signal in relationship to the process time.

Figure 1 illustrates an embodiment of a processing arrangement. The arrangement 1 comprises a reactor 2 for enclosing semiconductor devices 20 such as wafers. For example, the reactor 2 is suitable for enclosing a number of up to 50 wafers piled up in a suitable clamping device. The reactor 2 can be formed as a bath or as a closed reactor with pipes connected to it. The wafers 20 have at least one deposited layer of photoresist material which is to be removed. In the reactor 2 also a fluid 40 having ingredients 41 and 42 for removing the photoresist material from the wafers 20 is enclosed. This type of process fluid is also known as so-called piranha bath.

Further, the arrangement 1 comprises a fluid pipe system 9 for inserting the fluid 40 into the reactor 2 and for removing the fluid 40 from the reactor 2. The fluid pipe system 9 comprises a pump 91 for performing a fluid stream 10 through the fluid pipe system 9. Further, the fluid pipe system 9 comprises a filter 92 for extracting particles out of the fluid 40 and a degasifier 93 for removing bubbles from the fluid 40, each of them placed at exemplified positions. The fluid pipe system 9 then comprises a heater 94 for heating the fluid 40 up to e.g. 130 °C. An optical sensor system 3 is integrated into the fluid pipe system 9.

The fluid 40 comprises sulphuric acid 41 (H2SO4) and hydrogen peroxide 42 (H2O2) as fluid ingredients. Additionally, ozone (03) can be inserted as further ingredient of the fluid 40. With a spiking pump 6 the insertion of hydrogen peroxide 42 into the fluid pipe system 9 is controlled. The spiking pump 6 is connected to a controlling circuit 4 comprising a data processing unit 5. The spiking pump 6 is controlled by the controlling circuit 4. The data processing unit 5 and respective controlling circuit 4 are connected to the optical sensor system 3 by means of an optical cable 8. The controlling circuit 4 further comprises a radiation source 7 for emitting optical radiation which is transferred via the optical cable 8 to the optical sensor system 3 to detect the transparency of the fluid. For example, the radiation source 7 comprises a laser device or a light emitting diode for emitting optical radiation with a single wavelength. When insufficient processing is indicated with the signal of system 3, the process may automatically be repeated.

Figure 2 shows a more detailed view of the optical sensor system 3. The pipe of the fluid pipe system 9 containing the fluid 40 is arranged in the center of the optical sensor system 3. The optical sensor system 3 comprises a sender 31 for emitting optical radiation 32 towards the fluid 40. The optical radiation 32 is provided by an optical fiber 81 which is part of the optical cable 8 according to figure 1. The radiation 32 which is to be emitted is transmitted from the radiation source 7 via the optical fiber 81. The optical sensor system 3 further comprises a receiver 33 for receiving of emitted optical radiation. Especially, the receiver 33 is designed for receiving of the optical radiation 34 which is transmitted through the fluid 40. The received optical radiation 34 is transmitted to the controlling circuit 4 via the optical fiber 82 as part of the optical cable 8.

Figure 3 schematically shows a cross-sectional view of one of the wafers 20 enclosed in the reactor 2 according to figure 1. The wafer 20 is deposited with layers 21 and 22. Layer 22 is formed of photoresist material irradiated in a lithographic process. The photoresist layer 22 forms a mask for etching the layer 21. After the etching process, trenches are formed as shown in figure 3. In a following process, the remaining portions of the photoresist layer 22 have to be removed by inserting the wafer 20 into the reactor 2 according to figure 1.

With the optical sensor system 3 the transmission of light (preferably wavelength of about 550 nm) through the fluid 40 is measured. When removing the photoresist material 22 from the wafer 20 the fluid 40 changes its transparency (visible as a colour change). When the removed photoresist material is fully oxidized, the fluid 40 is nearly as clear as at the beginning of the process. The described change of transparency can be measured by means of the optical sensor system 3. If minima signal peaks of the received optical radiation intensity are filtered the result of the measurement is not influenced by induced bubbles which lead to scattering of the transmitted radiation.

Figure 4 shows a diagram of a measured sensor signal I in relationship to the process time t. The sensor signal I is derived from the detected optical radiation intensity measured by the optical sensor system 3. In this example, the optical radiation intensity is detected and transferred into a voltage signal. The data curve and respective signals are measured and processed with a very high signal-to-noise-ratio. The diagram schematically shows an example of a process for removing photoresist materials deposited on a wafer 20. Preferably, the data curve is smoothed when processing the measurement data.

The process begins with the time t0. The large decrease of the curve K1 at the beginning of the process denotes the extent of the dissolution of the photoresist material to a high number of reaction products. Since a relative high amount of unoxidized portions in the stripped-off photoresist material occurs at the beginning of the process, the absorption of the transmitted light through the fluid 40 is quite large. Therefore, the transparency is quite low, the signal I decreases. With the inserting of hydrogen peroxide 42 into the fluid 40 the process of oxidation of the photoresist material is established. This is illustrated with the increase of the curve K1. The time t1 denotes the end of the process.

Figure 5 shows a diagram of the measured sensor signal I in relationship to the process time t in case the photoresist hasn't been removed completely until process end. Again, the process begins with the time t0. The large decrease of the curve K2 at the beginning of the process denotes the extent of the dissolution of the photoresist material to a high number of reaction products. During wafer removal from the bath after process end (time t1) a concentrated dark piranha/resist mixture resulting from resist residues is dropping back into the fluid of the bath. This results in a clear turning point TP1 of the curve, a local maximum point MAX of the curve and a local minimum point MIN of the curve each being significantly different from signal noise. The concentration of the reaction product and, consequently, the colour of the bath changes quite rapidly when removing the wafer having unoxidized portions of photoresist.

According to the invention, with querying for at least one of a turning point TP1, a local maximum point MAX or a local minimum point MIN of the curve K2 after process end an incomplete removal of the photoresist deposited on the processed wafer can be detected. This information serves as a basis for deciding on further processing of the wafer dependent on the query result. In case at least one of the turning point TP1 of the curve, the local maximum point MAX or the local minimum point MIN of the curve K2 is detected the wafer is inserted back into the bath for continued processing. Alternatively, the wafer is marked as faulty processed and sorted out of the manufacturing process.

In another embodiment of the invention a query for at least two turning points TP1 and TP2 of the curve K2 after removing the wafer from the fluid is established. This information can serve as a precise basis for distinguishing the curve K2 (wafer having deposited resist residues) from curve K1 shown in figure 4.

List of reference signs

1

arrangement

2

reactor

3

optical sensor system

4

controlling circuit

5

data processing unit

6

spiking pump

7

radiation source

8

optical cable

9

fluid pipe system

10

fluid stream

20

wafer

21, 22

layer

31

sender

32

optical radiation

33

receiver

34

optical radiation

40

fluid

41

sulphuric acid

42

hydrogen peroxide

81, 82

optical fiber

91

pump

92

filter

93

degasifier

94

heater

I

sensor signal

t

process time

t0, t1

time

K1, K2

curve

TP1, TP2

turning point

MAX

local maximum point

MIN

local minimum point