We present the application of quadrupole mass spectrometry (QMS) with threshold ionization mass spectrometry (TIMS), optical emission spectroscopy (OES) with an actinometry method, and Langmuir probe (LP) as an integrated technique for in situ plasma characterization of the electron cyclotron resonance chemical vapor deposition (ECR-CVD) of hydrogenated amorphous silicon (a-Si:H). The main aim of this study was to determine the relationship among the process parameters, plasma characteristics, and thin film properties, including the microstructure parameter (R.), hydrogen content (CH), and a-Si:H film growth rate. The physical properties of plasma, such as the electron density (Ne), electron energy (Te), and sheath potential (Vs), were studied using an LP and OES. In addition to the general plasma properties, the ion density (Ni) and plasma electronegativity were examined. The results indicate the ECR plasma source has high potential for producing a high-quality a-Si:H film because of the availability of few high silanes (SinH2n+2, n > 2); furthermore, the results indicate an obvious difference in the QMS analysis results between plasma and gas. Therefore, the consumption of parent molecules must be considered. The TIMS method was applied to estimate the relative concentration of ground-state silane radicals (SiHx, x < 4) and consumption of SiH4. The results indicate that, in addition to the degree of ionization, the consumption of SiH4 and Vs are key factors affecting the thin film growth rate. When microwave power density increases, the values of Ni, Ne, and Vs increase considerably, but the plasma electronegativity remains unchanged. Furthermore, we compared our results with the TIMS analysis reported by other authors and found that the ratio of SiH2 to SiH3 obtained using QMS and TIMS can be considered an indicator of film quality for R. in the ECR-CVD process.